DIFFERENTIAL PHOSPHORUS RESPONSES OF LEGUMINOUS COVER CROPS ON SOILS WITH VARIABLE HISTORY

The response of 8 leguminous cover crops to phosphorus (P) application (7.5 mg P2O5 kg(-l) soil or 15 kg (PO5)-O-2 ha(-1) to the depth of 15 cm) on soils with variable history was evaluated in a pot trial suppl emented with a field experiment in 1993. The soil from a livestock far mer's field showed higher total organic carbon content and extractable cations compared to that from a non-livestock farmer's field. In the pot trial, P application, on average, increased shoot, root, nodule dr y matter and nitrogen (N) accumulation of the legumes by 82%, 45%, 871 %, and 900%, respectively, compared to the control. Cajanus cajan, Cro talaria ochroleuca, Centrosema pascuorum, and white-seeded Mucuna prur iens showed a higher P response than Centrosenza brasilianum and Chama ecrista rotundifolia. The legumes grown on the manured soil showed not only higher biomass and N accumulation, but also higher increase (110 % and 117%) in total dry matter and N accumulation because of P applic ation than those grown on the un-manured soil (27% and 45%). In the fi eld experiment, spreading legume groundcover at 16 weeks after plantin g was increased by 40% in the un-manured soil and by 31% in the manure d soil. Centrosema brasilimmm even showed a negative response of groun dcover to P application. There was little response in erect legume hei ght to P, except for measurements at 6 and 8 weeks after planting, whe n P increased plant height for Crotaktria on un-manured soil. Results imply high returns can be expected when P is applied to leguminous cov er crops in fairly fertile soil. The relatively low response under the field conditions, compared to pot, suggests caution is needed when P is recommended for legumes grown under environmentally stressed condit ions.