Improving the soil test phosphorus calibration for calcareous soils

The increased concern about cost and potential environmental impact of fertilizers emphasizes the importance of improving the accuracy and soil specificity of fertilizer recommendations. Buffering capacity is considered to be a key factor in understanding phosphorus (P) availability in the soil and crop response because it controls the rate at which P is supplied or depleted from the soil solution. Phosphorus buffering indices were determined from sorption isotherms developed using 18 soils from the Abda, Chaouia, and Ben Sliman arid and semiarid zones of Morocco. Soil characteristics were also used to develop prediction equations of these buffer indices. The greenhouse study was also conducted where wheat (Triticum aestivum) was grown in 13 soils under four P rates (0, 3.4, 6.7, and 13.4 mg P kg-1 soil). Critical soil test P levels for wheat under greenhouse conditions were determined. Maximum buffering capacity (MBC) of individual soils was incorporated into the Mitscherlich equation which was modified to determine the P fertilizer requirement. Our soils showed a contrasting ability to adsorb P. Maximum P adsorption (Xm) varied from 146 to 808 mg P kg-1 soil. Averaged across regions, soils from Chaouia adsorbed more P at maximum adsorption as compared to the Abda and Ben Sliman soils, suggesting that the soil from each region has to have specific P recommendations. Maximum buffering capacities ranged from 35 to 404 mg P kg-1. Differences between soils to react with P was also demonstrated in a kinetic study we conducted. The inclusion of MBC in fertilizer P recommendations did not significantly increase the precision of current applied P at infinity 0.05. However, calculations of P requirement, assuming the soil test in all of our 13 soils was 3 mg P kg-1 showed that the P fertilizer required to achieve 90% of the maximum yield varied from 1 to 15 mg P kg-1 depending on the MBC of each soil. However, the fertilizer P recommendation determined by the usual method was 12 mg P kg-1 for all soils with a soil test of 3 mg P kg-1. These results show that incorporation of buffer indices into a P requirement model should increase the accuracy of P recommendations and overcome the problem of overfertilization in sandy soils and under-fertilization in clay soils.