Summary
Green manuring was simulated in the laboratory by adding six-week-oldVica faba residues (C/P ratios between 123 and 506) to a calcareous soil (Kingston series) at a rate equivalent to 8 t C ha−1 15 cm. The mixtures were incubated at 25°C for 42 days. Only residues with the lowest C/P ratio increased NaHCO3-extractable inorganic P (Pi) above the control (5.8 compared to 3.9 μg P g−1 soil at 42 days). This increase was small relative to the total extractable P in the soil-residue mixture of 38.4 μg g−1 (=76.8 kg P ha−1 15 cm) at the start of decomposition. Plant P appeared to be rapidly adsorbed by the soil. In the first 24h, when microbial uptake was negligible, 54 and 64 percent of the total extractable P in the low C/P ratio and high C/P ratio mixtures, respectively, was mobilized.
There was no evidence for mineralization of plant organic P that was not extractable in 0.5M NaHCO3, except with the residues of lowest C/P ratio which released about 1 μg P g−1 soil (=2kg ha−1 15cm) in 42 days, corresponding to a mineralization rate constant of 0.008±0.002 day−1. Nor could any increase in the extractability of native soil Pi be detected when32P-labelled residues were incubated with the soil at a rate equivalent to 8 t C ha−1 15 cm. In general, it would seem that the efficacy of green manure residues in improving the P supply to a subsequent crop depends primarily on the crop's ability to use residue P that is adsorbed by the soil or immobilized by microorganisms.
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White, R.E., Ayoub, A.T. Decomposition of plant residues of variable C/P ratio and the effect on soil phosphate availability. Plant Soil 74, 163–173 (1983). https://doi.org/10.1007/BF02143607
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DOI: https://doi.org/10.1007/BF02143607