A comparison of the results of phosphorus analyses of alkaline and calcareous soils amongst South African laboratories

Abstract

Large areas of alkaline and calcareous soils are irrigated in South Africa. For these soils, proper fertilisation programs based on soil phoshorus (P) analyses are required. Little is known about P analyses of alkaline and calcareous soils. The study aimed to compare P analyses by three laboratories, each using Bray 1, Mehlich 3 and Olsen extraction methods. Six soils with a pH of 7.1 to 8.2 that contain 0.6 to 12.4% of CaCO₃ were incubated for two months after application of mono-ammonium phosphate (MAP) at levels equivalent to 0, 16.8, 33.6, 50.4, 67.2, 84.0 and 100.8 kg P ha⁻¹, subjected to several wetting and drying cycles. The P contents that were reported differed in many instances between the laboratories. The differences were sometimes of such an extent that it will influence P fertiliser programs significantly. This should be a concern for farmers and their advisors. This is probably inter alia due to the chemical composition of the extractants. An in-depth investigation into a larger number of laboratories’ P analyses of alkaline and calcareous soils is warranted to confirm the results.