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Sensitivity of Isotopically Exchangeable Phosphate in Soil Suspensions to the Supporting Solution

INRA-Agronomie, BP 81, 33883 Villenave d'Ornon cedex, France

J. Torrent

Departamento de Ciencias y Recursos Agricolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain

*Corresponding author (morel{at}bordeaux.inra.fr).

ABSTRACT

The isotopically exchangeable quantity (E_t) of phosphate measured in 1:10 soil/solution suspensions with deionized water has been identified as the reserve of plant-available phosphate in agricultural soils. We studied the effects of two supporting solutions, 0.01 M CaCl₂ and deionized water, on E_t values in 27 calcareous agricultural soils. For three soils, E_t was also measured after adding increasing phosphate quantities as KH₂PO₄, and equilibrating for 24 h, to assess changes in E_t with phosphate concentration in solution (C_p). In unamended and phosphate-enriched soils, C_p(CaCl₂) was smaller than and related to C_p(H₂O). The initial rapidly and the subsequent slowly isotopically exchangeable phosphate were both affected by the use of CaCl₂ solution. Since both the rapid and slow isotopically exchangeable phosphate are C_p dependent, the overall effect of CaCl₂ solution was analyzed considering two components: (i) a nonspecific effect attributed to the decrease in C_p and (ii) a specific effect attributed to phosphate reactions (i.e., adsorption or precipitation) in calcareous soils. Modifications in E_t are the result of both nonspecific and specific effects. The change in E_t due to the use of 0.01 M CaCl₂ instead of deionized water varies with the soil sample, and can be positive, negative or nil depending on the time of exchange. For instance, after 24 h of exchange, the ratio {[E_24h(CaCl₂) – E_24h(H₂O)]100/E_24h(H₂O)} ranged from –30 to 340% for the 27 soils. The 0.01 M CaCl₂ solution thus introduces complex and unpredictable changes in isotopic exchanged phosphate, and its use is not recommended to estimate plant-available phosphate in soil.

Received for publication February 19, 1996.

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