EFFECT OF SAMPLING DEPTH ON BICARBONATE SOIL-PHOSPHORUS TEST VALUES

In three field experiments involving five levels of application of phosphorus (P) on three different lateritic ironstone gravel soils in south-western Australia, soil samples were collected from dry soil, one year after P application in one experiment or two years after in the other two experiments, to measure the amount of P extracted by sodium bicarbonate (soil test P). The soil samples were collected to either 5 or 10 cm. The standard depth is 10 cm. For the experiment on a low P sorbing sand, soil test P was similar for both depths. However for the other two experiments, on soils with larger capacities to sorb P, soil test P was consistently larger for the 5 cm samples. When soils are dry and hard-setting it is often difficult to penetrate to the standard 10 cm depth. Inadvertently collecting soil samples to a shallower depth to measure soil test P could lead to large errors in estimating optimum P fertilizer application levels.