Effects of long-term cultivation on phosphorus (P) in five low-input, subtropical Australian soils

Despite the importance of P for cropping, there remains uncertainty regarding the effects of long-term cropping on P behaviour in soils. Using five soils (Billa Billa, Cecilvale, Riverview, Thallon, and Waco) from subtropical Australia cropped for ≤70 y with low rates of P-fertilizer inputs, changes in soil P (organic, inorganic, and total) were assessed and related to P removal in wheat grain. For four of the five soils, concentrations of organic P decreased significantly with increasing period of cropping, decreasing between 20 and 36%. In contrast, concentrations of inorganic P generally remained relatively constant. Despite the decrease in organic P, the C/P ratio remained relatively constant in all five soils (average of 99), indicating that organic P is protected within the soil organic matter in a manner similar to the organic C. Furthermore, although increasing period of cropping decreased soil organic P concentrations, grain P concentrations decreased significantly only for Billa Billa and Thallon. Finally, although the rate of P removal in grain was low (1.6 to 9.3 kg ha−1 y−1), the rate of removal exceeded inputs from P-containing fertilizers. Thus, there is a need to consider the long-term P-fertilization to ensure adequate P nutrition of the crops. This study provides important information regarding changes in P within long-term cropping systems and will assist in their sustainable management.