INCREASING PLANT-AVAILABLE PHOSPHORUS IN AN ULTISOL WITH A YARD-WASTECOMPOST

Beneficial uses of a yard-waste compost as a soil amendment and plant- growth medium were evaluated on a highly weathered, acid soil (Ustic K anhaplohumult), using corn (Zea mays L.) seedlings as a test crop. Fir st, factors responsible for the soil's infertility were identified in a greenhouse experiment consisting of five phosphorus (P) rates (0, 75 , 150, 300, and 600 mg/kg) with or without 1.0 g calcium (Ca)/g (2 ton s Ca/ha) as gypsum (CaSO4.2H2O) or calcium hydroxide [Ca(OH)2]. At no or low P additions, severe growth restrictions (low dry matter product ion and shoot P concentration <0.10%) irrespective of the Ca amendment s indicated that P deficiency was the most growth limiting factor in t his tropical soil. Subsequently, P sorption isotherms were constructed for the soil, the compost, and soil:compost mixtures by equilibrating 1.0 g soil in 20 mL 0.001M calcium chloride (CaCl2) containing variou s P concentrations. The results showed that in the unamended state, th e soil supported only 0.01 mg P/L, the compost 9.5 mg P/L, and the mix tures containing 25%, 50% or 75% compost by volume 0.04, 0.06, or 0.10 mg P/L in the soil solution, respectively. Approximately 300 mg P/kg must be added to the unamended soil to maintain 0.20 mg P/L in the soi l solution. Finally, effects of the compost amendment were studied by growing corn in various volumetric mixtures containing 0, 25, 50, 75, and 100% compost. Best growth was obtained when compost fractions were > 75%, corresponding to approximately 0.20% P in the plant shoots and 40 mg/kg Mehlich-1-extractable P.