Nitrogen and Sulfur Supply to a Degrading Signal Grass Pasture: Soil and Plant Concentrations.
Francisco A. Monteiro and Edna M. Bonfim-Silva. Soils and Plant Nutrition Dept, Luiz de Queiroz College of Agriculture, Univ of São Paulo, Avenida Pádua Dias, 11 - P. O. Box 9, Piracicaba, Brazil
Nitrogen is the main nutrient for forage grass production and sulfur may be necessary when nitrogen fertilizer is applied. The objective was to study the effects of the fertilization with nitrogen and sulfur for soil and plant responses, in a forage grass under degradation in an Entisol. Cylinders having 15 cm diameter and 20 cm height were collected with Signal grass (Brachiaria decumbens) and soil, and adapted to the greenhouse environment at Piracicaba, State of São Paulo, Brazil. The collected soil had 28 g kg-1 organic matter, 1.75 g kg-1 total nitrogen and 4.32 mg dm-3 sulfate-sulfur (extracted by the 0.01 mol L-1 calcium phosphate solution). The experiment was carried out during the Spring and Summer seasons, with five rates of nitrogen (0; 100; 200; 300 and 400 mg dm-3) and five rates of sulfur (0; 10; 20; 30 and 40 mg dm-3) combined in a modified central composite design, as 52 fractionated factorial, with four replications. The 13 combinations between those nitrogen and sulfur rates, respectively, were: (in mg dm-3): 0, 0; 0, 20; 0, 40; 100, 10; 100, 30; 200, 0; 200, 20; 200, 40; 300, 10; 300, 30; 400, 0; 400, 20 and 400, 40. Plants were homogeneously thinned before only one fertilization with ammonium nitrate and calcium sulfate. The grass was grown for three 30-days growth periods. At each harvest, soil (0-20 cm) was sampled and diagnostic leaves (two recently expanded leaf lamina) were collected. The interaction between nitrogen and sulfur rates was not significant (P>0.05) for the measured soil and plant parameters, whereas both nitrogen and sulfur had significant (P<0.05) effects on those response variables. Total nitrogen in the soil and in the diagnostic leaves were linearly increased by the nitrogen rates. At the highest nitrogen rate, soil total nitrogen was 1.2, 1.4 and 1.2 times higher, at each harvest, than in the no–nitrogen supply. Nitrogen in the diagnostic leaves collected at the first, second and third harvests, respectively, ranged from 8.98 to 25.41, 15.33 to 28.17 and 13.92 to 30.99 g kg-1, between the lowest and highest nitrogen rates, indicating that this grass was nitrogen deficient when the nitrogen containing fertilizer was not applied. Sulfate-sulfur in the soil (calcium phosphate extraction) and total sulfur in diagnostic leaves linearly increased with the supplied sulfur rates. At the first, second and third soil sampling, sulfate-sulfur was, respectively, 2.9, 6.3 and 3.2 times higher with the supply of the highest sulfur rate than with no-sulfur application. When sulfur was not applied, soil sulfate-sulfur was 7.2, 15.1 and 22.0 mg dm-3 at those three sampling times, suggesting great sulfur mineralization during the experiment. Total sulfur in diagnostic leaves changed with sulfur rates from 1.10 to 1.58 and 1.87 to 2.19 at the first and second harvests, and was not changed by sulfur fertilization for the leaves picked at the third harvest. Signal grass diagnostic leaves followed the changes in total nitrogen and sulfate-sulfur that occurred in the soil following ammonium nitrate and calcium sulfate fertilizations for the recovery of this grass in a degrading pasture.