Nitrogen-Sulfur Relations in Soft White Winter Wheat. II. Initial and Residual Effects of Sulfur Application on Nutrient Concentration, Uptake, and N/S Ratio

Although sulfur is routinely applied over much of the limited-rainfall area of the northwestern United States, present diagnostic criteria for determining the need and frequency of S fertilization of white winter wheat [Triticum aestivum ssp. vulgare (Vill., Host) and T. aestivum ssp. compactum (Host)] are not highly successful. Our objective was to evaluate the initial and residual availability of applied S by investigating a) S uptake and distribution in wheat and b) the success of S concentration and N/S ratio as diagnostic criteria. Sulfur uptake, S concentration, and N/S ratios were compared with yield to measure S response over a period of 7 years after S application to a typical S-deficient soil. Four rates of S (0, 17, 34, and 68 kg/ha) were applied as gypsum in factorial combination with four rates of N (0, 45, 90, and 180 kg/ha) as ammonium nitrate. Residual S effects were then evaluated in three subsequent wheat crops grown in a wheat-pea (Pisum sativum L.) rotation with optimum N fertilization. Sulfur uptake and concentration in the first wheat crop was proportional to the rate of S applied, but accumulated primarily in vegetative tissue when present in excess of the amount required for grain protein. Uptake from residual S was much lower than uptake from the fertilizer application and S levels in grain were maintained at the expense of S in straw. Excellent residual availability of S (i.e., two wheat crops for a 17 kg/ha application) was attributed to the low amount of S needed for the additional grain yield (< 1 kg S/ha), recycling of substantial quantities of S in straw residues, and negligible leaching of S. The initial N fertilization significantly increased S uptake from the S source by increasing both S concentration and dry matter. Nitrogen applied without S increased dry matter but not S concentration. In contrast, S fertilization had little effect on N concentration in grain and straw and influenced N uptake only through its effect on dry matter production. Grain yield responses to S were poorly correlated with S concentration or N/S ratio in tissue because of the inconsistency of S accumulation in the plant-parts and the dominating effect of N on yield. Decreased grain yield in wheat responding to S was associated with < 0.12% S in grain, <0.08% S in mature plants, and with N/S ratios > 16 in mature plants.