THE INFLUENCE OF IONIC STRENGTH AND MONOVALENT CATION SOURCE IN A LEACHING SOLUTION ON PHOSPHORUS EXTRACTABILITY IN A COARSE-TEXTURED SOIL

Abstract

The downward movement of phosphorus is significant for irrigated coarse-textured soils that are low in organic matter content. We conducted a laboratory study with columns of Valentine loamy sand (Typic Ustipsamment) to determine the effects of ionic strength and monovalent cation source of a leaching solution on P distribution in a soil with depth. Solutions of the chlorides of Na, Li, and K at concentrations of 1.0, 10−2, 10−4, and 10−6 mol L−1 were added to the columns after an application of 0.3 g diammonium phosphate on the surface of each column. Approximately two pore volumes, 500 ml of solution, were added to the columns, which measured 50 by 4.5 cm (cross-sectional area = 15.9 cm2), after which the columns were sectioned, and the amount of Bray-extractable P in each section was measured. Increasing ionic strength of the leaching solution resulted in an increase in the total amount of extractable P per column. Extractable P in the columns increased with depth as the ionic strength of the leaching solution decreased. Salt source of the leaching solution did not influence either the total amount of Bray-extractable P per column or the distribution of extractable P within the column. Dissolved organic carbon (DOC) in the leachate was found to increase with decreasing ionic strength. Changes in the distribution of extractable P with depth were attributed to increased competition for P adsorption sites by various organic molecules that were released into bulk solution when the organic matter coatings underwent a transition from the spherocolloid to the linear flexible molecule configuration. These transitions are believed to be caused by changes in the ionic strength of the leaching solution.