Liming Effects on Nitrate Adsorption in Soils With Variable Charge Clays and Implications for Ground Water Contamination

The present research sought to determine the extent and distribution of nitrate accumulation in the subsoils of central Oahu, and to measure the consequences of surface applied amendments on nitrate mobility. Deep bore holes collected from pineapple fields in central Oahu were analyzed for nitrate and nitrate adsorption, and a lime incubation experiment and column study were conducted. Three and one half to 11 T/ha of nitrate-N have been measured in subsoils and the deep. saprolitic vadose zones of pineapple fields located in central Oahu, Hawai’i. Nitrate adsorption isotherms confirmed the high nitrate retention capacity of the materials in the subsoil and saprolite. Since large areas of former sugarcane land are being converted to diversified crops requiring amendments to reduce acidity, a lime incubation study was conducted on soils from former sugarcane land to investigate the effect of lime on nitrate adsorption. When lime was added to acid subsoils, nitrate retention was decreased by up to one half of the native adsorption capacity. A leaching experiment found that surface applied lime and gypsum increased nitrate mobility through soil columns 50 cm in length. The data indicate that large quantities of nitrate have accumulated in the subsoil overlying the Pearl Harbor aquifer. Mobility of nitrate through the subsoil is retarded, but a laboratory column study shows that nitrate mobility can be increased by adding lime and gypsum to the surface soil. Field studies need to be conducted to evaluate the potential for ground water contamination as a result of surface applied amendments