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Availability of Phosphate Applied to Calcareous Soils of West Asia and North Africa

ICARDA, P.O. Box 5466, Aleppo, Syria

J. Torrent^*

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

*Corresponding author.

ABSTRACT

The relative influence of different soil components on P availability in calcareous soils is contradictory in the literature. For this reason, P-incubation experiments were conducted on 19 soils from different agricultural areas of West Asia-North Africa. The soils contrasted widely in clay, CaCO₃, and Fe oxide contents. The availability index (AI), the ratio between the increase in Olsen P and the P applied, was determined between 30 and 240 d after applying rates between 20 and 500 mg P kg^–1 soil. The Al at 20 mg P kg^–1 (AI₂₀) changed little with time after 60 d. The AI₂₀ at 180 d was negatively correlated to the Fe oxide content and to the cation-exchange capacity of the soils. In a stepwise regression procedure, the variance of AI₂₀ that was accounted for increased significantly from 44 to 56, and then to 70% by including Fe oxides, cation-exchange capacity, and CaCO₃ content. At a rate of 500 mg kg^–1, AI₅₀₀ decreased more markedly with time and was negatively correlated, at all times, with the amount of CaCO₃ but not correlated with Fe oxides, cation-exchange capacity, or clay content; at some times, a positive correlation was found between AI₅₀₀ and organic matter. These results (i) support the idea that P reactions in calcareous soils differ according to the P application rate, and (ii) raise caution about the incubation conditions in the laboratory that are used to estimate the loss of P availability in calcareous soils.

Joint contribution from ICARDA (International Center for Agrícultural Research in the Dry Areas) and the Departmento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba.

Received for publication December 31, 1991.

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