Variations in Within-Season Nitrogen and Sulfur Interaction Effects on Forage Grass Response to Combinations of Nitrogen, Sulfur, and Boron Applications

Abstract

Soil and plant analyses were conducted on samples from a field trial with spring nitrogen (N), sulfur (S), and boron (B) fertilizer applications on orchardgrass (Dactylis glomerata L.) in order to determine the reason for the variable yield response within the growing season, examine effects of the fertilizers on forage quality and explore possible methods (plant or soil analyses) to predict nutrient deficient or excess situations. Yield results and plant nutrient analyses showed that the soil was generally N deficient resulting in crop response to N applications throughout the three-cut growing season. Sulfur was seasonally deficient such that there was crop response to S applications in the spring (cut #1) and late in the growing season (cut #3). The S deficiency in the spring was particularly dramatic probably because of low available S after over-winter leaching. There was apparently a sufficient rate of mineralization of soil organic S to satisfy crop requirement during mid-season (cut #2), but at an insufficient rate late in the season (cut #3). The reason for the synergistic N × S interaction in cut #1 and the antagonistic N × S interaction in cut #3 appears to have been affected by a different nutrient requirement by the grass during the two parts of the growing season in relation to nutrient availability in the soil. The protein content of herbage, as shown by total N, was increased by the application of N and S. Herbage quality was further improved in cut #1 by combined N and S applications, which reduced the accumulation of nitrate that occurred in the plants. Boron did not affect yield in this study but did have a small effect on plant N and S concentrations. Extractable soil inorganic N tended to be relatively low in the spring and after each cut, making this measurement of limited usefulness for predictive and evaluation purposes. The different chemical solutions to evaluate measurement of available soil S extracted various proportions of solution, adsorbed and organic sulfate, but all had limited usefulness for predictive and evaluation purposes. Plant sulfate-S varied within the season and hence was generally not useful for measuring deficient or excessive situations.