The relationship between Bray's P₁, modified NaHCO₃, New Mehlich and NH₄F. HF extractants for P in savannah soils of western Nigeria

Abstract

Sixty surface (0–15 cm) soil samples and 20 field sites with different cropping histories were selected from the major soil types of the western Nigeria basement complex savannah. Available P was evaluated with six different extractants, namely Bray's P₁, 0.1N AHDF (pH 4.1–4.3), 0.1N AHDF (pH 5.0), 0.05N AHDF (pH 7.0), New Mehlich and modified Olsen's 0.5M NaHCO₃ (pH 8.5).

The highest amount of P was extracted by 0.1N AHDF (pH 4.1–4.3), while 0.05N AHDF (pH 7.0) extracted the lowest. In the greenhouse, the New Mehlich extractant had the best correlation with P uptake (r = 0.95,p < 0.001), which was not different from Bray's P₁, 0.1N (pH 4.1–4.3), 0.1N AHDF (pH 5.0) and modified 0.5M NaHCO₃ extractants. Field experiments in the various locations showed a significant correlation between relative maize yield and soil tests. The 0.5M NaHCO₃ had the highest correlation (r = 0.70,p < 0.01) while 0.1N AHDF (pH 4.1–4.3) had the lowest (r = 0.53,p < 0.05).

All extractants seem to extract the same forms of P. Although Fe-P contributes the highest amount of active P extracted with the Chang and Jackson procedure, its utilization by the crop seems to be relatively lower than the other P forms. The 0.1N AHDF (pH 4.1–4.3) extracted 38% of this active P form while Bray's P₁, New Mehlich, 0.1N AHDF (pH 5.0), modified 0.5M NaHCO₃ and 0.05N AHDF (pH 7.0) extracted 29, 27, 22, 21 and 7% respectively. It is therefore concluded that any of the following four extractants, namely New Mehlich, 0.5M NaHCO₃, 0.1N AHDF (pH 5.0) and Bray's P₁, could be adopted for routine soil testing for P in the savannah zone of western Nigeria.