1. Dried cell substance of various micro-organisms—nine fungi, one actinomycetes, two bacteria—were subjected to decomposition in soil and sand. In neutral garden soil, the addition of microbial substance gave rise to a more or less abundant, but always temporary, development of bacteria and actinomycetes; the latter group of organisms was often stimulated to a very conspicuous degree.

2. The development of micro-organisms was accompanied by a more or less abundant production of nitrate. After 60 days 19–61 per cent. of the added N had been nitrified, and after 120 days the proportion had in most cases not increased very materially.

It is important to bear in mind that the investigation outlined in this paper was designed primarily to test the effect of periodic applications of sulphate of ammonia on the yield and composition of good permanent pasture, well supplied with reserves of lime, potash and phosphate. The results have no bearing on the possible effect of manurial treatment on the yield and composition of inferior types of pasturage. The present account embraces the findings of the first two seasons of experiment, namely, 1929 (characterised by droughty conditions) and 1930 (season of ample rainfall).

The limitations of the “constant” saturation capacity as a factor in the characterisation of types of soils, rather than individual soils, are discussed. An attempt has been made to overcome these limitations by distinguishing between the contributions made to saturation capacity by the organic and inorganic colloidal components of three widely different tropical soil types.

1. Numerical evidence is given which shows that saturation capacity is very closely related to the content of clay and organic matter of soil (irrespective of the proportions in which they are present), but not to the fine silt fraction. Nevertheless, the correlation between saturation capacity and (a) organic matter, and (b) clay, is significantly greater for the tropical type containing the smallest amounts of these components than for the type containing the largest amounts.

The decomposition of the organic matter of the soil by means of hydrogen peroxide is shown to be a function of the hydrogen-ion concentration of the soil, alkaline soils permitting of a very low degree of oxidation, while with acid soils up to 90 per cent, of the organic matter is destroyed.

The results obtained with a range of soils suggest that a portion of the organic matter is oxidised irrespective of the soil reaction, while the oxidation of the remainder is a definite function of the hydrogen-ion concentration, the relation being expressed by the equation

The proportion of the organic matter oxidised further appears to be a definite function of the clay content of the soil. Alkaline soils containing free manganese dioxide also show small losses.

The effect of rainfall on the wheat crop grown on Broadbalk has been investigated, special attention being paid to the time of application of the nitrogenous manures.

In the years 1854–77, when plots were manured solely in the autumn, the average decrease in yield caused by each additional inch of rain is greater than the average decrease from 1854–1918. The difference in this case cannot be ascribed to the change in manurial treatment.

The period of 24 years was too short to allow seasonal variation in the effect of rain to appear, as it was only possible to compare such seasonal variation with that of 1854–1918 in the case of one plot. In this case it was observed that summer rainfall was more detrimental to autumn-dressed than to spring-dressed plots; this result was confirmed by studies of the effect of rain on the difference in yield of two plots which differed only in the time of application of their manures.

The preparation of natural humic acids from soil, peat (Dopplerite) and “Adco,” and of artificial “humic” acids from sucrose, cellulose, dextrose and glycine (Maillard), furfural, hydroquinone and lignin, and their purification is described.

Their elementary composition and their behaviour under conductometric titration with ammonia have been studied. The artificial products from sucrose and furfural did not behave as acids, but all the natural products, and the artificial products from cellulose, hydroquinone and lignin possessed the properties of colloidal acids.

Preliminary investigations into the “humification” of furfural and ω-hydroxymethyl furfural, and into the interaction of dextrose with amino bodies, are described.

1. The consolidation in a light soil produced by sheep folding has been investigated.

2. The soil affected extended to a depth of 10 cm., the maximum compression occurring at a depth of 3–4 cm.

3. The consolidation produced was not totally destroyed by ploughing and was still apparent 5 weeks later when the seed was drilled.

1. A method for measuring the heaviness of a sample of soil is described which has the following advantages:

(i) The test can be completed in about 7 min; no initial drying, sieving or weighing is involved.

(ii) The apparatus is simple to construct and comparatively easy to work. The resultant figures are fundamental in nature, and not dependent on the dimensions of the apparatus used.

1. The equivalent parallel capacity and conductance of a series of soil blocks have been measured for decreasing moisture content.

2. The results obtained depend on the electrodes used. With mercury, all soils give curves of the same general type for the variation of the capacity with moisture because the capacity effects associated with the soil electrode interface are large compared with those due to the soil. The electrical properties of the interface have been shown to exhibit marked changes in their variation with moisture at certain moisture contents.

In this paper the general theory of the coagulation of dilute clay suspensions is discussed. The more important of the points considered are summarised below.

1. Two separate mechanisms causing collisions between suspended particles are considered, namely their Brownian motion and the mass motion of one group of particles relative to another group. Following Wiegner, these two types of collision are called perikinetic and orthokinetic collisions respectively. The rate of coagulation of a suspension is then shown to depend on the rate of collisions between particles and on the probability of adhesion between them when they collide. If this probability is unity, that is if every collision between two particles results in their adhesion, the suspension is undergoing rapid coagulation. There is excellent agreement between theory and experiment for this type of coagulation whenever the mathematical equations involved can be solved. But if the probability of adhesion is less than unity, the suspension is undergoing slow coagulation, and there is as yet no theory capable of giving the rate of coagulation of such a system.

1. Measurements on the loss of ignition of four soils of different physical type are not affected by changes in the temperature of furnace, provided that the temperature is above 600°. Errors in the determination of the loss on ignition are of the order of 1 per cent. of the true value.

2. The loss in weight of the soils can be ascribed in the main to loss of free and interstitial water up to about 100°; to destruction of organic colloids between 100° and 250°; and to destruction of inorganic colloids at higher temperatures.

Thanks are due to Miss T. Alper, M.A. (Cape Town) for assistance in making some of the preliminary measurements.

The moisture equivalent has been measured (by a technique requiring only small quantities of soil) for a number of samples comprising Natal and Sind soils. It is concluded that while with the latter (alkaline and saline) soils the moisture equivalent gives valuable information, it adds little to the data obtained by other methods for the Natal soils. The xylene equivalent of the Natal soils has also been measured; from the moisture equivalent and the xylene equivalent, the imbibitional water can be calculated if the specific gravity of the soil is known.

Equations expressing the moisture equivalent and the xylene equivalent of the Natal soils in terms of their loss on ignition and mechanical composition are obtained, and the significance of the relative values of the numerical coefficients in these equations is discussed.

Several factors affecting the electrical conductivity of soil suspensions have been studied leading to the development of a simple and rapid technique for its measurement, and suitable for both moist and air-dry soils.

The possible use of the method in soil fertility studies necessitated examination of the effects of cultivations, manures, crop, meteorological variations, etc., on the conductivity measurements. This involved periodical determinations of initial conductivity (i.e. the specific conductivity of a 1 to 5 aqueous soil suspension determined at 25° C.) and the 7 days’ increase (i.e. the rise in specific conductivity of the same aqueous suspension after standing for 7 days in a thermostat at 25° C.). Soil samples were examined from several Rothamsted classical and nonclassical plots (over twenty plots were investigated for 15–20 months), and samples from experiments were also utilised during two growing seasons.

Vol. XX, Part 4, Oct. 1930

p. 539, line 6 for “1N sulphuric acid”

read “10N sulphuric acid”

Vol. XXI, Part 4, Oct. 1931

p. 764, line 14 for “mallee”

read “mallet”

p. 777. The last three lines belong to the legend to Fig. 3 on p. 770.