FERTILITY OF COOK ISLAND SOILS

Abstract

The fertility of some major agricultural soils of the Lower Cook Group was assessed from greenhouse experiments and soil analyses. Using a “subtractive” design the soils were cropped with green panic, Panicum maximum var. trichoglume, over six harvests to assess the response to N, P, K, S, and Mg. Relationships between yield of green panic from subtractive treatments and nutrient levels in soils before cropping were examined using regression analysis. All soils became nitrogen deficient after a short period of cropping. The younger soils of Rarotonga, formed from basalt, and those soils of the outer islands, formed from basalt alluvium and coralline sand and limestone, have high base saturations (high exchangeable Ca and Mg) and have higher available phosphorus levels than the older soils from basalt on the outer islands. Exchangeable potassium (Ke) and reserve potassium (Ke) are low in all soils except on two soils from Rarotonga. The marked fall-off of Ke with depth indicates that in topsoils it is associated with the organic cycle. Available sulphur was adequate in most soils apart from those with high base saturations. Phosphorus extracted with 0.1M H2SO4 (Bondroff P) and adsorbed sulphur both provided good indexes of available P and S whereas Ke and Kc were not as well related to yields of green panic.