Hernán Acuña P.1, Patricio Soto O.1, Germán Klee G.1, Nicasio Rodríguez S.1, Carlos Ovalle M.1 y Germán Martínez R.1
The experiment was carried out during four growing seasons (1982/83 to 1985/86) on a subterranean clover sward, established in 1981 on a "trumao" soil (Typic dystrandepts). The effect of four levels of P (0,22,44, and 132 kg/ha/year). with and without K (84 kg/ha/year). on total D.M. yield, botanical composition, and availClble N, P and K in the soil, was studied. Plots of 350 m2 , under cattle and sheep grazing, were used. The D.M. yield was measured by difference, evaluating the residual D.M. after grazing and the D.M. availability before the next grazing, by the comparative yield method. The botanical composition was studied in fixed lines, using the point quadrat technique. The response curves to P per growing season are presented. The maximum yields were reached with around 80 kg/ha of P/year. The highest response per kg of P applied would be between 22 and 44 kg/ha/year. There were no effects of K and of P x K interaction. Clover contribution increased when P application was increased, in the first growing season. In the fourth growing season, clover contribution decreased when 132 kg/ha of P/year were applied. The response to P of grasses and other species increased when N levels in the soil increased. The average available N in the soil (0 to 5 cm) increased from 34 ppm in 1983, to 176 ppm, in 1986. The extractable P increased from 10 to 18 ppm, when P applied was 22 or 44 kg/ha/year, and from 10 to 40 ppm, when P applied was 132 kg/ha/year, between 1982 and 1986. The use of small plots under grazing, would have some advantages compared with smaller plots under cutting, for evaluating the response to P in th is k ind of sward.
1 Estación Experimental Quilamapu (lNIA), Casilla 426, Chillán, Chile.