EFFECTS OF SOIL TEMPERATURE ON PHOSPHORUS EXTRACTABILITY. I. EXTRACTIONS AND PLANT UPTAKE OF SOIL AND FERTILIZER PHOSPHORUS

Abstract

The effect of 10, 15, 20 and 25 °C soil temperatures on the extractability of soil and fertilizer phosphorus (P) was examined in two soils, one containing free carbonate (pH 7.8) and the other non-carbonated (pH 6.9). The time course of fixation and desorption reactions were monitored. The extractability of P was also assessed using sodium bicarbonate (NaHCO3) extractions, desorption curves, and short-term uptake by wheat seedlings. Phosphorus-32 was used throughout. Opposing effects of temperature were found. An increase in incubation temperature from 10 to 25 °C decreased the amount of applied P extracted probably due to accelerated fixation reactions. This effect was established 1 day after the P was applied and persisted for 57 days. An increase in extraction temperature over the corresponding incubation temperature increased the extractability of P, indicating endothermic desorption reactions. This effect was established 1 h after the extraction began and persisted for 48 h. Hence, the net effect of temperature on the extractability of P will depend upon the balance of these opposing processes. The time course of these processes had two phases. The effects of temperature were established during the initial phase (< 1 day) of each process. The reactions continued more slowly after the initial phase but the later phases were not significantly affected by temperature. The P-desorption buffer capacity increased as temperature increased in the fertilized, carbonated soil. Isotopic exchange of applied 32P with native soil 31P increased as temperature increased. Isotopic exchange appeared to be more extensive when measured by plant uptake as opposed to NaHCO3 extraction, suggesting that the plants had access to a larger pool of soil 31P. Key words: Temperature, soil phosphorus, fertilizer, incubation, desorption, plant uptake, wheat