Relationships between Ambic 1 and Bray 1 extractable phosphorus in some South African arable soils

Abstract

South African analytical laboratories use different extractants for P in soils of which the most important are Bray 1, Ambic 1, Bray 2, Truog and Olsen. The fertilizer industry prefers to use Bray 1 extractions for fertilization recommendations and the ARC-Grain Crops Institute, Ambic 1 extractions for calibration research. Therefore it became necessary to establish relationships between the P extracted by the two methods in specific soils and if possible to relate differences in these relationships to soil properties. Soil samples collected to a depth of 150 mm from phosphorus fertilization trials at 12 localities were used for this study to ensure a large gradient in extractable P. The clay content of the 150 mm topsoil at these localities ranged between 8.4 and 47.0%. More than 3000 soil samples were analyzed for extractable P using both the Ambic 1 and Bray 1 methods. Relationships between Ambic 1 and Bray 1 extractable P for each locality and for 12 localities in total were determined. In addition, multiple and simple regression analyses were carried out to establish relationships between the slopes of Ambic 1 versus Bray 1 relationships and some soil properties of the different localities. Excellent relationships between Ambic 1 and Bray 1 extractable P with Revalues ranging from 92 to 98% have been established at nine localities. At the remaining three localities the Revalues ranged from 75 to 88%. Although the relationship between Ambic 1 and Bray 1 extractable P over all 12 localities was statistically significant with a R²-value of 91 %, the slopes of the different localities differed significantly from each other. Most of the variation between these slopes was attributed to exchangeable calcium (R² = 80 and 83%), degree of leaching based on clay content (R² = 80 and 71%), pH(KCI) (R² = 70 and 69%), effective cation exchange capacity (R² = 68 and 74%), degree of leaching based on silt-plus-clay content (R² = 68 and 63%), pH (H₂O) (R² = 62 and 68%), Mn- oxides (R² = 60 and 72%) and the exchangeable Fe content (R² = 60 and 54%) of the soils. It is suggested that at least some of these soil properties should be considered in the development of an universal equation for the conversion of extractable P values from one to the other.