Accumulation of organic matter in some New Zealand soils under permanent pasture

Abstract

Allophanic soils, compared with non-allophanic soils, are shown to contain more aluminium extractable by oxalate, more organic matter, and higher phosphorus retention capacities, but lower apparent specific rates of mineralisation. This difference in mineralisation rate is considered to be real, to be related to differences in the type of mineral colloid, and may determine the differing organic matter contents found in these soils. Annual rates of immobilisation of nitrogen, sulphur, and phosphorus in organic forms may be quantitatively important when compared with the amounts needed to maintain good pasture growth. Allophanic soils should be ploughed infrequently and shallowly in order to reduce subsequent immobilisation of available nutrients. The cropping of both allophanic and non-allophanic soils, appears to be equally possible provided that the organic matter content is at a near steady-state.