Effect of sowing density and flooding on the comparative phosphorus requirement of Trifolium balansae and Trifolium subterraneum

The effect of sowing density and flooding on the phosphorus (P) requirement of Trifolium balansae and Trifolium subterraneum, two pasture legumes grown in Western Australia, is not known and was compared in two pot experiments and one field experiment. The average weight of one seed was 0.5 mg for T. balansae in all experiments, compared with 7.0 mg for T. subterraneum in the pot experiments and 5.0 mg in the field experiment. Either the same weight or number of seed of T. balansae as T. subterraneum was sown. The soils in the pot experiments were either maintained near field capacity or flooded. The P requirement was determined from the amount of P required to produce 90% of the largest or maximum dry herbage yield. In the pot experiments, T. balansae sown using 63 mg seed/pot (126 seed/pot) required the least P, followed by T. subterraneum sown using 63 mg seed/pot (9 seeds/pot). T. balansae sown using 4.5 mg seed/pot (9 seed/pot) required the most P. Flooding had no effect on the P requirements of the two species in one soil. However in the other soil, flooding increased, by about 36%, the P requirements of both species sown using 63 mg seed/pot compared with a 4% increase for T. balansae sown using 4.5 mg seed/pot. In the field experiment, T. subterraneum sown at 100 kg seed/ha required the least P, followed by T. balansae sown using 100 kg seed/ha, with T. balansae sown using 10 kg/ha (i.e. the same number of seeds as T. subterraneum) requiring the most P. However, differences decreased with increasing time from sowing and there was little difference at 133 days after sowing. In the pot experiments, production of dried tops per plant was up to five times greater for the low plant density treatments for both the field capacity and flooded treatments. It is concluded that sowing density and flooding can affect the comparative P requirements of T. balansae and T. subterraneum.