Relationships between commonly-used South African and international soil phosphorus extraction tests on pristine and cultivated soils

The availability of soil phosphorus (P) norms for various plant available P tests is limited due to the cost of establishing these norms. Consequently a need exists for a means of converting between the values of commonly used South-African and international soil P extraction tests for advisory purposes or comparing the results of soil P studies from different regions. Therefore, the aim of this study was to compare and correlate eight widely used soil P extraction tests on a wide variety of South African soils (48 pristine and cultivated samples). It was found that the relative effectiveness of total P extraction was a follows: 1:2 H₂O &lt; Olsen &lt; Colwell &lt; Bray I &lt; Ambic I &lt; Bray II &lt; Mehlich II ≈ citric acid. The strongest linear correlations were observed between Mehlich III, Bray I and Bray II (<i>r</i> = 0.92–0.99, <i>p</i> &lt; 0.001), Ambic I and citric acid (<i>r</i> = 0.96, <i>p</i> &lt; 0.001), and Olsen and Bray I, Bray II and Mehlich III (<i>r</i> = 0.85–0.89, <i>p</i> &lt; 0.001). Thus, it is possible to directly convert between the soil P values of these tests with more confidence using linear regression equations derived in this study.