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

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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 managment on the calcareous oils of this region. This research was undertaken to evaluate the suitability of five soil P extractors predicting the need for P fertilization on 15 important soils in this region. The Olsen (0.5 M NaHCO3), the Soltanpour (1.0 M NH4HCO3 + 0.005 M DTPA) and their dilution modifications, and CaCl2 solution 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 CaCl2 0.20 mg P Kg-1. A second degree polynomial method indicated the critical P values to be: Olsen 8.2; modified Olsen 14.0; Soltanpour 1.1; modified Soltanpour 5.1 and CaCl2 0.12 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.