Grazing studies on the Guadalcanal Plains, Solomon Islands. 1. Climate, soils, and soil fertility assessment

Abstract

Summary: The Guadalcanal Plains encompass approximately 460 km² with both forest and grassland phases. Mean annual rainfall is 2160 mm, with a drier season from June to October, but soil moisture deficits seldom occur. Soils are freely to imperfectly drained brown loam to clays (Ustropepts). Soil analyses show available P concentrations of 15–17 μg/g, adequate K, Cu, Zn and Mn, and high Ca and Mg, but sulphate sulphur is low. A nutrient omission trial gave significant negative responses to the omission of P, K, S and Mo. However, responses to P applications were not obtained in the field with Centrosema pubescens, but significant responses to K and S were found under a cut and remove system. Yield response in the grasses Brachiaria decumbens and B. mutica to applied N was markedly limited in the absence of applied S, even though B. mutica was able to maintain S concentrations in herbage at 2–3 times those of B. decumbens. Without applied S, S contents and N:S ratios in B. decumbens were inadequate for beef cattle, but adequate in B. mutica except at high N inputs (300 kg/ha/year). Application of 40 kg S/ha to the legume C. pubescens increased yields by approximately 50%, reduced incidence of Cercospera leaf spot, and gave increased N concentrations. For establishment of sown mixed pastures on the Guadalcanal Plains we recommend the basal application of 20 kg P/ha, 50 kg K/ha, 30 kg S/ha and 0·15 kg Mo/ha, with 30 kg S/ha applied every 2 years.