EFFECTS OF PLACEMENT OF ELEMENTAL-S AND SULFATE ON THE GROWTH OF 2 RICE VARIETIES UNDER FLOODED CONDITIONS

Sulfur deficiencies are increasing in rice producing areas as a result of the increasing use of S free fertilizers. The presence of oxidized and reduced soil layers in flooded soils affect S availability to ric e plants. A glasshouse experiment was conducted where S-35-labelled su lfate or elemental S was either broadcast onto the surface soil soon a fter transplanting or incorporated in the 7-21 cm soil layers 30 days before transplanting. The soil used was an S deficient Aquic Haplustal f. Two rice varieties C4-63 and Pulu Bolong, which had high and low O2 diffusion rates from roots respectively, were used in the study where plants were harvested at 42 days after transplanting (dat) and at mat urity. Rhizosphere oxidation and root growth were factors contributing to the higher fertilizer S uptake at 42 dat in C4-63 than in Pulu Bol ong. Filled grain yield was higher in C4-63 than Pulu Bolong. At 42 da t shoot yields, and at maturity straw yields, were highest in the surf ace sulfate (SS) treatment. Uptake of S derived from the fertilizer, r elative to total S taken up by the plant averaged over the two varieti es was 82% in the SS, 52% in surface elemental (SE), 34% in deep sulfa te (DS) and 8% in the deep elemental (DE) treatments. At maturity the values were 51% (SE), 42% (SS), 30% (DS), and 5% (DE). The lower value s for deep placement were due to lower oxidation of elemental S and de creased root activity at depth. There was a significant time x fertili zer placement x soil layer x variety interaction in the amount of fert ilizer S recovered in the KH2PO4-extractable sulfate and sulfide soil pools at 42 dat and maturity. At maturity, 34 to 74% of the fertilizer S was recovered in the soil sulfate and sulfide pools and in the alga e and surface water in the presence of plants, and 20 to 34% in the po ts without plants. The data indicate that surface application results in greater utilization of fertilizer S and suggest that combined P/S f ertilizer sources are likely to be more efficient than N/S sources.