Specific‐Gravity Effects on Fertilizer Leaching from Surface Sources to Shallow Water Tables

Abstract

AbstractSubirrigated sands with surface‐applied fertilizer bands are susceptible to considerable gravity‐induced convection of fertilizer salts whenever the water table approaches within 30 to 45 cm of the soil surface and induces rapid fertilizer dissolution. Laboratory studies have been conducted on two sands from Manatee County in the vegetable and citrus production region along central Florida's Gulf Coast. Peak saturated‐zone electrical conductivity (EC) values of 14 and 9 dS m−1 were attained within 3 to 5 d at the 60‐cm depth for columns of local sand having a water table maintained 2.5 and 22.5 cm below the soil surface, respectively. Such movement is far too rapid to be ascribed to reasonable diffusional or traditional convective‐flow effects, and is instead attributed to gravity‐induced convective flow, or fertilizer “dropout”. Sectioned‐column studies demonstrated near‐surface EC values of 25 to 35 and 70 to 90 dS m−1 for water tables maintained at the 15‐ and 45‐cm depths, respectively. These same columns evidenced solute‐pulse movement to the 8‐ to 10‐cm depth and the 15‐ to 20‐cm depth after 6 and 24 h, respectively, for water tables maintained at the 45‐cm depth. Lower hydraulic conductivity, quite possibly as a result of increased soil organic matter content for these otherwise virtually identical fine sands, moderated such effects somewhat. Implications are discussed with respect to early‐season fertilizer management in vegetable production regions of the southeastern Coastal Plain.