Quantities of phosphorus fertilizer required to raise the soil test value

Abstract

To calculate nutrient requirement it is necessary to know how much of that nutrient must be applied to overcome any buffering effects and raise the test value to a desired level. A glasshouse study was conducted on a wide range of Natal soils to establish this information for phosphorus. The upper 200-mm depth at 54 sites was sampled. Soils varied greatly in texture (4–83% clay), organic carbon (0.2–9.2%) and clay mineralogy (kaolinitic-sesquioxic through to illitic and smectitic). Soil from each site was treated with four levels of P and taken through a number of wetting and drying cycles over a six-week period. Analysis for P using the Ambic, Bray no. 1 and Truog extradants showed that the quantity of applied P (kg ha−1) required to raise the soil test value by a unit amount (1 mg L−1), i.e. the P requirement factor, varied greatly for the different soils. The range in values for these three extradants was 2.5–37.9; 2.0–17.7; and 2.3–30.3, respectively. The level of P sorption was found to be strongly related to clay content, and soils rich in 2:1 minerals showed surprisingly high P sorption. © 1991 Taylor & Francis Group, LLC.