Improving the soil test phosphorus calibration for calcareous soils

The increased concern about cost and potential environmental impact of fert ilizers emphasizes the importance of improving the accuracy and soil specif icity 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 deplet ed from the soil solution. Phosphorus buffering indices were determined fi om sorption isotherms developed using 18 soils from the Abda, Chaouia, and Ben Sliman arid and semiarid zones of Morocco. Soil characteristics were al so used to develop prediction equations of these buffer indices. The greenh ouse study was also conducted where wheat (Triticium aestivum:) was grown i n 13 soils under four P rates (0, 3.4, 6.7, and 13.4 mg P kg(-1) soil). Cri tical soil test P levels for wheat under greenhouse conditions were determi ned. Maximum buffering capacity (MBC) of individual soils was incorporated into the Mitscherlich equation which was modified to determine the P fertil izer requirement. Our soils showed a contrasting ability to adsorb P. Maxim um P adsorption (Xm) varied from 146 to 808 mg P kg(-1) soil. Averaged acro ss regions, soils from Chaouia adsorbed more P at maximum adsorption as com pared 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 prec ision of current applied P at infinity 0.05. However, calculations of P req uirement, assuming the soil test in all of our 13 soils was 3 mg P kg(-1) s howed that the P fertilizer required to achieve 90% of the maximum yield va ried from I to 15 mg P kg(-1) depending on the MBC of each soil. However, t he fertilizer P recommendation determined by the usual method was 12 mg P k g(-1) for all soils with a soil test of 3 mg P kg(-1). These results show t hat incorporation of buffer indices into a P requirement model should incre ase the accuracy of P recommendations and overcome the problem of over-fert ilization in sandy soils and under-fertilization in clay soils.