Wheat P requirements on calcareous moroccan soils: I. A comparison of olsen,soltanpour and CaCl2soil tests

Abstract

Wheat Triticum aestivum(L.)] is the major cereal crop of the Chaouia (dryland) region of Morocco. Efforts for self‐sufficiency in wheat will require improved fertility management on the calcareous soils of this region. This research was undertaken to evaluate the suitability of five soil P extractors for predicting the need for P fertilization on 15 important soils in this region. The Olsen (0.5MNaHCO³), the Soltanpour (1.0MNH₄HCO₃+ 0.005MDTPA) and their dilution modifications, and CaCl₂solution P method were evaluated in laboratory and greenhouse experiments.

Ten of the 15 soils responded to P fertilization. The five soils for which a significant P response was not measured were from the P plateau of the Chaouia region. The P extractors performed equally to predict the need for P fertilization. The critical levels below which a response would be highly probably and above which no response would be anticipated varied by extractor and method used to interpret the soil test values. A graphic method indicated the critical P values to be: Olsen 10.2, modified Olsen 18.0; Soltanpour 2.0; modified Soltanpour 12.5 and CaCl₂0.20 mg P kg^‐1. A second degree polynomial method indi cated the critical P values to be: Olsen 8.2; modified Olsen 14.0; Soltanpour 1.1; modified Soltanpour 5.1 and CaCl₂0.12 mg P kg^‐1. This limited data will permit the interpretation of soil test values as to whether a response to P fertilization is highly probable for the dominant soils of this wheat region.

Results from this study suggest that any of the five P extractors evaluated could be selected to calibrate P response under field conditions. It might be best to concentrate future research efforts on the Olsen test because most field P calibration data published with similar soils, climate and crops are with this extractor