Contribution of soil organic sulphur fractions to Marandu palisadegrass sulphur nutrition

Sulphur (S) deficiency in plants is widespread throughout the world. In soil S can be found in organic and inorganic forms. The contribution of the S organic fractions in meeting plant requirements in S is not well known. This study aimed to evaluate the variation of the mineral and organic S fractions in an Entisol where Marandu palisadegrass (Brachiaria brizantha) was fertilized with N and S. A fractionated 52 factorial experiment was carried out in a greenhouse, in a randomized block design, with four replications. Combinations of five N rates with five S rates, supplied as NH4NO3 and CaSO4.2H2O, were studied. Plants were harvested three times and the soil was sampled at each harvest. The total S content in the plant material was determined. Four soil S fractions were determined: inorganic SO4 S, ester bonded S, carbon-bonded S, and residual S. The 400-40 mg dm-3 N-S combination resulted in more S removed from the soil by the grass than the treatments with N or S supply alone. Inorganic SO4 S in the soil was increased by S application. The carbon-bonded S did not change in the soil, in any sampling time, with any N-S combination. The ester bonded S was increased, and apparently did not follow any N and S fertilizations. Except for the combination 400N-0S, the residual S fraction decreased as the time progressed. The variations of the S organic fractions in soil indicate that part of the S requirement of Marandu palisadegrass may be supplied by the residual S fraction