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This article in SSSAJ

  1. Vol. 61 No. 4, p. 1044-1052
     
    Received: Feb 19, 1996


    * Corresponding author(s): morel@bordeaux.inra.fr
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doi:10.2136/sssaj1997.03615995006100040009x

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

  1. C. Morel  and
  2. J. Torrent
  1. INRA-Agronomie, BP 81, 33883 Villenave d'Ornon cedex, France
    Departamento de Ciencias y Recursos Agricolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain

Abstract

Abstract

The isotopically exchangeable quantity (Et) 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 Et values in 27 calcareous agricultural soils. For three soils, Et was also measured after adding increasing phosphate quantities as KH2PO4, and equilibrating for 24 h, to assess changes in Et with phosphate concentration in solution (Cp). In unamended and phosphate-enriched soils, Cp(CaCl2) was smaller than and related to Cp(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 Cp dependent, the overall effect of CaCl2 solution was analyzed considering two components: (i) a nonspecific effect attributed to the decrease in Cp and (ii) a specific effect attributed to phosphate reactions (i.e., adsorption or precipitation) in calcareous soils. Modifications in Et are the result of both nonspecific and specific effects. The change in Et 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 {[E24h(CaCl2) − E24h(H2O)]100/E24h(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.

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