Evaluation of the contribution of residual fertilizer phosphorus to phosphate status and plant growth under conservation agriculture practice

Conservation Agriculture (CA) practice is reported to increase the level of total P in soil after a few years of cultivation with fertilizer application. Measures to ensure efficient utilization of the residual P in soil can, therefore, of great importance, especially in acid soils where the amounts of residual P which is not plant available can be significant. This study was conducted to establish the levels of residual fertilizer P in soils that had been under CA for five years and to investigate whether the residual fertilizer P could be made available to plants by applying, agricultural lime or organic matter or a combination of the two to the soils. Two acid soils under CA were used as test soils in the study. Total P contents and the chemical fractions of P in the cultivated and adjacent non-cultivated soils were determined. The residual fertilizer was calculated as the difference between the total P levels in the cultivated and non-cultivated adjacent soils. A greenhouse crop trial was carried out to investigate whether the release of the residual fertilizer P could be enhanced by applying agricultural lime or organic matter or a combination of the two to soils. This was laid out as a completely Randomized Design with four treatments, namely, soil alone(control), soil with organic matter, soil with agricultural lime, and soils with organic matter and agricultural lime with four replicates. The levels of total P in cultivated soils were significantly (p<0.05) higher than those in the adjacent non-cultivated soils and P was mainly associated with Fe and Al compounds. The cultivated soils had significant amounts of residual fertilizer P which ranged from 7 to 38 % of the total P. Applying organic matter or agricultural lime or combination of these significantly (p < 0.05) increased the levels of plant available P ,P uptake and dry matter yield. The order of the increase in available P, plant uptake of P and maize dry matter yield due to treatments were as follows: Agricultural lime + OM > agricultural lime alone > OM alone > the control. Further confirmation of the positive effect of soil amendments on the release of residual P was provided by significant positive correlations between plants available P in the soils and maize dry matter yield. Results of this study have demonstrated that applying agricultural lime and organic matter to soils enhances the release of residual fertilizer P in soils and can therefore, be used to increase fertilizer P use efficiency in CA cropping systems.