AGRONOMIC EVALUATION OF PHOSPHATE ROCK AND SLAG ON AN UPATA ACID SOILIN BOLIVAR STATE, VENEZUELA

An agronomic evaluation of phosphate rock and slag on the acid soils o f Upata, Bolivar State was conducted to measure their effect on the ch emical properties of the soil and growth of Brachiaria decumbens. A fi eld experiment was established using a split plot design with three re plications. The treatments were, i) three sources of phosphate rock (R iecito and Lizardo of Falcon State and Monte Fresco of Tachira State) at four rates of P2O5/ha (0, 50, 100, and 200), ii) one source of high -solubility P [triple superphosphate (TSP) at the same rates], and iii ) three levels of calcium applied as basic slag from the Orinoco Steel Company (0, 300, and 600 kg Ca/ha). A basic fertilization was applied to all plots (217 kg/ha Urea, 50 kg/ha KCl, and 78 kg/ha magnesium su lphate). The pasture species used was Brachiaria decumbens. Pasture an d soil samples were taken at 2, 3, 5, 7, 12, 14, 16, 17, 24, 27, and 3 1 months after planting. The pasture samples were analyzed for dry mat ter production (DM), and percentage content of nitrogen (N), phosphoru s (P), potassium (K), calcium (Ca), and magnesium (Mg). Soil samples w ere analyzed for pH, cation exchange capacity (CEC), and available cal cium (Ca) and phosphorus (P). The relative agronomic effectiveness (RA E), calculated using the formula: RAE (%) = DM of the rock - DM of the check plot x 100/DM of TSP - DM of check plot was applied for each ha rvest time. The best RAE values at each sampling time were obtained wi th the Monte Fresco phosphate rock at the rate of 200 kg P2O5/ha and 6 00 kg Ca/ha as slag. There was a tendency for the RAE value to improve with each sampling apparently due to the slow release of phosphorus f rom the phosphate rock and its residual effect. There are also values of RAE that show better dry matter production than the high soluble so urce of TSP. Soil available P and Ca increased with phosphate rock and slag application from 5 and 200 mg/kg to 25 and 400 mg/kg, respective ly, after 2.5 years of the experiment. Phosphorus uptake changed from 0.1% in the check plot, which is insufficent to supply the P requireme nt of grazing cattle in the tropics to a value higher than 0.2%, depen ding on the sampling time. Soil pH increased slightly from 5.2 to 5.6 with phosphate rock, and to 6.2 when basic slags were applied.