6.2.1 Controlling Excessively High Soil Temperatures

Large areas of West Africa are covered by Alfisols that have high sand and gravel content in the topsoil (see Fig 4.10 in Chapter 4). The thermal diffusivity of these horizons is much lower than that of clayey topsoils. Consequently, they can retain large quantities of heat, particularly when dry. Studies in Ibadan, Nigeria, have shown that temperatures in such soils can reach 42 °C at 5 cm depth and 38 °C at 10 cm depth, when bare or recently planted (Lal Reference Lal1975). When topsoil temperatures reach 35–38 °C, they inhibit the emergence of yam sprouts and virtually stop the growth of maize and soybean seedlings (Lal Reference Lal1974). Furthermore, Lal observed that nutrient and water uptake were severely affected at such temperatures.

The solution to this problem consists of mulching with residues from a previous crop. Mulching decreases soil temperatures down to 20 cm depth, as shown in Fig. 6.2. The unmulched plots had excessively high temperatures down to that depth during early stages of growth. As crops develop a canopy soil temperatures decrease, but if a severe drought occurs later in the season, the mulch can prevent the return of excessively high temperatures (Lal Reference Lal1974).

Fig. 6.2 Mulching decreases high soil temperatures and increases soil moisture in a loamy, gravelly Alfisol of Ibadan, Nigeria (Lal Reference Lal1974).

With permission from Plant and Soil

Mulching also increases soil moisture storage throughout the rainy season, as shown in Fig. 6.2. These increases in soil moisture are equivalent to about 8–12 percent of the rainfall received. In this example, the overall effect of mulching was to increase maize yields from 3.6 to 5.4 t/ha, and to decrease weed growth to one-third of that observed in the unmulched plots (Lal Reference Lal1974). The practical problem is where to get biomass for mulching, but this can be addressed by conservation agriculture as discussed later in this chapter.

6.2.2 Controlling Low Soil Temperatures

In the tropical highlands, with isothermic or isomesic soil temperature regimes, many crops suffer from low-temperature limitations, particularly during wet periods. A wet soil also has low thermal diffusivity. Such is the case with certain pineapple plantations on Oxisols in the highlands of Hawaii. Low soil temperatures exert a negative influence on nutrient uptake, and consequently result in longer growth duration and lower yields (Ravoof et al. Reference Ravoof, Sanford, Young and Silva1973).

The solution to this problem is also mulching, but with clear plastic. Unlike the straw mulch discussed before, clear-plastic mulch is a vapor barrier that increases soil temperatures through a greenhouse effect. Transparent polyethylene mulches have also increased mean soil temperatures and decreased irrigation requirements of potatoes in northern India, where low temperatures in the late fall season limit potato growth. Grewal and Singh (Reference Grewal and Singh1974) observed that potato yields increased from 14 t/ha to 20 t/ha when such mulch was used. This increase was positively correlated with the rise in minimum soil temperatures caused by the clear mulch. They are commonly used in tropical highland regions in China for early planting, so that their fruits and vegetables reach the markets earlier when the prices are high. In addition to mitigating temperatures, plastic mulches also control weeds.

6.3.1 Aggregate Formation

The processes involved in aggregate formation and destruction are shown in Fig. 6.5, divided among three processes for soils with permanent charge and a fourth process for soils with variable charge (Six et al. Reference Six, Elliott and Paustian1999, Reference Six, Conant, Paul and Paustian2002b, Reference Six, Feller, Denef, Ogle, Sá and Albrechtc).

Fig. 6.5 Aggregate formation and destruction. The processes are: (A) fungal and bacterial activity, (B) active root growth, (C) earthworm activity and (D) iron and aluminum oxide interactions in variable-charge soils. UA = unstable aggregate, WSA = water-stable aggregate.

Adapted from Six et al. (Reference Six, Callewaert, Lenders, De Gryze, Morris, Gregorich, Paul and Paustian2002a). With permission from Johan Six

The first phase of aggregate formation is a biological one, starting with bacteria and fungi that begin to bind individual sand, silt and clay particles as well as pieces of plant- and root-derived organic matter (stage A). Afterwards, active root growth further binds particles by exudates and mycorrhizal associations (stage B). Earthworm activity and other macro- and meso-fauna further contribute to the binding of primary particles and particulate plant-derived organic material into unstable macroaggregates (stage C). These three biological processes act simultaneously; their relative roles depend on the microbial population, type of plants, macro- and meso-faunal population and soil pH (see Chapter 10). Low soil pH promotes fungal activity over bacterial activity. In variable-charge soils a fourth process occurs – the binding of clay particles with iron and aluminum oxides (stage D).

The second phase of macroaggregate formation consists of the aging of the components, with further interactions between microbial activity, sand, silt and clay particles and organic materials (along with iron and aluminum oxides in the variable-charge soils). Wetting and drying cycles strengthen the macroaggregates, making them water-stable, and forming microaggregates inside them. Organic inputs are converted into slow and passive soil organic carbon (SOC) pools (Chapter 11). In calcareous soils (b soils in the Functional Capability Classification [FCC] system), which are permanent-charge soils, carbonate and bicarbonate precipitates help to strengthen aggregates in arid environments (Bronick and Lal Reference Bronick and Lal2005).

6.3.2 Aggregate Destruction

The principal aggregate destruction process is tillage, which can physically destroy macroaggregates, “pulverizing” the topsoil. A decrease in organic inputs to the soil also fosters aggregate destruction because it decreases microbial and macrofauna activity, weakening the bonds created by fungal hyphae, root exudates and mycorrhizae. Lack of plant cover also increases soil temperatures and stops the organic additions to the soil, weakening aggregates. In soils subject to sodification (n soils in the FCC system), large amounts of sodium (Na⁺) ions can cause aggregate destruction by flocculation. The bottom of Fig. 6.5 shows the effects of the aggregate destruction processes.

6.3.3 Influence of the Three Core Factors

Texture, mineralogy and SOM determine soil aggregation. In sandy and loamy sand textures (< 15 percent clay), the primary particles tend to remain single-grained because the large macropores between them make microorganisms rapidly respire away organic inputs as carbon dioxide, instead of decomposing them slowly to form SOM that binds the primary particles into aggregates. It takes at least loamy (15–35 percent clay) and clayey textures (> 35 percent clay) to make aggregates. The choice of texture as the highest category of the FCC system (Chapter 5) indicates the importance of this core factor, not only in physical properties but in chemical and biological ones as well.

Clay minerals are also a determinant of the strength of aggregation in loamy and clayey textures. Soils with variable-charge clays (1:1 clay minerals, iron and aluminum oxides, i soils and allophane in volcanic soils (x soils in the FCC system) form tight bonds with primary particles and SOM, which give a strong structure to Oxisols and Andisols. A review by Six et al. (Reference Six, Feller, Denef, Ogle, Sá and Albrecht2002c) found higher aggregate stability in those soils than in soils with permanent charge dominated by 2:1 clay minerals.

SOM plays a major role in aggregation. It is an excellent binding agent with primary particles as well as with iron and aluminum oxides in variable-charge clay minerals. Soils dominated by such minerals show stronger aggregation at lower SOM levels than soils with permanent-charge minerals (Denef et al. Reference Denef, Six, Merckx and Paustian2002). Six et al. (Reference Six, Feller, Denef, Ogle, Sá and Albrecht2002c) found that a decrease in SOM contents results in a smaller decrease of aggregate stability in soils with variable-charge clays than in those with permanent-charge clays.

What is considered a good structure depends on the desired speed with which air and water move through the soil. A good structure for growing flooded rice is that attained by puddling (the destruction of aggregates), to eliminate downward water movement (Sanchez Reference Sanchez1973). For other tropical systems, a good structure is one which maintains aggregate stability upon abrupt changes of soil moisture and intense rainfall (Pereira Reference Pereira1956).

Soil pores are where the action is in terms of air and water dynamics, as well as biological activity. While originally measured indirectly, it is now possible to quantify the soil pores directly, including pore connectivity and tortuosity, through non-destructive image analysis techniques (Moran and McBratney Reference Moran and McBratney1992). For example, Pires (Reference Pires2009) provided insights of the spatial arrangement of Oxisol pores in response to compaction.

6.3.4 Measuring Soil Structure

There is no single parameter that adequately measures soil structure, although water-stable aggregate stability is often used in the laboratory (Pereira Reference Pereira1956, Six et al. Reference 175Six, Elliott and Paustian2000a, Reference Six, Paustian, Elliott and Combrinkb), and is qualitatively estimated in the SoilDoc field kit (Weil Reference Weil2011). Pretreatment of soil, previous water content and differences in sand size distribution among soils can confound interpretation of these measurements.

Field tests using double-ring infiltrometers, when adequately replicated, are perhaps the most pragmatic means of evaluating soil structure, by estimating saturated hydraulic conductivity. Field infiltration measurements integrate many of the physical variables and provide a soil volume large enough to reduce the variability caused by insect and root channels. Lugo-López et al. (Reference Lugo-López, Juárez and Bonnett1968) applied this technique to 749 soils in Puerto Rico and found that infiltration rates during the first 3 hours depended on the initial soil moisture content. After the fourth hour, infiltration rates became constant and approximated the saturated hydraulic conductivity. A summary of their results by soil order appears in Table 6.1. The high maximum and minimum infiltration rates of Oxisols, Ultisols and Mollisols reflect their good structure, the very high maximum values in Alfisols and Inceptisols probably reflect sandy topsoil texture, and the low range of Vertisols indicates their high content of expanding 2:1 clays. It is important to note the wide range of infiltration rates within soil orders, which indicates that generalizing at the order level tells you only that Vertisols have lower infiltration rates than Oxisols, Ultisols and Mollisols.

6.3.5 Structure of Major Soils of the Tropics

Taking the idea of core soil properties (texture, mineralogy, SOM), I use examples from some of the most extensive tropical soils to describe the range in soil dynamics commonly found in the tropics.

Oxisols, and oxidic or rhodic groups and subgroups of Ultisols and Alfisols (Nitisols in the World Reference Base (WRB) system), generally possess excellent and strong granular structure. These are the Ci soils in the FCC system. Many Oxisols can be plowed a day after heavy rain with little aggregate disruption.

The excellent structure of Ci soils is caused by primary particles aggregated into very stable, sand-sized macroaggregates (shown in brown in Table 6.2). Their stability is due to their high clay content and coatings of amorphous iron oxides. Soil-organic-matter content is also correlated with aggregate stability in Oxisols, as it plays a cementing role between clay particles. When estimating soil texture by hand, clayey Oxisols initially feel like sandy loams. As one continues to work the moistened soil with one’s fingers, it gradually feels more clayey as the macroaggregates are progressively destroyed.

Table 6.2 Thin sections of soils with contrasting physical properties. Thin sections from Eswaran and Drees (Reference Eswaran and Drees2002) and Stanley Buol (personal communication, 2016).

When Oxisols are subjected to particle size analysis without pretreatment with sodium dithionate to remove free iron oxides, the clay contents read lower than when iron oxides are removed. In an example given by Moura and Buol (Reference Moura and Buol1972), the clay content of a Eutrustox increased from 40 percent to 83 percent when iron oxides were removed. The actual clay content of such Oxisols can be estimated by multiplying the water content at –1500 kPa by a factor of 2.5 (Soil Survey Staff 1970). This is a classic pedotransfer function.

The above generalizations apply to other soils carrying the Ci designation, mainly oxidic and rhodic groups and subgroups of Alfisols (Nitisols), including well-known ones like Terra Roxa Estruturada in Brazil and Kikuyu Red Loam in Kenya.

These generalizations do not hold throughout the Oxisol order. Uehara et al. (Reference Uehara, Flach and Sherman1962) observed different structural characteristics in Oxisols of similar clay contents, and mineralogical composition (primarily kaolinite and amorphous iron and aluminum oxides). Differences in aggregate stability could not be correlated with free iron oxide contents beyond the 5 percent level. Cagauan and Uehara (Reference Cagauan and Uehara1965) then determined that these differences were associated with the degree of anisotropy (clay orientation) inside the aggregates. A more oriented system of kaolinite particles cemented or coated with iron oxides provides stronger aggregates than a less oriented arrangement. The greatest degree of anisotropy has been found in ustic soil moisture regimes.

Oxisols in udic regimes exhibit less clay orientation, as is found in the well-researched Latossolo Amarelo, textura muito pesada (Yellow Latosol, very heavy texture, Acrudox), with over 80 percent clay throughout the profile, and which covers extensive areas of the Brazilian humid tropics, north of the Amazon River, and the Congo rainforest in southern Cameroon. They do not show granular structure (Chauvel et al. Reference Chauvel, Grimaldi and Tessier1991). Soils in continuously humid climates do not retain as strong a structure as those with marked seasonal moisture fluctuations (Denef et al. Reference Denef, Six, Merckx and Paustian2002).

Consequently, clay and SOM content, free iron oxides and the orientation of kaolinite within the aggregates are the main factors associated with the excellent structure of Oxisols, but the degree of manifestation of this structure varies.

Ultisols and Alfisols have less desirable structural properties because of a generally sandier texture in the A horizon. Clay skins, formed by clay migration from the A horizon, coat aggregates in the argillic B horizon. They are anisotropic and provide increased aggregate stability in the subsoil. Briones and Veracion (Reference Briones and Veracion1965), working with Philippine “red” soils (Alfisols and Ultisols), found that the aggregate stability of these soils increased linearly with clay content up to about 50 percent clay. This is shown in Fig. 6.6.

Fig. 6.6 Aggregate stability increases with clay content in “red” soils (Alfisols, Ultisols) in Luzon, Philippines (Briones and Veracion Reference Briones and Veracion1965).

The coarser-textured topsoils of Ultisols and Alfisols have lower aggregate stability than Oxisols, partly due to the differences between the sandy or loamy topsoils and the loamy or clayey argillic horizons, shown in the thin sections of Table 6.2. Alfisols with gravelly topsoils or with shallow subsoil depths are common in West Africa and parts of southern Africa (Kayombo and Lal Reference Kayombo and Lal1993). These gravelly horizons have poor structure, low water-holding capacity and excessively high soil temperatures, which can have a negative effect on root development and crop yield (Lal Reference Lal1975). Soils with more than 10 percent gravel (2–25 cm in diameter) in the top 50 cm cover 145 million hectares or 3 percent of the tropics (Chapter 5).

Structurally inert soils (not shown in Table 6.2) have topsoils low in SOM, but with sufficient fine-sand and silt content (about 10–20 percent) that can bind larger sand and gravel particles, and behave very differently from other Alfisols. They are widespread in semiarid regions of the Sahel, southern Africa and Australia and are termed structurally inert by Lal (Reference Lal, Lal, Sanchez and Cummings1986). These soils are treated separately from the more conventional sandy Alfisols and Ultisols of the humid and subhumid tropics, and are identified as z soils in the FCC system (Chapter 5).

Andisols also have extremely stable soil structure. The x attribute in the FCC system identifies such soils. The high SOM content of Andisols is intimately associated with allophane and imogolite, the main amorphous mineral species, providing strong aggregation. The macroaggregates in Andisols are somewhat larger than those in Oxisols, but their stability is similar. A study of 18 volcanic soils from Pasto, Colombia showed that 81 percent of the water-stable aggregates were greater than 2000 µm in diameter – very large macroaggregates (Escobar et al. Reference Escobar, Jurado and Guerrero1972).

Andisols usually have very low bulk density (0.4 to 0.8 t/m³) because of their mineralogy, high SOM content and high porosity (see thin section in Table 6.2). The absence or near absence of layered silicate minerals in many of these soils makes them difficult to disperse because of the strong bonding between allophane and organic matter. Attempts to estimate clay content by multiplying the –1500 kPa water content by 2.5 failed in these Andisols.

A few volcanic soils are essentially devoid of crystalline materials, such as the Hydrudands of Hawaii. They have thixotropic properties: upon force, the soil suddenly becomes fluid (Swindale Reference Swindale1964). In most Andisols, however, thixotropy does not occur.

The structural properties of other tropical soils, dominated by permanent-charge clays – Vertisols, Mollisols, Aridisols – are different from those observed in most of the previously mentioned soils and more typical of temperate-region soils. In such soils, increased anisotropy means lower aggregate stability, because little or no iron coats are present.

Vertisols and vertic subgroups (v soils in the FCC system) have poor aggregate stability, which is associated with low soil-organic-matter content and expanding, smectitic clays (Lugo-López and Juarez Reference Lugo-López and Juárez1959, Lugo-López and Pérez-Escolar Reference Lugo-López and Pérez-Escolar1969). Vertisols can be tilled adequately at a much narrower moisture range than other soils. When they are dry, shrinkage and cracking breaks the topsoil into massive clods of such strength as to make most tillage practices difficult (see thin section in Table 6.2). When wet, they are soft and sticky. One can certainly feel differences in clay mineralogy by driving on unpaved roads of “red” kaolinitic soils (Alfisols, Oxisols) and on “black cotton soils” (Vertisols) in Africa, often getting stuck in the Vertisols.

Aridisols have a notoriously weak structure. Because of their low SOM content and predominantly 2:1 mineralogy, aggregates easily fall apart when wetted. Those high in sodium saturation (n soils in the FCC system) disperse readily. Free carbonates and bicarbonates of calcareous soils (b soils) bind soil aggregates less strongly than SOM and iron and aluminum oxides.

In contrast, Mollisols have a good structure due to their high SOM content in both the A and B horizons.

The physical behavior of Entisols and Inceptisols cannot be generalized at the order level.

6.3.6 Changes with Tillage

Soil structure often changes with tillage. The degree of structural change varies with the core soil properties (texture, mineralogy, SOM), and with management.

The effects of intense cultivation on the size distribution of water-stable aggregates were determined by Grohmann (Reference Grohman1960) in an Oxisol and an Ultisol from Brazil. The results in Table 6.3 indicate that cultivation reduced the percentage of macroaggregates larger than 2000 µm by about half in both soils, which were converted into smaller macroaggregates (210–2000 µm). But the percentage of microaggregates (< 210 µm in this example) did not increase in the Oxisol, while it increased drastically in the Ultisol, with a less strong structure. These microaggregates can clog the macropores between the larger aggregates and decrease infiltration.

Moura and Buol (Reference Moura and Buol1972) compared the effects of 15 years of annual cropping in a Eutrustox and observed that infiltration rates decreased from 82 cm/h to 12 cm/h (Table 6.4). The decrease in infiltration was due to a sharp decrease in macropores (> 50 µm) in both the A and B horizons, whereas the micropores (< 50 µm) remained essentially unchanged. Compaction by machinery was the cause of the decreased macroporosity. Moura and Buol also observed that water-dispersible clay contents decreased in the A horizon with cultivation but increased in the B horizon. Some clay translocation took place and this reduced macroporosity. The observed drop in infiltration rate, however, could be considered beneficial because of reduced percolation and potential leaching.

One aspect that is seldom considered is that mineral fertilizer applications, when properly managed, generally improve soil aggregation, in comparison to unfertilized soils (Subbian et al. Reference Subbian, Lal and Akala2000, Hamza and Anderson Reference Hamza and Anderson2005). When crops are well nourished, they have higher root volume, protect the soil from erosion with their crop canopy, and provide more fresh organic inputs and then SOM through root decomposition and incorporation of crop residues.

6.4.1 Surface Sealing and Crusting

Soil surface sealing or crusting is a major management problem that has received considerable attention in semiarid West Africa (Fig. 6.7), southern Africa and Australia. Surface sealing occurs when the soil is wet, while crusting refers to seals that have dried and hardened (Roth Reference Roth, Summer and Stewart1992). The sealed layer at the soil surface is about 5 mm thick (Fig. 6.8).

Fig. 6.7 Surface sealed sandy Alfisol in Burkina Faso.

Fig. 6.8 A thin, sealed layer of about 5 mm from the surface of a structurally inert sandy Alfisol of Segou, Mali.

Surface sealing is most acute in structurally inert Alfisols. Surface sealing does not occur in loamy sands and pure sands because these soils are single-grained – there is nothing to compact. It seldom occurs in Oxisols and Andisols because of their strong granular structure, nor in smectitic silty and clayey Aridisols and Vertisols. Again, the three core factors, texture, SOM and mineralogy, play a determinant role in this process.

Surface sealing prevents seedling emergence, sharply reduces infiltration, decreases water storage in the soil and triggers runoff, and hence erosion. Unlike other kinds of soil compaction, surface sealing is largely due to raindrop impact.

Valentin and Bresson (Reference Valentin and Bresson1992) recognized three types of crusts, structural, erosional and depositional, in the Sahel. They are formed in sequence. Structural crusts begin to develop during the first minutes after high-intensity rains hit a dry topsoil. Erosional crusts consist of an exposed silt–clay layer after the removal of sandy particles by wind or runoff. Depositional crusts are formed by translocation of clay particles and their subsequent deposition away from the original location.

Mineralogy plays a major role in defining crust formation. In topsoils with smectitic clay mineralogy, structural crusts mainly result from aggregate breakdown. They are caused by entrapped air compression when a dry soil is suddenly and thoroughly wetted. The advancing water compresses air in the soil pores until small air explosions occur, causing the breakdown of macroaggregates into primary particles (Yoder Reference Yoder1936). This process is called slaking, and occurs in aggregates that are not strongly bound by iron and aluminum oxides or by SOM.

Valentin and Bresson (Reference Valentin and Bresson1992) described a swelling crust, commonly observed in tilled arid loamy soils with smectitic clay mineralogy. Upon wetting, clay lattices expand and fill the interstices between cracks. This process can trigger runoff within a few minutes of rainfall. The swelling process then combines with the rapid slaking of aggregates, creating a crust. Upon drying, the crusts crack and curl. They are often photographed as the effects of global warming on increasing droughts, which is usually unrelated.

In the structurally inert Alfisols, structural crusts are made of a layer of loose sand and gravel-sized grains, overlaying a layer of finer material. They develop primarily as a result of raindrop impact, which forms microcraters, the walls of which present a clear, vertical sorting of particles. A lack of smectitic mineralogy prevents slaking and swelling (Valentin and Bresson Reference Valentin and Bresson1992). Such crusts are common in the Sahel.

Erosional crusts consist of one rigid, thin and smooth surface layer, rich in fine particles, a result of the destruction of structural crusts. Voids are generally restricted to some cracks and vesicles, and in arid environments they are often referred to as “desert pavement.”

Depositional crusts are typically very compact. Separation between fine particles (plasma) and coarse particles (skeleton) induces alternate microbeds. Still, “depositional crusts” form in standing water and develop where surface flow is hindered. In puddles, the larger grains sink rapidly and form the bottom layer, whereas the finer grains deposit more slowly and form the top layer (Valentin and Bresson Reference Valentin and Bresson1992).

The presence of algae and fungi strengthens both erosional and depositional crusts. Particularly under schlerophyllic (cactus-type) vegetation, they become hydrophobic or water-repellent (McKenzie et al. Reference McKenzie, Coughland and Cresswell2002).

Surface sealing has a devastating effect on the hydrology of the Sahel. It was amazing to see, at the beginning of the rainy season when the soils are surface-sealed, a 5-cm layer of water flowing on flat terrain and eventually emptying into the Niger River, with minimal infiltration (Fig. 6.9). Not only is rainfall marginal in the Sahel, but also much of it does not get inside the soil.

Fig. 6.9 Runoff flowing on a level, surface-sealed Alfisol near Niamey, Niger.

After some dry weather, farmers plow with cattle and break the crust, allowing water to infiltrate. In the meantime, much of the rains that were supposed to replenish moisture in the soil profile are lost. A feasible partial solution is live fences with leguminous shrubs such as Ziziphus mauritiana, and some Acacia species that can survive the 9-month dry season and prevent surface sealing by providing a leaf canopy under which the soil remains porous and runoff water infiltrates, replenishing some of the field subsoil moisture. In addition, these live fences attenuate wind and water erosion, fix nitrogen, produce high-value fruits, in the case of Ziziphus, and replace dead fences made from sorghum or millet stems (see Chapter 19).

Surface crusts can be reversed when vegetation and soil fauna recolonize crusted areas (McKenzie et al. Reference McKenzie, Coughland and Cresswell2002). Simply leaving twigs or straw on the soil attracts termites in the Sahel, where the resulting “biological tillage” breaks the surface seal (John Wendt, personal communication, 1980). But the need to use every bit of biomass for fuel and animal feed in semiarid areas prevents this from becoming a realistic intervention, at scale. Therefore, sealed surfaces have to be broken with tillage, be it biological tillage via termites or mechanical tillage by plowing. But they return after the next rain.

6.4.2 Hardsetting

Another form of soil compaction, unrelated to surface sealing or the application of force to the soil surface, is hardsetting. It occurs when much of the entire soil becomes so hard as it dries that it is virtually impossible to cultivate until it is rewetted (Mullins et al. Reference Mullins, MacLeod, Northcote, Tisdall and Young1992). Hardsetting in a cultivated soil usually involves destruction of soil aggregates during wetting, and an increase in bulk density and soil strength in subsequent drying. Hardsetting happens without the application of an external force, making this process a unique form of natural soil compaction.

Hardsetting occurs in soils with little or no water-stable aggregates, in textures ranging from loamy sands to clay loams, with kaolinitic or hydrous mica mineralogy and low SOM content. When thoroughly wetted, such soils lose their structure by the slaking process described previously, and a subsequent slumping of dispersed soil particles. As the soil dries, the whole profile hardens from the surface downwards, reaching extremely high bulk densities (1.8 t/m³) and soil strength in the top meter (Mullins et al. Reference Mullins, MacLeod, Northcote, Tisdall and Young1992). Figure 6.10 shows how difficult it is to auger a hardset, sandy Alfisol during the dry season. When a hardset soil is wetted again by rainfall or irrigation, it loses soil strength and decreases its bulk density.

Fig. 6.10 Trying to auger through a hardset, sandy Alfisol in Mali.

Photo courtesy of Markus Walsh

Hardsetting occurs in large areas of the semiarid tropics with ustic soil moisture regimes. Hardsetting occurs (again) in sandy soils with a high fine-sand and silt content, low SOM and kaolinitic clay mineralogy. It also occurs in Alfisols, Entisols and Inceptisols of the Sahel, Botswana and Australia (Nicou and Charreau Reference Nicou and Charreau1980, Peltier Reference Peltier1993, McKenzie et al. Reference McKenzie, Coughland and Cresswell2002) in soils that have no water-stable aggregates. Hardsetting is common in “duplex” soils of Australia (Natraqualfs), where only the A horizon hardsets, aided by high sodium saturation (n soils in the FCC system). Hardsetting does not happen when smectitic clays are present because they shrink and swell with changes in moisture. It also does not happen in coarse sands (Quartzipsamments) because they have no structure as such, nor in Oxisols, Andisols and Mollisols, which have mostly water-stable aggregates. Such soils and many Entisols and Inceptisols with water-stable aggregates remain friable when dry. Hardsetting does not happen in udic soil moisture regimes.

Management practices that minimize soil disturbance and add crop residues can prevent hardsetting. Mullins et al. (Reference Mullins, Young and Bengough1987, Reference Mullins, MacLeod, Northcote, Tisdall and Young1992) report that some maize farmers near Kabwe, Zambia incorporate chopped cereal stalks into the topsoil, which prevents hardsetting. When there is a short rainy season in Botswana, farmers delay planting, having to wait until the first rains have sufficiently softened the soil, which can diminish yields (Willcocks Reference Willcocks1981).

6.4.3 Plow Pans

Tillage and farm machinery traffic can also induce subsoil compaction in the form of a plow pan. The depth of the pan varies from 10 cm to 60 cm and is usually several centimeters thick. Plow pans can develop in most tropical soils except Vertisols, where shrinking and swelling through the profile prevents their formation.

Although Oxisols in subhumid savannas (Ustox) are the strongest-structured tropical soils, excessive tillage can produce plow pans that lead to classic land degradation symptoms. The Brazilian Cerrado was opened with heavy disk harrows, used to incorporate lime to a depth of at least 30 cm, as an effective way to eliminate aluminum toxicity and allow for crop root development to that depth (González-Erico et al. Reference Gonzalez-Erico, Kamprath, Naderman and Soares1979). Due to the large farm size, the disk harrows used became larger, and after a few years a plow pan developed at a depth of 12–15 cm, where water-dispersible clays have moved to from the harrowed topsoil (Lopes and Daher Reference Lopes and Daher2008). They are locally called pé de grade (foot of the harrow). Movement of water-dispersible clays, a product of partial aggregate breakdown, helps form these plow pans.

Plow pans form at a depth of 15–20 cm in loamy and clayey Alfisols and Ultisols of the African savanna, and in semiarid regions with intensively mechanized cultivation, resulting in restricted infiltration and root penetration, and lowered grain yields. The situation is worse with 25-cm-deep moldboard plowing (Kayombo and Lal Reference Kayombo and Lal1993).

6.4.4 Managing Soil Compaction

Perhaps the best management practice with such soils, other than the appropriate tillage, is to prevent compaction, by protecting the soil surface with mulch or plant canopies that decrease the energy of raindrop impact and the rate of SOM decomposition as well. A bare soil is very exposed to surface compaction. Lal (Reference Lal1975) showed that mulching also prevented excessively high soil temperatures, increased soil water storage, decreased weed infestations and inhibited runoff and erosion in loamy textured Alfisols of Nigeria. Evidence from sandy textured Ultisols of the Amazon supports Lal’s findings (Wade and Sanchez Reference Wade and Sanchez1983). In traditional shifting cultivation systems, the soil is never tilled but it is protected with an ash mulch and, later, with a constant crop canopy.

Mulching can prevent surface sealing in most soils. In practice, the only practical mulch is the one that is produced in situ by leaving crop residues on the soil surface. Transporting mulch from outside the fields is expensive, in capital or labor, and only feasible for very small areas, growing high-value crops. Leaving the land fallow so that some grass and tree seedlings establish helps to break crusts down. Valentin et al. (Reference Valentin, Rajot and Mitja2004) found that physical properties were restored after a fallow period of 10 years on a clayey loam soil in Cote d’Ivoire, whilst crusts persist over a longer time on sandy soils.

Consequently, management to minimize or prevent the different kinds of soil compaction varies with the core soil properties. As mentioned previously, sands (Quartzipsamments) have no aggregation and therefore are not subject to compaction. At the opposite end of the soil spectrum, Vertisols with cracking and swelling smectitic clays do require conventional tillage and this must be done during a relatively narrow range of moisture content. Soils that are highly susceptible to surface sealing or hardsetting require tillage every year. In most other soils, minimum or no tillage is feasible and should be the preferred option, both in easily compacted soils as well as the most highly aggregated soils, Oxisols and Andisols. Kondo and Dias Júnior (Reference Kondo and Dias Júnior1999) showed that resistance to compaction increased with increasing clay content in Oxisols. Many loamy Oxisols suffer readily from compaction (by definition, there are no sandy Oxisols).

6.5.1 Soil Water Retention

Soil water (soil moisture) is divided into three classes according to its potential energy, or soil moisture tension, which is now expressed in negative kilopascals (kPa) and previously in bars (1 bar equals –100 kPa). A saturated soil has zero tension (0 kPa). The first class, gravitational water, is that which is held in the macropores, normally between 0 kPa and –10 to –30 kPa. This tension range is called the “field capacity,” defined as the range at which the downward movement of water ceases. Secondly, the available water to plants resides mainly in the micropores, as films, where most of the nutrient uptake and ion exchange reactions take place. In soil chemistry, we call these films the soil solution. Soil moisture tension increases as the soil dries and the films become thinner. The available water is that which is held between the field capacity and the permanent wilting point (–1500 kPa). Residual water, the third class, is held at tensions lower than –1500 kPa, but it is not relevant to plants (McKenzie et al. Reference McKenzie, Coughland and Cresswell2002). However, some gravitational water is also available to crops, so the term “available water” is often an underestimate.

The differences in structure between broad kinds of tropical soils result in dramatic differences in their water-retention properties. Figure 6.11 shows the classic soil moisture characteristic curve of a sandy soil and two clayey soils, one with oxidic and the other with smectitic mineralogy, and an Andisol with amorphous mineralogy. The sandy soil empties its pores of gravitational water at tensions close to –10 kPa, while the smectitic clay does this at –50 kPa. This is what is traditionally known as field capacity, but the tensions vary from the classic 0 to –30 kPa definition.

Fig. 6.11 Moisture-retention curves of a sandy soil, a clayey Oxisol, a clayey Vertisol and an Andisol.

The moisture-retention pattern of a clayey, well-aggregated Oxisol is a hybrid of these two (Fig. 6.11). These Oxisols hold as much water as smectitic soils, up to approximately –10 kPa, but drain their macropores at about that tension, thus reaching field capacity, as sands do. Oxisols act like sands at low tensions because of their strong, sand-sized macroaggregates, but hold water like smectitic soils at higher tensions. Therefore, they have a narrower available-water range than other clays. The most commonly accepted value for Oxisol field capacity is about –10 kPa (Ruiz et al. Reference Ruiz, Ferreira and Pereira2003).

Many Oxisols have drought problems that are completely out of proportion to their clay contents. The micropores in the microaggregates may be close to saturation, while the macropores between the aggregates are totally depleted of available moisture to crops. These unique properties are due to the aggregate size distribution. Figure 6.12 shows that the rapid drainage at soil moisture tensions between –1 kPa and –10 kPa is due to the predominance of sand-sized water-stable aggregates (2 mm) in these soils. At moisture tensions greater than –20 kPa, the aggregate size is irrelevant because it is the micropores within the aggregates that are being drained.

Fig. 6.12 Soil-moisture retention curves of different aggregate fractions of an Oxisol (Sharma and Uehara Reference Sharma and Uehara1968).

Andisols also possess unique water-retention characteristics. They hold considerably more water at lower tensions than other soils because of the high macroporosity and low bulk density of their aggregates. If the water content is calculated on a weight basis, the figures may range from 100 percent to 300 percent water. If calculated volumetrically (the correct way), their moisture contents may run as high as 84 percent water at –30 kPa field capacity and 45 percent water at the wilting point, –1500 kPa. Andisols, therefore, hold more water at a given tension than most other soils, as shown in Fig. 6.11 Although the quick attainment of field capacity is due to the drainage of large pores between very stable microaggregates, the high microporosity inside these aggregates accounts for their particular water-holding capacity (Forsythe and Diaz-Romeu Reference Forsythe and Diaz-Romeu1969, Swindale Reference Swindale1969). Andisols generally have more water available to plants than Oxisols because organic matter–allophane microaggregates are more porous than kaolinite–iron oxide microaggregates.

Ultisols and Alfisols with oxide coatings on kaolinitic clays seem to have better moisture-retention properties than Oxisols in general. The large areas of tropical soils with conventional 1:1 and 2:1 layer-silicate mineralogy have moisture-retention curves similar to those presented in standard textbooks.

6.5.2 Water-Holding Capacity

The definition of available water as the difference in volumetric water (percent water by volume) held at field capacity (–30 kPa) and that at the permanent wilting point (–1500 kPa) is still widely used. The original work of Briggs and Shantz (Reference Briggs and Shantz1914), using one plant (sunflower) and one loamy soil, has become a general principle. There are important limitations to this principle, both conceptually and quantitatively. Available water is not uniformly available in soils; roots do not extract it uniformly, and the replenishment of water lost by transpiration in soil close to the roots may be slow enough to cause wilting in the presence of ample water. Rice plants may be observed with wilted leaves, growing in a flooded soil on hot sunny days. There are also rather large differences between crops and varieties in their ability to extract water at different soil moisture tensions.

A study with several Oxisols, Vertisols, Inceptisols and Entisols in Zambia, by Maclean and Yager (Reference Maclean and Yager1972), showed that soil moisture tensions between –10 kPa and –20 kPa are better approximations of the actual field capacity than the classic value of –30 kPa. When the available water was calculated as the difference between –30 kPa and –1500 kPa, the result was an underestimation of the available-water range by 35 percent compared with actual field measurements in these soils.

Sharma and Uehara (Reference Sharma and Uehara1968) in Hawaii and Wolf (Reference Wolf, Bornemisza and Alvarado1975) in Puerto Rico have shown little difference in water content between –100 kPa and –1500 kPa. A series of in situ studies on Oxisols and Ultisols in Puerto Rico and Brazil led Wolf (Reference Wolf, Bornemisza and Alvarado1975) to recommend that the available-water range of these soils should be estimated as the difference in water content between –10 kPa and –1500 kPa. Brazilian scientists now consider the available water range in Oxisols to be that held between –6 kPa and –100 kPa (Resck et al. Reference Resck, Ferreira, dos Santos Júnior, da Sá and Figueiredo2008), and the old –30 kPa to –1500 kPa field capacity to wilting point range is no longer applicable to these soils. The field capacity limit has also been lowered from –30 kPa to –10 kPa in Australia, where most soils are not Oxisols (McKenzie et al. Reference McKenzie, Coughland and Cresswell2002).

Table 6.6 shows some representative available-water ranges, albeit using the old soil moisture tension limits and expressed gravimetrically. Regardless of texture, the available-water range of Oxisols is low, less than 10 percent. Ultisols generally have higher available-water capacities, Andisols hold much more water, and the two permanent-charge soils (Aquept and Ustert) hold about twice the available soil water than Oxisols.

Access to available soil water by plant roots depends on how deep they can grow, and this also depends on management. A 22-year experiment in a Cerrado Oxisol indicates that the available water stored in the top 100 cm ranged from 576 mm to 672 mm (Resck et al. Reference Resck, Ferreira, dos Santos Júnior, da Sá and Figueiredo2008). Maize roots can reach this depth only after lime and gypsum applications that promote calcium leaching into the subsoil. Otherwise, they are limited to the water held in the topsoil (Chapter 9).

Available-water capacity is one of the most important soil properties. Pedotransfer functions have been developed to estimate this capacity using commonly measured physical properties. The functions that seem to fit best for the tropics are those that include texture, mineralogy and SOM. Gaiser et al. (Reference Gaiser, Graef and Cordeiro2000) obtained better predictions when they subdivided soil samples from northeast Brazil and Niger according to whether they had low-activity clays (< 24 cmol_c/kg clay) or permanently charged clays of smectitic and mixed mineralogical families. Pedotransfer functions for estimating volumetric available water capacity are based on a direct regression between the soil water content at field capacity minus the soil water content at the permanent wilting point as the dependent variable; and clay, silt and organic carbon as independent variables. Gaiser and co-workers found that pedotransfer functions for the low-activity clay soils differed considerably from those for the high-activity clay soils, with respect to the contribution of total organic carbon and silt content to the prediction of soil water content at field capacity and wilting point. The overall pedotransfer functions for volumetric available-water capacity (AWC) were:

For low-activity clay soils

For high-activity clay soils

Pedotransfer functions have also been developed for other Brazilian soils (Tomasella and Hodnett Reference Tomasella and Hodnett1998, van den Berg et al. Reference van den Berg, Klamtb, van Reeuwijkc and Sombroek1997) with higher clay contents than Gaiser’s data set, which are more typical of clayey Oxisols. All pedotransfer functions have to be locally validated, but the message is that clay mineralogy, texture and SOC have to be considered.

Bulk density is also needed to convert the usual gravimetric water contents to volumetric ones, which is what roots really use. Since bulk density is seldom measured, it may have to be modeled using assumptions.

6.5.3 Leaching

Infiltration in excess of the soil’s available water-holding capacity causes nutrient losses through leaching. Estimates of leaching losses from selected tropical locations are summarized in Table 6.7. The amounts vary considerably with annual rainfall, soil properties and crops grown.

The Colombian study in Table 6.7 was carried out in highly porous Andisols, high in weatherable minerals, and under high annual rainfall, which produced high losses of nitrates, potassium, calcium and magnesium from leaching (Suárez de Castro and Rodríguez Reference Suárez de Castro and Rodríguez1955, Reference Suárez de Castro and Rodríguez1958). The data from the Philippines was obtained on an older soil, grown with flooded, but not puddled, rice, at two controlled leaching rates (Reyes et al. Reference Reyes, Galvez and Nazareno1961). The low nitrogen values reflect the absence of nitrates under reduced conditions. The differences between the two controlled leaching rates suggests a similar composition of the leachate.

The three West African sites representing older Alfisols and Ultisols were obtained with in situ lysimeters (Charreau Reference Charreau1972). They show progressively higher nutrient losses with increasing annual rainfall. The high leaching losses of nitrogen and potassium in the Côte d’Ivoire soil are probably due to the fertilization of the banana plantation, as compared with the rubber plantation, which was not fertilized.

6.5.4 Runoff

When high-intensity rainfall events exceed the infiltration capacity of soils, or when some soils become surface-sealed, rains often cause considerable runoff (water running over the soil), even on gentle slopes. Table 6.8 shows the average values for annual runoff in four West African locations under forest, cultivation and bare soil conditions. Soil cover is the predominant factor affecting runoff, in spite of differences in slope and annual rainfall. The lower runoff values in cultivated soils than in bare soils underscores the need to keep a constant soil cover.

In a well-aggregated Andisol from Chinchiná, Colombia, considerable runoff amounting to 62 percent of the annual rainfall was measured by Suárez de Castro and Rodríguez (Reference Suárez de Castro and Rodríguez1955, Reference Suárez de Castro and Rodríguez1958) on bare soils with slopes of 22 percent. Table 6.9 shows that large quantities of calcium and magnesium were washed away in the process. When pastures or coffee plantations were established, runoff, erosion and leaching losses decreased drastically. Terracing reduces runoff in young coffee plantations, but an old and very steep coffee plantation without terraces produced the least runoff, probably because the coffee canopy was well developed.

Runoff losses from experimental high-intensity rainstorms were evaluated by Barnett et al. (Reference Barnett, Carrecker and Abruña1972) on steep Ultisols and Inceptisols in Puerto Rico. Substantially lower runoff and nutrient removal was observed in the Ultisols, which were better aggregated.

Runoff losses are often deposited in the same fields, resulting in no net losses of water or nutrients at the field scale, although important local effects are created. In other cases, runoff steals most of the rainfall from surface-sealed Alfisols in the Sahel, as previously mentioned.

6.6.1 Rainfall Erosivity (R)

Erosivity is the potential ability of rain to cause erosion (Lal Reference Lal1976). It is independent of total annual rainfall, its seasonal distribution and the properties of soils on which it falls. Rainfall erosivity is best measured as the product of the kinetic energy of the storm (in mega joules per hectare [MJ /ha]) times the 30-minute maximum intensity (in mm/h), which is the highest average rainfall intensity during a 30-minute period. This measure (the erosivity index, EI₃₀) is considered the best estimate of rainfall erosivity (Wischmeier et al. Reference Wischmeier, Smith and Ukland1958, Lal Reference Lal1976). Rainfall intensity is also used as a measure of rainfall erosivity, ignoring the more difficult to measure kinetic energy. For example, Emerson (Reference Emerson1995) suggested that rainfall intensities of 25 mm/h are sufficient to start erosion in Australia.

Rainfall intensity is generally higher in the tropics than in temperate regions because higher air temperatures provide more energy. The exceptions are subtropical areas (~30° N), often affected by hurricanes or typhoons. Rainfall intensity in the tropics is often sustained at 150–200 mm/h for 10–15 minutes, with large drops having a median diameter of 2.5 mm (Kayombo and Lal Reference Kayombo and Lal1993).

A range of rainfall erosivity indices is shown in Table 6.10 as a function of latitude. High erosivity indices, EI₃₀ > 7000, occur in the tropics, with the exception of the highlands in the coffee zone of Colombia, and a site at sea level in Hawaii. Low EI₃₀ values are found in São Paulo state in Brazil and in Germany. The United States shows variability of two orders of magnitude, including very high levels found in rainstorms in the Midwest and other temperate regions.

The low rainfall intensities of Germany and much of Western Europe led European soil scientists, when confronted with tropical rainstorms, to assume a great difference between tropical and temperate regions in rainfall intensity. American scientists, more used to a wide range of rainstorm intensities in their temperate regions, were less ready to make such an assertion.

6.6.2 Soil Erodibility (K)

Erodibility is the quantitative index of inherent susceptibility of soil to erosion by water. It is measured by soil losses from Wischmeier plots, 22.2 m long, on a uniform 9 percent slope, under continuously bare soil (El-Swaify and Dangler Reference El-Swaify and Dangler1982). Soil erodibility is then calculated as tons per hectare of soil loss divided by EI₃₀. This is known as the K value, relating soil loss per unit of rainfall erosivity.

Some examples in Table 6.11 show that soils dominated by low-activity clays and iron oxides (Oxisols, Ultisols) as well as allophane (Andisols) have generally lower erodibility than soils with high-activity clays (Aridisols, Mollisols, Vertisols). The high erodibility of loose marl deposits in Morocco is probably indicative of the extreme erosion that is found in similar deposits in Haiti.

Vertisols are particularly susceptible to gully erosion. Specific vertic properties, such as low hydraulic conductivity, slumping due to slickensides and subsurface piping through cracks, facilitate gully head formation (Deckers et al. Reference Deckers, Spaargaren, Nachtergaele, Syers, Penning de Vries and Nyamudeza2001).

Although erodibility is largely determined by the soil, the underlying parent material can play a role. One of the worst-case scenarios I have seen is when Vertisols are underlain by loose unconsolidated lake sediments, such as in the Nyando Basin near Lake Victoria in Kenya. At the start of the rainy season, cracks in Vertisols allow highly erosive rains to penetrate these extremely erodible sediments, rapidly creating huge gullies.

6.6.3 Slope Length and Gradient (LS)

Topography has major effects on soil erosion. The LS factor refers to slope length and gradient. The worldwide availability of Shuttle Radar Topography Mission (SRTM) satellite data at 90-m resolution makes the assessment of these parameters straightforward, although higher resolutions are needed for flat areas. Examples of the dimensionless LS values are included in Table 6.12, showing the values increasing with slope as well as slope length.

6.6.4 Vegetative Cover (C)

As previously indicated, the principal cause of soil erosion is the lack of soil cover. In a comprehensive study in mountainous Andisols near Pereira, Colombia, Hoyos (Reference Hoyos2005) found that the cover management factor C was the preponderant one, overriding the effects of topography in steep lands. Examples of C factors in West Africa are shown in Table 6.13. Roose (Reference Roose1975) gave the bare soil a C value of 1, the ungrazed savanna 0.01, and the dense forest 0.001.

General estimates of the magnitude of erosion losses in West Africa, from the semiarid Sahel to the humid tropics (Charreau Reference Charreau1972), underline the importance of keeping a cover over the soil, as Table 6.14 shows.

6.6.5 Protective Structures (P)

Terraces and other protective structures and practices (represented by the P factor) can have major positive effects on erosion control. This is dealt with in the next section.

6.6.6 Measuring Soil Erosion

The USLE is the primary model used for large-area spatial risk assessment in the tropics. But this model often overestimates erosion losses due to the small size of the plots – about 22 m long and 3–5 m wide. These small plots can also underestimate erosion when the slope or other factors not present in plots are evident in the landscape (Fig. 6.13).

Fig. 6.13 These erosion plots in Indonesia ignore the topography of this catchment.

Courtesy of Meine van Noordwijk

The USLE is an empirical model, developed to assess plot-scale soil loss, but not sediment detachment and transport at the watershed scale. The USLE has rarely been validated using ground observations of soil degradation.

Cohen et al. (Reference 172Cohen, Shepherd and Walsh2005) evaluated the effectiveness of models to predict erosion in a small watershed of western Kenya using standard remote assessment methods based on interpolated spatial data layers. They found that diagnostic models based on geographic information systems produced validation accuracies of 78 percent, while the association between USLE-predicted and observed erosion was only 38 percent.

Many process models exist on how to put all this information together. Probably the most commonly used one is the Erosion-Productivity Impact Calculator (EPIC), which goes beyond erosion and is often used for crop modeling (Williams and Renard 1985). The Water Erosion Prediction Project (WEPP) developed by the United States Department of Agriculture National Soil Erosion Research Laboratory (Flanagan and Nearing Reference Flanagan and Nearing1995) is being successfully applied in the tropics (Ella Reference Ella, Coxhead and Shively2005), combining characteristics of small watersheds (2000–10 000 ha) with soil erosion processes.

Land degradation in many policy studies focuses only on soil erosion, which economists quantify in terms of the value of soil loss or the cost of soil conservation measures. To our shame, many economists use the USLE soil-loss data from the Wischmeier plots as representative of real conditions at the field, watershed and even national scales. Most soil scientists know very well that this methodology overestimates or sometimes underestimates erosion losses due to the small size of these plots. But what is eroded from one spot is most frequently deposited in another one in the same field, often leading to no losses at the field scale.

Stefano Pagiola (personal communication, 1990), a natural resource economist, showed how erosion data, when cited from paper to paper, becomes conventional knowledge. He showed in a presentation that soil erosion rates in El Salvador average 130 t/ha per year. He dug back in the literature and found the original study – in one region of the country where Wischmeier erosion plots in four sites produced soil losses ranging from 10 to 130 t/ha per year. Guess which one became conventional knowledge.

Soil erosion is better estimated at the watershed (or catchment) scale. The differences that can occur between plot and watershed scales are shown in Table 6.15, where both types of measurements were done in the same land-clearing study in Sabah, Malaysia. Sidle et al. (Reference Sidle, Ziegler, Negishi, Nik, Siew and Turkelboom2006) found a 20-fold difference in erosion estimates between the two methods, the larger numbers at the watershed scale. In this case, the USLE underestimated soil erosion.

Perhaps the best example of how to measure erosion in the tropics was done by Lal (Reference Lal, Lal, Sanchez and Cummings1986), using catchments ranging from 2.6 to 15 hectares, in a land-clearing experiment near Ibadan, Nigeria (Table 6.16), where the effects of land-clearing methods and tillage were studied.

This experiment is one of the largest and best-run in the tropics, using different catchments that were necessarily of different area due to topography. The data show how both runoff and erosion increase with more intense land-clearing methods.

These data contrast sharply with Lal’s results from the USLE plots in the same soil, which he used a decade before (Lal Reference Lal1976), and are shown in Table 6.17. The USLE plots overestimated erosion losses for the catchment at 11.2 t/ha per year, compared with 4.8 t/ha per year (Table 6.16), in treatments cleared manually, tilled conventionally and planted to maize. If the bare fallow soil is used to calculate the erodibility factor K in the Wischmeier plots, the result, 106.6 t/ha per year, is an order of magnitude higher than all erosion experienced in the catchment plots.

In spite of their limitations, the Wischmeier erosion plots permit comparisons among soils in different ecosystems and management systems, but as shown they can underestimate or overestimate the more realistic catchment-based data. Other estimates from many countries in the tropics, compiled by Lal (Reference Lal, Lal, Sanchez and Cummings1986), are in the range of 4–3600 t/ha per year.

The relationship between soil erosion and crop yields is complex because there are many other factors involved. Figure 6.14 shows the effect of a high-intensity storm on a cultivated landscape; the crop cover was unable to prevent erosion. In addition, the question of how to predict crop losses is difficult. For sub-Saharan Africa, Lal (Reference Lal1995) estimated annual crop yield reductions of 6 percent (3.6 million tons of cereals) in 1989, and estimated an increase, to 15 percent, by 2020.

Fig. 6.14 Effects of a high-rainfall erosivity event on a crop production landscape in Rwanda. ICRAF file photo.

There are many assumptions in such estimates, but these numbers suggest that crop productivity losses due to soil erosion are not overwhelming, in comparison, let’s say, with post-harvest losses during crop storage, which average 30 percent (Sanchez Reference Sanchez2015).

When gully erosion is unchecked, it can lead to catastrophic circumstances where no cultivation is possible, such as in Fig. 6.15.

Fig. 6.15 Gully erosion has made this field in Koraro, Ethiopia, impossible to cultivate.

6.6.7 Civil Engineering Erosion

While traveling in the tropics one often sees catastrophic erosion along rural roads. Ziegler and Giambelluca (Reference Ziegler and Giambelluca1997) found that saturated hydraulic conductivity on unpaved roads in Thailand was ten times lower than adjacent soils, therefore increasing runoff and erosion along roadsides. This “civil-engineering erosion” is often photographed as if it has been caused by cultivation, and decorates many publications about soil degradation.

6.7.1 Small-Scale Structures in the Field

Many small-scale farmers in Africa till their land with hoes and without animal or mechanical power. In well-structured, gently sloping Oxisols and oxic Alfisols of western Kenya, this works relatively well (Fig. 6.16). In less-well-structured soils, such as loamy Alfisols in Malawi and most Alfisols in subhumid areas of both West and East Africa, most farmers make ridges and furrows on just about any slope, planting crops on top of the ridges (Fig. 6.17). Often they use tied ridges, earthen bunds that are constructed at right angles to the furrows, at intervals of 1–4 m, effectively making small, shallow ponds to allow more time for water to infiltrate (Pereira et al. Reference Pereira, Hosegood and Dagg1967, Hulugalle et al. Reference Hulugalle, de Koning and Matlon1987).

Fig. 6.16 Hand hoeing in a clayey Oxisol in Sauri, Kenya, the only land preparation practice. Fields are usually separated by earthen fanya juu bunds (not shown).

Fig. 6.17 Ridge and furrow system during the rainy season in Mwandama, Malawi, in a loamy Alfisol. Ridges are rebuilt every year.

In semiarid northern Ethiopia, farmers use trench bunds, about 1–2 m long, 50 cm wide and 30 cm deep, located where there is plenty of runoff (Fig. 6.18). These trench bunds allow water that would run off to slowly infiltrate into the soil; this is one of many ways to store water in the subsoil. Tied ridges increase soil moisture storage, which can help avert the short-term droughts that are common in rainfed systems. Figure 6.19 shows the increased water availability with tied ridges. Sorghum, maize and cotton yields increased by 26–45 percent in a 6-year trial in Zimbabwe (Nyamudeza et al. Reference Nyamudeza, Mandiringana, Busanagavanye and Jones1991, Reference Nyamudeza, Hussein, Matibiri, Syers, Penning de Vries and Nyamudeza2001).

Fig. 6.18 Trench bunds in Koraro, Ethiopia, in a sandy Inceptisol (Ustept).

Fig. 6.19 The positive effects of tied ridges (tied furrows) on profile soil moisture in a Vertisol of Zimbabwe.

Adapted from Nyamudeza et al. (Reference Nyamudeza, Mandiringana, Busanagavanye and Jones1991)

Throughout the semiarid tropics, microcatchments are commonly used to capture surface runoff. Zai holes are shallow pits surrounded by a hemisphere of stones that form a small catchment (about 3–10 m in diameter) where a valuable tree is often planted (Fig. 6.20). There are many variations of these techniques. Trash lines of organic materials are also commonly used to slow down runoff. The overall strategy of these practices is to hold water for a longer time (Kayombo and Lal Reference Kayombo and Lal1993).

Fig. 6.20. Zai holes in Burkina Faso.

6.7.2 Contour Cultivation

Planting along contours, instead of up and down the slope, is usually a feasible and sound practice, but often a difficult one to introduce to farmers who are used to plowing up and down slopes. All over the tropics, farmers indicate that when they tried contour cultivation, intense rains broke the contours and erosion got worse.

Results from Kunming, China, provide valuable insights (Table 6.18). At a gentle slope of 6 percent there were no major differences in soil loss between contour or downslope cultivation, with losses lower than 1 t/ha per year. On a steep 20 percent slope, downslope tillage produced slightly higher soil loss. It was at the extreme 50 percent slope where downslope tillage produced more than twice the soil loss than contour tillage. The advantage of contour cultivation is more evident for steep slopes.

Contour cropping, an inexpensive and simple technology, was adopted and maintained in the Dominican Republic after the research demonstrations had stopped (Carrasco and Witter Reference Carrasco and Witter1993), while in neighboring Haiti, downslope cultivation is still widespread on steep slopes, with devastating results.

6.7.3 Contour Hedges

Planting trees or tropical grasses along contours provides effective barriers that protect the soil against erosion. Evidence of the positive impacts of these practices is well established (Roose Reference Roose1970, Lal Reference Lal1989, Young Reference Young1989, Juo et al. Reference Juo, Caldwell and Kang1994, Kiepe and Rao Reference Kiepe and Rao1994, Alegre and Rao Reference Alegre and Rao1996, van Roode Reference van Roode2000). Closely spaced trees on slopes reduce soil erosion by water, as they provide a physical barrier of stems, low branches, superficial roots and leaf litter, as well as allowing higher water infiltration due to better soil aggregation under the trees (Kiepe and Rao Reference Kiepe and Rao1994). If leguminous, these hedges also fix nitrogen. Trees, however, do not deliver these functions until they are well established and have developed a litter layer, and they must be planted closely. Once established, most trees protect the soil indefinitely, provided they are well managed and the litter layer is not removed.

The perennial nature of tree and grass root systems provides a dependable source of carbon for microorganisms, which in turn promotes better soil structure and often higher macrofaunal activity, which in turn promotes higher porosity (Sanchez Reference Sanchez1995). When contour hedges are composed of fodder trees (Calliandra calothyrsus, Leucaena leucocephala, for example) and grasses such as Napier or elephant grass (Pennisetum purpureum), they also provide livestock feed, without competing for area with crops. Vetiver grass (Vetiveria zizanioides), although capable of controlling erosion, is seldom used because it has no value as forage. Fruit or timber trees grown as an upper storey in such contours also provide additional income and ecosystem services (Garrity Reference Garrity1993, Reference Garrity, Ong and Huxley1996; Garrity and Mercado Reference Garrity and Mercado1994).

Njoroge and Rao (Reference Njoroge and Rao1994) developed a summary of multi-year trials across the tropics (Table 6.19). Under a variety of soils and slopes, contour hedges show their effectiveness in minimizing soil loss and runoff. It is important to space the contour hedges at least 4–5 m wide, to largely eliminate the competition with the crops in between, or else you begin encountering the limitations of alley cropping (Sanchez Reference Sanchez1995; Chapter 19). The Malawi example, although effective in controlling erosion, resulted in low maize yields, partly due to competition between the crop and the trees, with only 0.9 m between hedges (Banda et al. Reference Banda, Maghembe, Ngugi and Chome1994).

In semiarid climates, spacing between contours can be wider. Contour hedges in an essentially flat slope in Hyderabad, India, predictably did not have an erosion control effect. The effect is more pronounced with an increasing slope angle of up to 30 percent, with over 80 percent reduction in soil loss. At more pronounced slopes, this reduction in soil loss is less pronounced, at 40 percent for a 75 percent slope. For such steep slopes bench terraces may need to be constructed to adequately control soil erosion (Njoroge and Rao Reference Njoroge and Rao1994).

Controlling soil erosion with planted hedges has an additional advantage, which is that the slope between the hedges becomes less steep and eventually flat in some instances, forming, in effect, bench terraces. These “biological terraces” are produced by taking advantage of the erosion process within the contour hedges, with the tree or grass growth keeping up with the higher soil surface at the lower end, something stone bench terraces cannot do. With time, soil from the upper end of the terrace moves downslope and is captured by the lower hedge (Fig. 6.21).

Fig. 6.21 Contour hedge terraces gradually become flat. Rwerere, Rwanda, ~2500 m elevation. Wheat is the crop and Grevillea robusta the tree, stunted at this high elevation. Drs. Erika Styger and Amadou Niang.

Loss of topsoil from the upper third of these terraces can occur often with lower crop yields in relation to the more fertile lower end of the terrace where the topsoil accumulates (Garrity Reference Garrity, Ong and Huxley1996, Walle and Sims Reference Walle and Sims1999). One solution is to plant nitrogen-fixing tree fallows in the upper third of the terrace for 2 years, which, in the steep highlands of southwestern Uganda, added enough nutrients and SOM to even out the fertility gradient (Siriri and Raussen Reference Siriri and Raussen2003). Furthermore, Agus et al. (Reference Agus, Cassel and Garrity1997) also found lower plant available water at the top of a terrace under alley cropping in Mindanao, Philippines, confirming the competition for water between closely spaced hedgerow species and the crop. That is one reason why alley cropping is not recommended (Sanchez Reference Sanchez1995, Chapter 19).

6.7.4 Earth Terraces

During the colonial period in Africa, farmers were often forced to construct narrow bench terraces, a practice quickly done away with in many countries after independence, and not particularly effective. In its place, steep upslope terraces (fanya juu in Kiswahili, loosely meaning “throw upwards”) are now widespread in East Africa.

Fanya juu terraces are mainly responsible for the environmental rehabilitation of Machakos District in Kenya (Tiffen et al. Reference Tiffen, Mortimore and Gichuki1994). These terraces, constructed after consultation with villagers instead of being imposed on them, along with extension and credit programs, created the “more people, less erosion” paradigm. Tiffen and co-workers concluded that the Machakos District has sustained agricultural intensification and improved soil conservation and crop production for the last 40 years in spite of a population growth rate of 3 percent per annum. Fanya juu terraces have been found more effective in reducing soil loss and runoff than contour hedges in Machakos (van Roode Reference van Roode2000) because they present a solid barrier, unlike trees, at ground level.

The opposite strategy is to dig the furrows along the contour, but build the terrace downslope, and plant grasses. These are called raised bed terraces in Indonesia, where they have decreased soil erosion in steep slopes in Central Java from 106 t/ha to 34 t/ha per year (Adiningsih and Syarifuddin Karama Reference Adiningsih and Syarifuddin Karama2004). However, the authors considered bench terraces made of stone to be more economically attractive in spite of higher labor costs.

6.7.5 Stone Bench Terraces

Indigenous tropical farmers have produced spectacular and long-lasting bench terraces made of local stones, which have controlled erosion for decades and centuries. The Inca civilization spread stone terraces called andenes (where the name Andes probably comes from), which are where their descendants grow tropical highland crops like potato, quinoa, wheat and barley, and raise cattle, llamas and alpacas. An iconic example is Machu Picchu in Peru. Gravity irrigation canals and drainage ditches, along with the terraces, showed excellent engineering by a culture that had not yet discovered the wheel. Many of these structures are now abandoned in many parts of the Andes, but many also retain their productivity.

Southeast Asia has similar examples in the Philippines (Fig. 6.22); Java, Sumatra, and Bali, Indonesia (Siebert and Belsky Reference Siebert and Belsky1990); Sri Lanka; and other countries. Wetland rice is the main crop in these terraces, requiring soils to be flooded, which decreases soil strength and requires more maintenance. I have seen such stone terraces in West Sumatra that are wide enough to plant only two rows of rice, about 50 cm wide.

Fig. 6.22 Sagada rice terraces, Mountain Province, Philippines.

6.7.6 Stone Bunds

In semiarid highlands of the Tigray region of Ethiopia, most soils are stony Entisols and Inceptisols, and farmers have used stones to construct a plethora of soil and water conservation structures since the 1980s. A 30-year study by Nyssen et al. (Reference Nyssen, Munro, Haile, Poesen, Descheemaeker, Harteweyn, Moeyersons, Govers and Deckers2007) shows that sheet and rill erosion rates have decreased, while infiltration, spring discharge, vegetation cover and crop production have improved.

The first intervention is making stone bunds on the contour (Fig. 6.23). Rock fragments larger than 5 mm can cover as much as 57–85 percent of the surface soil in this region. By following the contour in slopes greater than 15 percent, the soil is displaced downslope, and in effect makes the slope flattened in about 8 years (Nyssen et al. Reference Nyssen, Poesen, Haile, Moeyersons and Deckers2000), like contour hedges and fanya juu terraces in other areas. Stone bunds have been shown to increase productivity in the semiarid Tigray region (~650 mm per year), but not in the humid Amhara region (~1980 mm per year), suggesting a soil moisture saving effect (Kassie et al. Reference Kassie, Pender, Yusuf, Kohlin, Bluffstone and Mulugeta2007).

Fig. 6.23 A landscape in Tigray region, Ethiopia, with stone bunds along the contour.

Stones left on the fields have actually increased rainfall infiltration by protecting the soil from compaction by raindrop impact, and decreased surface sealing and crusting; they also reduce evaporation from the topsoil directly underneath the stones (Nyssen et al. Reference Nyssen, Haile, Poesen, Deckers and Moeyersons2001, Reference Nyssen, Poesen, Moeyersons, Lavrysen, Haile and Deckers2002). These “stone mulches” save soil moisture and are shown in Fig. 6.24.

Fig. 6.24 Stone mulch, Axum, Ethiopia. The two stones in the center were overturned before taking the photo.

Halfway across the tropics, contour bunds made from duripan fragments in steep Andisols in Ecuador, when reinforced with grasses, were found superior to bench terraces or infiltration ditches in controlling erosion, requiring less labor and not as much loss of arable land (Dehn Reference Dehn1995).

6.7.7 Rehabilitating Eroded Landscapes by Community Action

Gully-ridden landscapes force farmers into extreme poverty, resulting in the classic “badlands” (shown above in Fig. 6.15). But when the community is mobilized and supported by science-based advice, the results can be remarkable. Community-based work during the past decade in the semiarid (500 mm annual rainfall) Koraro Millennium Village, in Hawzen Woreda in the Tigray region of Ethiopia, transformed much of an extremely degraded landscape, with stunted, extremely poor people, into a thriving community – also due to successful interventions in soil fertility, health, education, electricity and roads.

The area has a sharp sandstone escarpment to the south, which captures rains that reach the village as intermittent storm flows, causing gully erosion and reducing the arable land available. A comprehensive landscape rehabilitation plan was developed by a local agricultural engineer, Gigar Kebede, together with the community, supported by a government food for work program and the Millennium Villages Project (Chapter 2). Several technologies not only deviated a major gully from crop fields, but resulted in the village now having 43 hectares of irrigated land where there was none before. Some of these technologies are described in the following sections.

6.7.8 Controlling Gully Erosion

The restoration of gullied land involves larger-scale efforts. Figure 6.25 shows the construction of check dams, including the assembly of gabions (a cubic heavy wire mesh structure, filled with stones), the finished check dam and the captured sediments behind the dam that progressively fill the gullies. The gully bottoms are generally fertile and are often planted with palatable grasses and even tree legumes like Sesbania sesban. To that extent, it might be thought that the gully sides should also be planted, but often they are too steep.

Fig. 6.25 A stone check dam constructed with gabions in Koraro. Top left: gabions. Top right: construction. Bottom left: the finished dam, which is narrower because the gabions penetrate sideways into the gully wall. Runoff flows over the center of the dam. Bottom right: sediment is deposited behind the dam, which can be planted with useful trees like Sesbania sesban along the sides.

6.7.9 Runoff Storage in the Subsoil

When cropland is next to hills or scarps, the potential for capturing runoff and storing it in the subsoil can be very significant. Percolation ponds are strategically located to capture runoff and allow it to percolate into the subsoil. Water stored in the subsoil is then retrieved downstream with shallow wells, and pumped to individual farmer fields (Fig. 6.26). Under these conditions, the subsoil is a great place to store water for subsequent irrigation.

Fig. 6.26 Percolation pond (left) captures runoff from the scarp at Koraro. The water is allowed to percolate into the subsoil, and is pumped from shallow wells lower in the landscape where farmers have their fields (right).

Another way to capture water is via the ditches along roads, many of which are lined with cement in steep areas. It is simple to deviate the flow into storage ponds. Some of these ponds are underlain by strong polyethylene sheets or made with cement. Ethiopia now has hundreds of thousands of these ponds, used for gravity or bucket irrigation (Nega and Kimeu Reference Nega and Kimeu2002, RELMA 2005, Malesu et al. Reference Malesu, Oduor and Odhiambo2007).

6.8.1 Principles of Conservation Agriculture

Conservation agriculture was based originally on four principles: (1) minimum or no mechanical soil disturbance; (2) crop residue returned to the soil, to cover the soil surface throughout the year; (3) weed control by herbicides; and (4) crop rotation. To this, Vanlauwe et al. (Reference Vanlauwe, Wendt, Giller, Corbeels, Gerard and Nolte2014) added a very sensible but often forgotten fifth principle: (5) the appropriate use of fertilizer to enhance crop yields and crop residue production. All must be simultaneously undertaken.

Conservation agriculture, therefore, is an umbrella term that encompasses a range of cultural practices, based on these five components: Conservation agriculture is often equated to minimum or no tillage, ignoring the three other components, and causing confusion in the literature. The application of the five components is taken for granted in large-scale mechanized operations, but not so in smallholder farming situations, where residue return, herbicide application and even fertilizer use are not normal operations. For clarity, I will use the term no-till when only the first component is practiced.

Conservation agriculture represents a fundamental shift from the millennial paradigm: a clear seedbed with crop residues removed (Giller et al. Reference Giller, Witter, Corbeels and Tittonell2009). Rather dogmatically, the FAO (2001) definition states, “as long as a farmer plows at least one crop during the rotation, he does not practice conservation agriculture.” As we will see, occasional plowing is a requisite of some successful conservation agriculture practices in the tropics as well as in the temperate region.

The advantages and limitations of conservation agriculture have been debated not only in the scientific literature (Lal Reference Lal1989, Reference Lal1991, Six et al. Reference Six, Feller, Denef, Ogle, Sá and Albrecht2002c, Giller et al. Reference Giller, Witter, Corbeels and Tittonell2009, Reference Giller, Corbeels, Nyamangara, Triomphe, Affholder, Scopel and Tittonell2011, Palm et al. Reference Palm, Blanco-Canqui, DeClerck, Gatere and Grace2014, Powlson et al. Reference Powlson, Stirling, Gerard, Jat, Palm, Sanchez and Cassman2014, Pittelkow et al. Reference Pittelkow, Liang, Linquist, van Groenigen, Lee, Lundy, van Gestel, Six, Venterea and van Kessel2015) but in policy-oriented publications such as FAO (2001) and World Bank (2012), particularly as a tool to sequester carbon and mitigate climate change. In any analysis it is critical to know which of the five components of conservation agriculture are used.

6.8.2 Soils in Which Conservation Agriculture Works

Oxisols, Andisols, Nitisols, Oxic Groups of Ultisols and Alfisols (Ci, Lx Soils in the FCC system), on Gentle Slopes

These strongly aggregated soils are well suited for conservation agriculture. The Oxisol story in the Cerrado ustic savanna of Brazil illustrates this point. The Cerrado began its rapid and successful development in the 1970s. The first two decades focused on overcoming extreme acid soil infertility of the Oxisols, using disk plowing up to 30 cm depth to incorporate lime (González-Erico et al. Reference Gonzalez-Erico, Kamprath, Naderman and Soares1979). Gradually, the soil structure began to deteriorate, plow pans often developed and SOC decreased, while erosion losses and runoff increased (Lopes and Daher Reference Lopes and Daher2008). The large farm size (~1000 ha) aggravated these problems, forcing quick land preparation, leaving the pulverized soil exposed to the first intensive rains of the single rainy season.

Conservation agriculture, using heavy seeding machinery and herbicides, began in earnest, increasing from essentially nothing in 1990 to 11.5 million hectares in 2005. No-till practices reduced soil erosion by 75 percent and runoff by 25 percent, while increasing crop yields (Lopes et al. Reference Lopes, Wietholter, Guilherme and Silva2004, Resck et al. Reference Resck, da Silva, Lopes, da Costa, Lal, Kimble and Stewart2006). A 21-year comparison of several crop rotations in a Typic Hapludox of southern Brazil showed that no-till management systems had more stable aggregates and SOC than conventionally plowed fields; much of the SOC was in the 2 mm macroaggregate size class (Castro Filho et al. Reference Castro Filho, Lourenço, Guimarães and Fonseca2002). No-till planting in Brazil covered about 32 million hectares in 2015 (Motter and Almeida Reference Motter and de Almeida2015).

Still, other barriers remained. Year-round soil cover did not occur because cropland was not used during the 6-month dry season, resulting in weed growth and wind erosion. In lands devoted to grazing livestock, pastures degrade with time, and cattle production declines, with major live-weight losses, during the dry season. Embrapa researchers then developed an integrated crop–livestock system (integração lavoura–pecuária), one of the most intelligent rainfed tropical farming systems, which reached about 700 000 hectares in 2003 (Kluthcouski et al. Reference Kluthcouski, Stone and Adiar2003), over 1.5 million hectares in 2009 (Alfredo Lopes, personal communication, 2010) and 11.5 million hectares in 2017. The system, as I saw in a 1500-ha farm in Baldim, Minas Gerais, in 2008, is fully described in the chapter on livestock (Chapter 18) and summarized below.

Starting from a degraded pasture, farmers plow conventionally, incorporating the necessary amendments and fertilizers down to 30 cm, and planting maize. Seeds of Brachiaria ruziziensis are broadcast over the maize rows. Maize is harvested by small combines, leaving the crop residues on the surface, through which the stoloniferous Brachiaria grows. Cattle start grazing during the dry season, and continue for about 18–24 months. When farmers decide it is time to rotate back to crops, because the pasture begins to degrade, grazing pressure is increased to reduce pasture biomass. Soybeans are then planted with no-till machinery (Fig. 6.28), combine-harvested, and the cycle repeats itself with an average duration of 4 years. By creating a biological superstructure in and over the soil, comprised of deep-root biomass and a constant vegetative cover over the soil surface, a year-round productive land-use system becomes possible. It was remarkable to see well-nourished heifers gaining weight by grazing pastures that had received no rain in 5 months.

Fig. 6.28 No-till planting of soybeans in pastures, in an Oxisol of Baldim, Minas Gerais, Brazil.

Courtesy of Alysson Paolinelli

This is intelligent tropical agriculture and what conservation agriculture is all about. Notice that an essential component was to do conventional tillage first, in order to restore soil fertility, which could not be accomplished with strict conservation agriculture guidelines.

In the northern part of the Cerrado, conservation agriculture is practiced at a very large scale. The spectacular Fig. 6.29 shows the simultaneous harvesting of soybeans followed by no-till planting of cotton in Oxisols in Mato Grosso State, Brazil. This is another example of intelligent tropical soil management using conservation agriculture with high capital inputs in Oxisols, which were believed by many to be unsuitable for agriculture (Chapter 3).

Fig. 6.29 Combine harvesting of soybeans followed by no-till planting of cotton in Oxisols in Mato Grosso, Brazil.

Courtesy of Alysson Paolinelli

Soils with similar core properties (Ultisols, Oxisols and some Andisols) have also been found to be suitable for conservation agriculture in smallholder tropical farming systems. The conservation agriculture movement has also taken hold in what are called small farms in Latin America (5–20 hectares of cropland), but are really medium-sized from a pan-tropical perspective. Many systems are described by Buckles et al. (Reference Buckles, Triomphe and Sain1998aReference Buckles, Eteka, Osiname, Galiba and Galianob), Eilittä et al. (Reference Eilittä, Mureithi and Derpsch2004) and Goddard et al. (Reference Goddard, Zoebisch, Gan, Ellis, Watson and Sombatpanit2008), most of which occur in two regions, the hillsides of Mesoamerica and the subtropical state of Santa Catarina, Brazil, with adjacent regions of Paraguay.

A remarkable one is abonera, a maize–mucuna relay intercropping system, now widespread in the humid tropical hillsides of Mesoamerica, where the soils are mainly Ultisols (Triomphe and Sain Reference Triomphe, Sain, Eilittä, Mureithi and Derpsch2004). Farmers plant a fallow of Mucuna pruriens (velvet bean, frijol terciopelo) during the first rains, and later maize, using a planting stick in the slashed, thick mucuna mulch at the end of the rainy season. The fast-growing herbaceous legume reestablishes itself though natural reseeding near maize harvest time; it is allowed to regrow but it is always slashed prior to the next maize planting.

Mucuna provides a continuous cover throughout the year, prevents erosion in very steep slopes and controls weeds including the aggressive Rottboellia cochinchinensis. If well established, mucuna fixes nitrogen and reduces labor, and farmers are able to take advantage of the higher prices of the dry-season maize. But if mucuna is not slashed in a timely fashion, it can grow over the maize and increase labor at harvest. Maize yields increased by about 50 percent in these systems (Triomphe and Sain Reference Triomphe, Sain, Eilittä, Mureithi and Derpsch2004).

Loamy, Gravelly Alfisols (LCr+, SL, LC soils)

Loamy, gravelly Alfisols without steep slopes are very common throughout subhumid West Africa and are also suitable for conservation agriculture. In fact, Rattan Lal pioneered no-till research in the tropics in such soils (Lal et al. Reference Lal, Wilson and Okigbo1978, Reference Lal, Wilson and Okigbo1979, Lal Reference Lal, El-Swaify, Moldenhauer and Lo1985, Reference Lal, Lal, Sanchez and Cummings1986, Opara-Nadi and Lal Reference Opara-Nadi and Lal1987, Lal Reference Lal1989, Reference Lal1991, Kayombo and Lal Reference Kayombo and Lal1993, Lal Reference Lal1995, Reference Lal1999). Lal and colleagues observed that conventional tillage in these soils caused rapid runoff, erosion and in some cases compaction, while no-till avoided much of this damage (see Fig. 6.30).

Fig. 6.30 Profiles of the top 30 cm of the same loamy, gravelly Alfisol at IITA, Ibadan, Nigeria, after 6 years of conventional tillage (left) and no-till (right) in Rattan Lal’s plots. Conventional tillage caused the loss of primary soil particles from the topsoil, thus increasing the gravel content. The soil under no-till certainly looks better.

In these gravelly, loamy Alfisols, Lal (Reference Lal, El-Swaify, Moldenhauer and Lo1985) observed that water retention at –10 kPa increased from 14.7 percent to 17.5 percent in the no-till watershed after 6 years of continuous mechanized farming, whereas it decreased from 17.7 percent to 13.8 percent in the plowed watershed, both at 0–10 cm depth.

Loamy and Clayey Alfisols – The Zambian Model

A different form of conservation agriculture is being practiced by over 180 000 small-scale farmers (less than 5 hectares) in the subhumid to semiarid central and southern provinces of Zambia (Peter Aagard, personal communication, 2009). These are areas of gentle topography, which are dominated by loamy to clayey Alfisols, without gravel and with permanent charge. Farmers dig micro-basins with a specially designed implement (Fig. 6.31), laid out in a precise grid that disturbs about 10 percent of the field, thus meeting the definition of conservation tillage. These 30-cm-long basins are dug by hand with hoes, at the start of the dry season, when the soil is still soft. This reduces the labor needs at the busy start of the rainy season, allowing time for early planting and early weeding. The basins catch the first and erratic rains that allow early planting, and are dug up again in following years, becoming a permanent feature of these fields. A variant of the system is using ox-drawn rippers in lines, instead of hand hoeing, for the 10 percent of farmers that own oxen or can hire a tractor equipped with a reaper. In either case, crop residues remain in the field. Improved seeds, precisely placed fertilizers and crop rotations with legumes are also key components of the system.

Fig. 6.31 Implement used to dig the permanent planting basins near Shimubala, southern Zambia.

Maize yields increased by 1.5 t/ha, probably doubling the yields under conventional tillage. Haggblade and Tembo (Reference Haggblade and Tembo2003) broke down this yield increase as follows: 0.4 t/ha from early planting; 0.7 t/ha from water harvesting in the microbasins; and the remaining 0.4 t/ha from higher doses of fertilizer and high-yielding varieties. I saw that a nearby farmer who did not use conservation tillage and planted late, with no fertilizer, obtained basically no yield.

The curse of rainfed farms is late planting. By preparing the land during several operations during the dry season, applying the correct amounts of properly placed fertilizers, dry planting before the rains, having the basins to capture rainfall and controlling weeds with a herbicide that does not have a residual effect, these microbasins become permanent. In some systems, the maize is rotated with peanuts or soybeans, leaving the maize stover on top of the soil. This is probably the most practical form of conservation agriculture I have seen in Africa. It meets all five requirements of conservation – minimum tillage, keeping the soil covered, herbicide use, crop rotation and appropriate fertilization. This form of conservation agriculture is another example of intelligent agriculture.

6.8.3 Soils in Which Conservation Agriculture is Difficult

6.8.4 Small-Scale Farms in Africa

Considering the wide array of soils where conservation agriculture is feasible, as well as the instances where it is unlikely to work, why has this practice not taken root in smallholder tropical African farms as it has in large swathes of smallholder farms in tropical Asia and Latin America?

Conservation agriculture in smallholder African farms faces three major obstacles: the energy needs of planting equipment, keeping crop residues in the field and the need for herbicides. Results from African research are neither clear nor consistent (Giller et al. Reference Giller, Witter, Corbeels and Tittonell2009). What is clear is that conservation tillage decreases runoff and erosion (Lal Reference Lal, Lal, Sanchez and Cummings1986, Kayombo and Lal Reference Kayombo and Lal1993).

Giller et al. (Reference Giller, Witter, Corbeels and Tittonell2009), in their “heretic” review, indicate several tradeoffs that the smallholder African farmer often has to make to shift to conservation agriculture. Is mulching the most sensible, efficient or profitable use of crop residues? Is there a yield penalty in the short term and a yield gain in the long term? Does conservation agriculture save labor? Does conservation agriculture increase SOM and soil fertility? Also, can they afford herbicides and no-till machinery?

In my view, smallholder African farmers need to obtain no-till planting machinery (ideally the small ones used in southern Brazil, some of them pulled by cattle), keep at least half of the crop residues in the field and shift from hand weeding to herbicides. Farm service companies can rent suitable equipment to smallholders. In fact, there is an “Uber”-like company that rents farm equipment to smallholder farms in Tanzania (Cheryl Palm, personal communication, 2017).

The results of Aagard’s (Reference Aagard2009) southern Zambia model shows that it can be done in farms of 1–5 hectares and taken up at scale, avoiding the need to purchase no-till planting machinery. The labor required to plant by hoe (Fig. 6.16) or to make ridges and furrows every year (Fig. 6.17) is so deeply ingrained in farmers with 1 hectare or less that only a comprehensive effort will be effective. One that addresses not only tillage, but crop residue use, crop rotation and herbicide and fertilization, with a financial stimulus package to cover the additional costs and probably the initially lower yields.

Farmers get paid for soil conservation in many rich countries as an ecosystem service. Certainly the millions of smallholder African farmers can benefit in a similar way.

Conservation agriculture is a major and positive paradigm shift in world agriculture, and the larger farms of the tropics are already benefiting. The challenge is for the smallholder farmers to benefit as well.