FACTORS AFFECTING THE ABILITY OF PLANTS TO ABSORB PHOSPHATE FROM SOILS

Summary The relationship between the absorption of phosphate by plants and labile soil phosphate has been investigated in four soils ranging from a calcareous alluvium to basaltic soils of reputed high ‘phosphate‐fixing’ capacity. Labile phosphate has been determined both by isotopic exchange and by applying the principles of isotopic dilution to the results obtained when plants are grown in soils to which labile phosphate has been added (the method of Larsen). The two measures gave generally similar but not identical results. Aliquots of each soil type were enriched with varying quantities of phosphate to give series of soils similar in all respects other than their phosphate status. The soils were sometimes stored for periods of up to a year before further examination was carried out. A large fraction of the added phosphate always remained in a labile form. In each such series of artificially enriched soils a closely linear relationship was found between absorption by test plants and the labile soil phosphate. Soils of contrasting origin, however, showed no such correlation, but absorption was related to the reciprocal of the quantity of phosphate sorbed by the soils from phosphate solutions under standard conditions. An interpretation of the factors which determine the extent to which phosphate can be absorbed by plants from different soils is advanced in terms of the concepts of phosphate ‘potential’ and ‘capacity’ introduced by Schofield. The labile soil phosphate is not, however, regarded as homogeneous; evidence of wide variation in activation energy between different labile‐phosphate fractions has been obtained. When used to assess the phosphate status of agricultural soils, tracer methods appear to be subject to limitations similar to those of the older extraction procedures.