Sulfur Fertilization Of Rice

Sulfur (S) is an essential plant nutrient. In previous years S was delivered free to the farm by rain. Volcanoes and fossil fuel burning power plants release millions of tons of S annually. This S was then brought to earth with rain and snow. While the government cannot regulate volcanoes the EPA has restricted S emissions from fossil fuels. Sulfur deficiencies in rice are increasing each year. The University of Missouri recommends applying S only on soil with a CEC of less than 6.5. This would preclude sulfur fertilization for most rice production fields. This recommendation was developed during the early 1970's and reflects sulfur carried to the soil by rainfall. This study was conducted to evaluate Sulfur fertilization of rice.

Two pre-plant sulfur treatments were compared to the standard nitrogen fertilization for cultivating rice. A pre-plant urea treatment was included to verify if yield differences in the ammonium sulfate (NH₄SO₄) treatment were due to a sulfur or nitrogen effect. The total amount of nitrogen supplied to each plot was 150 lbs/acre. Nitrogen rates for pre-plant N treatments were balanced by deducting the corresponding N from the pre-flood rate. These treatments are listed in Table 1. Soil samples were collected from each plot at harvest. These samples were analyzed for pH and SO₄-S.

The yields for both sulfur treatments were numerically greater than for the urea treatments (Table 1). The average numerical yield advantage for the sulfur treatment was 19 bu/acre. At harvest SO₄-S soil test levels for both sulfur treatments were significantly greater than the urea treatments. SO₄-S levels for the 90% sulfur treatment were numerically greater than the NH₄SO₄ treatment. Post harvest SO₄-S levels indicate that sulfur build up should not be a problem. Soil pH levels were significantly lower for sulfur treatments at pre-flood. At harvest only the NH₄SO₄ treatment was significantly different. The NH₄SO₄ treatment lowered post harvest pH by 0.4 units.

While sulfur pre-plant fertilization did not significantly increase rice yields, the numerical yield advantage (19 bu/acre) for these treatments is compelling. At current prices for ammonium sulfate adding 12 of sulfur per acre (50 lbs NH₄SO₄) adds $1-$2 per acre to production costs. Rice producers should soil test for SO₄-S and consider pre-plant sulfur fertilization for fields testing low.