The Form and Availability of Slowly Available Phosphorus in Depleted Vertosols

Abstract

Vertosols are an important soil type used for cotton production in the northern grains region (NGR) of eastern Australia. Historically, cotton has received minimal phosphorus (P) input due to the high fertility of Vertosols. Over the past 30 years, P fertiliser use in the cotton industry has increased six-fold, due to the perceived decline in soil fertility in the NGR. However, when P is applied to cotton systems, the amount of P fertiliser recovered by the crop (PUE) has been low and unpredictable (0 - 67 %), and few studies have investigated why this is so. The unpredictability of PUE in cotton systems refers to the lack of cotton response when P fertiliser has been applied in some soils, which was based on recommendations from the commonly used Colwell soil P test. Long-term field trials suggest the quantity of readily available P measured using the Colwell extractant is being replenished by other soil phosphates not measured by the Colwell extractant. This may be due to the large amounts of Ca phosphates typically found in Vertosols, which may supply or replenish labile soil P pools in the 0 - 10 cm and 10 - 30 cm layers. Recent studies have used a dilute acid test (colloquially referred to as the BSES extractant) to measure Ca phosphates in Vertosols, however, little is known about the P pools removed by this extractant. The aim of this project was to understand the ability of Ca phosphates to supply or replenish plant available P in the cotton systems of the NGR.