Soil P availability as affected by the chemical composition of plant materials: implications for P-limiting agriculture in tropical Africa

Plant materials that can replace costly inorganic fertilizers as phosphorus (P) sources are needed in smallholder farming systems in tropical Africa, where P is often yield-limiting. The objective of this study was to determine the relationship between the biochemical composition (quality) of plant materials, described in terms of total P, total nitrogen (N), lignin (LIG), and soluble phenolics (Pp), and soil P availability ( P av) under laboratory incubation conditions. The materials were ground and added to the soil at a rate equivalent to 10 Mg ha −1 (DM) and the samples (including a control soil) were kept at 50% field moisture capacity and 25 °C. The anion exchange resin method was used to extract solution P periodically. Because P added was not balanced among the treatments, P av was expressed as percentage of total P of plant material (i.e. P av =100×[(P av amended soil −P av control soil )/ total P added ] . Some treatments showed net P release and others showed net P uptake. The pattern of P av was viewed in three phases: (i) an initial rapid P release from the sparingly soluble inorganic P fraction of the plant materials, (ii) a subsequent phase when P in solution comes from both soluble P and mineralization of plant materials, and (iii) a last phase when P in solution is influenced by its equilibrium with P sorption processes. Total P was the best predictor of P av with r 2 ( P≤0.05) ranging from 0.50 to 0.77. Predictive functions were developed to determine the critical quality levels for net P release and net P uptake. The critical quality levels ranged from 2.0 to 2.7 g kg −1 for total P; 156:1 to 252:1 for C/P ratio; and 7:1 to 14:1 for N/P ratio. Among the materials tested, Tithonia diversifolia and Croton megalocarpus which contained total P >3.0 g kg −1 of total dry weight were identified as having the potential to release adequate P to replenish solution P for crop uptake.