THE COMPARATIVE PHOSPHORUS REQUIREMENTS OF SOME TEMPERATE PERENNIAL LEGUMES

The phosphorus (P) responses of seven temperate perennial pasture legumes and two species of lupins were compared in a field trial over a range of nine P rates, from 0 to 800 kg ha-1. The two lupins produced more than 5 t ha-1 of dry matter in the absence of added P and showed no response to the fertiliser. In contrast, the pasture legumes initially failed to grow without added P and responded to applications of between 200 and 800 kg ha-1. At the higher rates of P, dry matter production of the pasture legumes was equivalent tothat of the lupins. In the first 2 years of the trial; the most productive pasture legume species at the higher rates of added P were also the most productive at the lower rates. Phosphorus requirements of 90% of maximum yield varied greatly between species, but were closely related to maximum yield. Thus species with low P requirments for maximum yield were not necessarily P-efficient species. In the third and subsequent years of the trial Lotus corniculatus performed better than the other pasture legumes at the lower rates of added P. In contrast to other studies Lotus pedunculatus showed no ability to outyield Trifolium repens at low rates of P. Critical P concentrations of the pasture species for the late spring-early summer period declined in the order Trifolium repens (0.34%) > Lotus pedunculatus (0.30%) > Trifolium pratense (0.28%) > Trifolium hybridum (0.27%) > Trifolium ambiguum (0.26%) > Lotus corniculatus (0.23%). Mineralisable nitrogen (N) levels were determined in soils under three species in the 7th year of the trial. At the lowest rates of added P, mineralisable N levels were much higher under Lupinus polyphyllus than under Trifolium repens or Lotus corniculatus. With increasing P rate, levels under the latter species increased, and at 100 kg P ha-1 were equivalent to those under the lupin with no added P.