The nature of available phosphorus in some acid pasture soils and a comparison of estimating procedures

Abstract

Several methods of evaluating the phosphorus status of soils for the growth of pasture plants were compared. NaHCO3 and NH4F in HCl were the most suitable extractants used. On these soils, differing widely in origin and fertilizer history, phosphorus values of 30 p.p.m, as determined by three of the methods distinguished responsive from unresponsive soils. For the other, the bicarbonate method of Olsen et al. (1954), the corresponding value was 15 p.p.m. The evidence suggests that in the soils examined, all inorganic forms of soil phosphorus, as determined by the method of Chang and Jackson (1957), contributed in some degree to the pasture plants needs. The principal source of available phosphorus, however, was the aluminium phosphates. On the other hand, the evidence suggests that the principal loss in the availability of residual phosphorus to the plant may be through its conversion to organic forms. Iron phosphates appeared to be the least available of the inorganic forms. The common ability of the four successful methods to efficiently extract phosphorus from the aluminium source appears to be the prime reason for their measuring the soil phosphorus status for plants equally well. The physical nature of aluminium phosphate, however, has a marked effect on its ability to supply the plant's phosphorus needs. A crystalline form was less available than an amorphous one. The greater availability of the amorphous forms, of both iron and aluminium phosphates, appeared to depend more upon their solubility than on their positional availability due to the greater specific surface presented. Fertilizer application enriches the aluminium phosphate content of the soils, but, in these experiments, the quantities found were related significantly to the total applications made in the previous three years only.