COMPARATIVE PHOSPHORUS REQUIREMENTS OF TRIFOLIUM-BALANSAE AND T-SUBTERRANEUM MEASURED AT THE SAME PLANT-DENSITY

The phosphorus (P) requirement of Trifolium balansae is unknown and wa s measured and compared with the P requirement of T. subterraneum in 2 glasshouse pot experiments and 3 field experiments. The same density of plants was used for each species in each experiment. Requirement fo r P was determined from the amount required to produce 90% of maximum yield of each species at each harvest. The yield produced at nil P or low levels of applied P was also used to compare the P requirement of the 2 species. As estimated from the amount of P required to produce 9 0% of maximum yield, 4 comparisons of P requirement were made in the p ot experiments. The 2 species had similar P requirements in 2 comparis ons, and T balansae required 40% more, and 50% less, P than T. subterr aneum on the other 2 occasions. In dense swards in the field experimen ts, T. balansae required 12-750% more P than T. subterraneum to produc e 90% of maximum yield, except for the first harvest (about 105 days a fter sowing) in 2 experiments when T balansae required 20-60% less P. In one field experiment, swards of T. balansae or T. subterraneum were either uncut or cut at weekly intervals from 110 to 166 days after so wing. Compared with the uncut swards. weekly cutting reduced dry herba ge yields of both species by about 5-15% for the 2 lowest levels of P applied and by 30-40% for the 3 highest levels.