Phosphorus soil test calibration for wheat on Moroccan soils

Phosphorus fertilizer recommendations for wheat in Morocco are based on the climatic regions disregarding soil P fertility. An improved fertilizer program must be based upon the soil test level. This research was conducted in order to select the best P soil test and calibrate it for wheat.; Greenhouse and field experiments were conducted to calibrate P soil tests for wheat (Triticum aestivum L.). Wheat was grown in a greenhouse, up to heading, on 11 soils from different regions of Morocco. The P rates were 7.7, 15.4, 30.8, and 61.6 mg P kg{dollar}sp{lcub}-1{rcub}{dollar} as monocalcium phosphate. Two and three field experiments, with increasing P rates, were conducted in the Tadla and the Gharb regions, respectively. A P adsorption study was conducted on 21 soils including the 11 soils used in the greenhouse experiment.; Dry matter production and P uptake were highly correlated with the NaHCO{dollar}sb3{dollar}-P soil level on a wide range of soils while the NH{dollar}sb4{dollar}OAc-P test only correlated well on soils low in lime content. The CaCl{dollar}sb2{dollar}-P and the H{dollar}sb2{dollar}SO{dollar}sb4{dollar}-P soil tests were not effective. The NaHCO{dollar}sb3{dollar}-P soil test was also correlated with wheat P requirement. Phosphorus adsorption was correlated with clay content, CEC/clay ratio, lime content, and organic matter content. Among the different soil P buffering capacity indices tested, the Salmon index was the most effective in predicting wheat P requirement when used concurrently with NaHCO{dollar}sb3{dollar}-P or Joret-Hebert soil tests. This index is easy to determine or can be predicted from the soil clay content.; Three of the five locations responded to applied P. The NaHCO{dollar}sb3{dollar}-P test of the responsive sites was lower than the critical level, 12 mg kg{dollar}sp{lcub}-1{rcub}{dollar}, as determined by Cate-Nelson graphical and analysis of variance methods. Square root and straight lines equations described well the yield data. The net return method was more accurate in determining the optimum P fertilizer requirement of wheat. The optimum P requirement was negatively related to the NaHCO{dollar}sb3{dollar}-P soil test level.; The use of the NaHCO{dollar}sb3{dollar}-P test will improve P fertilizer recommendations in Morocco. However, the fertilizer P recommendation accuracy can be improved by using, concurrently, the NaHCO{dollar}sb3{dollar}-P test with the soil P buffering capacity.