Rate‐controlling Processes in the Release of Soil Phosphate

Summary The isotopic exchange of phosphate ions in a deep river gravel soil, maintained at different pHs from 3.5 to 6.7 in a field experiment, established by the N.A.A.S. at Shardlow, was governed initially (<20 h) by simultaneous ‘first‐order’ and ‘bulk diffusion’ kinetics, but later by ‘bulk‐diffusion’ kinetics only. The ‘bulk‐’ or ‘intracrumb‐’ diffusion coefficient was independent of phosphate manuring, was least at pH 5.5 and increased on either side of this critical pH. The rate constant for the phosphate component exchanging initially with ‘first‐order’ kinetics increased significantly with phosphate manuring at pH 3.5, 4.4, and 5.5, although the constants for unmanured and manured soils did not change with soil pH. But at pH 6.7, the rate constants for the unmanured and phosphate‐treated soils were greater than those in the more acid soils, although the constant for the manured soil was just significantly greater than that of the unmanured soil. Residual phosphate (i.e. the difference between manured and unmanured soils) adsorbed on the soil was greatest in the more acid soils but water‐soluble residual phosphate increased with soil pH to a maximum at pH 5.5. Residual phosphate, exchanging initially, decreased to nothing above pH 6.5.