Assessing Subtropical Subsoil Cation Fertility Under Forage Bermudagrass.

Deep-rooted perennial forage species, such as bermudagrass, can mine subsoils of available nutrients. The overall status of subsoil cation fertility to support forage production in Florida is largely unknown. Soil profiles from three representative soil orders (Ultisol, Entisol, and Spodosol) commonly used for forage bermudagrass production in Florida were sampled at 15-cm increments to a depth of 120 cm. Exchangeable, hot 1 M HNO3-extractable, and total recoverable K, Mg, and Ca were evaluated. The difference between exchangeable and hot extraction was used to represent ‘fixed’ K (K in collapsed 2:1 clay mineral interlayers that is potentially available for plant uptake during clay mineral weathering). Two frequently used soil test methods in the southeastern US, Mehlich-1 and Mehlich 3, were also used to document cation fertility in the soil profiles. Subsoil K, Mg, and Ca were severely depleted in the Entisol and Spodosol, and they were low in the Ultisol, which had a small K reserve. Although the hot HNO3 extractant removed a portion of Fe and Al oxide coatings from all three soil orders, it did not appear to dissolve a significant portion of primary mineral silicates, as evidenced by the small amounts of extracted Si. Soil Mehlich-3 values after 3 years of different K application rates provided supporting evidence that bermudagrass roots can remove fixed K from subsoils (45 to 60 cm). A thorough state-wide soil inventory, including subsoil, is recommended in order to optimize fertilizer use efficiency and thereby, economic return from perennial forage systems.