ISOTOPIC EQUILIBRIA BETWEEN PHOSPHATES IN SOIL AND THEIR SIGNIFICANCE IN THE ASSESSMENT OF FERTILITY BY TRACER METHODS

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Abstract
The isotopic exchange of phosphorus has been studied between solutions of orthophosphate and the following solid phases: anion‐exchange resin charged with phosphate; dicalcium phosphate; rock phosphates; hydroxyapatite aged to varying extents; and natural soils.The nature of exchange reactions in the soil is discussed. It is considered that all inorganic phosphate fractions in the soil undergo exchange with added phosphates, though at different rates.Consideration is given to procedures proposed by earlier workers for the estimation of the phosphate status of the soil by tracer methods. In particular, the hypothesis that‘plant available’soil phosphate can be estimated by comparing the specific activity of phosphate added to the soil with that of phosphate absorbed by plants is examined. It is shown that the method is of limited application. It appears, however, that since‘plant‐available’soil phosphate undergoes isotopic exchange more readily than‘unavailable’forms, it should be possible to estimate available soil phosphate by observing isotopic exchange between solutions and soil suspensions under controlled conditions. The appropriate procedure will vary between soil types and must be determined by ad hoc investigations.The limitations which isotopic exchange and isotopic dilution effects impose on the estimation of fertilizer absorption by tracer methods are discussed.