AVAILABILITY OF PHOSPHATE APPLIED TO CALCAREOUS SOILS OF WEST ASIA AND NORTH-AFRICA

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, Al. 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 betweeen 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.