A natural rock phosphate, ‘Kodjari” (RP), sparingly
soluble, indigenous to Burkina Faso, and a
commercial single superphosphate (SSP) fertilizer were studied at Farako-Bâ in
the Northern Guinea
Savannah (NGS) and at Oipasse in the Sudan Savannah (SS) regions of Burkina
Faso. A 3-year crop
sequence either of cowpea, maize and sorghum or of cowpea, sorghum and
sorghum was used,
respectively, in the NGS and SS regions. The sorghum crop in the third
year was not fertilized with
any phosphorus (P) source. The objectives of the research were to study
the direct and residual effects
of the P fertilizers on soil fertility improvement in order to boost agricultural
productivity in both
regions.
Cowpea daylength-insensitive cultivars, in both regions, and maize and
sorghum in the NGS and
SS regions, respectively, responded more strongly to SSP than to RP fertilizer
treatments. The
optimum rate of SSP and RP source was 21·8 kg P/ha and 43·6
kg P/ha, respectively, for cowpea in
both regions. The optimum rates of phosphorus fertilizer applied in the
second year to maize in the
NGS and to sorghum in the SS region, in addition to the optimum rate of
P applied to cowpea the
previous year, was 10·9 kg P/ha of SSP or 43·6 kg P/ha
of RP, and 21·8 kg P/ha of SSP or
43·6 kg P/ha of RP, respectively. Both P sources had significant
residual effects for up to 2 years. The
agronomic effectiveness of RP relative to SSP, in the year of application
of both fertilizers, was
greater for cowpea than for maize in the NGS region and similar for cowpea
and sorghum in the SS
region; it increased markedly for the two subsequent cereal crops in both
regions. Cowpea
fertilization with both P sources proved, therefore, to be effective in
improving the soil fertility and
boosting the productivity of cereal crops in the 3-year crop sequence.