SPECIFIC-GRAVITY EFFECTS ON FERTILIZER LEACHING FROM SURFACE SOURCES TO SHALLOW-WATER TABLES

Subirrigated sands with surface-applied fertilizer bands are susceptib le to considerable gravity-induced convection of fertilizer salts when ever the water table approaches within 30 to 45 cm of the soil surface and induces rapid fertilizer dissolution. Laboratory studies have bee n conducted on two sands from Manatee County in the vegetable and citr us production region along central Florida's Gulf Coast. Peak saturate d-zone electrical conductivity (EC) values of 14 and 9 dS m(-1) were a ttained within 3 to 5 d at the 60-cm depth for columns of local sand h aving 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 reason able diffusional or traditional convective-flow effects, and is instea d attributed to gravity-induced convective Bow, or fertilizer ''dropou t''. Sectioned-column studies demonstrated near-surface EC values of 2 5 to 35 and 70 to 90 dS m(-1) for water tables maintained at the 15- a nd 45-cm depths, respectively. These same columns evidenced solute-pul se 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 dep th. Lower hydraulic conductivity, quite possibly as a result of increa sed soil organic matter content for these otherwise virtually identica l fine sands; moderated such effects somewhat. Implications are discus sed with respect to early-season fertilizer management in vegetable pr oduction regions of the southeastern Coastal Plain.