Short‐term Replenishment of Soil Solution Phosphorus

Depletion of soil solution P from zones around plant roots indicates that diffusive flux of P from the soil solution to the plant root is inadequate to keep pace with removal by plant roots. Consequently, the concentration and/or concentration gradients of solution P maintained under conditions of repeated removal over time are an important soil property that is a factor in determining the quantity of P diffusing to root surfaces. This study was conducted to determine the P release characteristics of five soils differing in P retention properties and pretreated with quantities of P equivalent to their sorption maxima. Five‐gram samples were sequentially extracted with 3.5‐ml aliquots of 10 −3 N CaCl 2 for 350, 15‐min cycles. Quantities of extracted P were 15 to 30% of the added P. Concentrations maintained during repeated extractions revealed three distinct phases of release presumably associated with different forms of soil P. Release persisted through approximately 25, 125, and 150 extraction cycles for the initial, intermediate, and final phases, respectively. For different soils, concentrations of P ranged from 0.5 to 1.1 µg/ml in initial phases and 0.02 to 0.08 µg/ml in final release phases. Though pretreatment of soils with P was based on retention properties, relative extractability and concentrations in solution during release phases appeared to be a characteristic of each soil. During the intermediate release phase, P concentration was very high and decreased rapidly for one soil, was moderately high and stable for another soil, and was very low and hardly discernible from the final phase for a third soil.