| Abstract: | Inorganic phosphorus (P) fractions and their availability under subsurface irrigation were investigated in a greenhouse planted for 5 years with tomato. Irrigation was applied when soil water conditions reached the predefined maximum allowable depletion (MAD) for different treatments, e.g., ?10 kPa, ?16 kPa, ?25 kPa, ?40 kPa, and ?63 kPa. Concentrations of five inorganic P fractions, which include of soluble / loosely bound P, aluminum (Al) P, iron (Fe) P, calcium (Ca) P, and occluded P, were obtained by following a sequential chemical fractionation procedure. Results showed that the effect of subsurface irrigation and schedules on inorganic P fractions was more pronounced in topsoil layers than in deep soils. The concentrations of soluble / loosely bound P, Al P, and Fe P generally decreased with soil depth, having the largest values at the depths of 0–10 cm and 10–20 cm. In all the irrigation treatments, Al P and Fe P were the dominant fractions at the depths of 0–10 cm, 10–20 cm, and 20–30 cm, whereas Ca P and occluded P were most predominant at the depths of 30–40 cm and 40–60 cm. Soluble or loosely bound P, Al P, and Fe P were the main sources contributing to plant-available P, whereas Fe P and Al P were the two most important sources for contribution to plant-available P. Frequent irrigation with small amounts of water (e.g., irrigation with MAD of ?16 kPa and ?25 kPa) yielded larger concentrations of soluble / loosely bound P, Al P, and Fe P, which are the main sources of plant-available P. However, infrequent irrigation with larger amounts of water in each irrigation event led to greater concentrations of Ca P and occluded P, which are relatively less available to plants. |
