ALTERNATIVE SULPHUR FERTILISERS IN NEW ZEALAND

Abstract

Interest in high analysis sulphur (S) fertilisers is centred on elemental S which is generally mixed with other materials for safety reasons and to improve effectiveness. Central to the effectiveness of S fertilisers is the rate of oxidation to sulphate-S (SO4S)) for plant uptake. Oxidation rates increase as particle fineness increases. The distribution of S particle sizes in mixtures therefore affects the effectiveness of the fertiliser in any environment. Rates of oxidation vary between environments and thus different S particle size distributions suit different environments. Currently the choice of alternative commercial available S fertilisers is restricted to screened 'agricultural's" and mixtures of S" with superphosphate. Of these, molten mixed S superphosphate is the most suitable material for most environments but especially where S oxidation is slow. Commercial production of alternative S" fertiliser mixtures in New Zealand is imminent. Mixtures of S and sodium bentonite offer the possibility of preparing fertilisers to suit different environments. Granulated ground S" mixed with bentonite clay can be expected to release SO4-S more slowly but this has advantages where leaching of sulphate is likely. Production of SO4-S from two improved Canadian S/bentonite materials is expected to be similar to or slower than that from granulated ground S. Two other imported products, S impregnated urea (20% S, 36%N) and 'Thiovit' (85%S) contain finely divided and ultra finely divided S" respectively. They are raptdly and very rapidly oxidised to sulphate and may have specialised uses in cropping and horticultural enterprises. Keywords: Sulphur fertilisers, elemental sulphur, sulphur/sodium bentonite, plant sulphate, oxidation rates