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 was 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 for 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 90% of maximum yield, 4 comparisons of P requirement were made in the pot experiments. The 2 species had similar P requirements in 2 comparisons, and T balansae required 40% more, and 50% less, P than T. subterraneum on the other 2 occasions. In dense swards in the field experiments, T. balansae required 12-750% more P than T. subterraneum to produce 90% of maximum yield, except for the first harvest (about 105 days after 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 sowing. Compared with the uncut swards. weekly cutting reduced dry herbage 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.