Original Articles

An improved phosphorus desorption index for the routine determination of the Ambic 2 phosphorus requirement factor of South African soils


A short (24 h) laboratory incubation study was conducted and plots of P desorbed in Ambic 2 extraction solutions versus added P were constructed for a wide range of soils of varying P fixing capacity representative of the province of KwaZulu-Natal, South Africa. The plots were found to yield straight lines and, using linear regression analysis, equations of best fit describing these desorption functions were established for each soil. A two point test, the Ambic 2 Phosphorus Desorption Index (PDIAmbic2), was found to correlate closely with the slopes of the desorption functions (R2 = 0.943). This result showed the proposed quick two point PDIAmbic2 test to be a reliable index of the slope of the multiple point desorption functions. It established that, although simpler, it could be used advantageously as a time saver to estimate the slopes of the desorption functions routinely. PDIAmbic2 was further shown to correlate significantly to the Ambic 2 Phosphorus Requirement Factor (PRFAmbic2) obtained from a previous long term (six weeks) glasshouse incubation study and designed to simulate the fate of additional P fertiliser under field conditions. PRFAmbic2 is a key factor used to convert soil-P deficits in quantities of additional fertiliser required by crops, and is a useful characteristic for making P fertiliser recommendations, albeit one that is difficult to measure routinely. Non-linear regression analysis was used to establish the function best describing the relationship between PRFAmbic2 and the more user-friendly PDIAmbic2 test. Confirmation of the predictive value of the equation of best fit was validated using data obtained from soils not originally included in the regression. Implications of this test for routine laboratories involved in P fertiliser advisory work are discussed.

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