Superphosphate and stocking rate effects on a native pasture oversown with Stylosanthes humilis in central coastal Queensland. 4. Phosphate and potassium sufficiency

Abstract

Soil fertility was studied over the last 5 years of an 11 -year period in which a native pasture (Heteropogon contortus dominant) oversown with Stylosanthes humilis (TS) was fertilized annually with either 125 or 250 kg ha-1 superphosphate. A severe deficiency of potassium was found to have developed since the improved pasture was sown, and correction of the deficiency increased legume yield 2- to 3-fold and increased beef production by 24%.Pastures fertilized annually with 250 kg ha-1 of superphosphate gave no dry matter response to additional dressings of phosphorus, but those fertilized with 125 kg ha-1 annually responded slightly even after ten applications, indicating that this rate of superphosphate was close to the optimum. Three annual applications of 31 kg K ha-1 increased exchangeable soil potassium significantly and results indicate that a slightly lower rate would satisfy pasture requirements when combined with 125 kg ha-1 superphosphate. The practical implications of potassium fertilizer use are discussed in relation to the large increase in beef production possible and the need for potassium to ensure long-term maintenance of legume production. The decision to correct the deficiency would depend on local conditions and current conditions of beef production prices. Results are considered in relation to diagnosis of phosphorus and potassium deficiencies in TS in the field. For TS sampled at early flowering in March, it is concluded that fertilizer responses may be expected if phosphorus in whole plant tops is below 0.14%, or if potassium is below 0.4%, while responses are unlikely when these values are 0.17 and 0.7%, respectively. Satisfactory diagnosis would also have resulted from adoption of soil standards of 20 ppm P (extracted by 0.01 N H2SO4) or 0.20 m.equiv K 100 g-1.