Grain Yield Response of Bread Wheat to Sulfur and Phosphorus as Sole-nutrients and Interacting with Nitrogen in Four Soil Types in Ethiopia

Abstract

Despite sulfur being widely deficient in arable-soils, its integration into Ethiopian crop-production systems is relatively recent. This study involved six field experiments designed to evaluate the response of bread-wheat to sulfur and phosphorus (S&P) applied individually and in combination with nitrogen (N). Eight-treatments involving S, P, and N were arranged in a RCBD with three-replications, using a nutrient-omission approach. Results indicated no significant grain-yield (GY) response to the application of S or P as sole-nutrients, nor to their combinations. In contrast, the response to sole-N was highly-significant (P≤ 0.001), with GY increases ranging between 160.8–279.7%, explaining about 75% of the observed yield-variation. Wheat showed a limited response to P, even in combination with other nutrients. However, a highly significant positive interactions were observed for N×S, N×P, and N×S×P, with decreasing levels of statistical-significance. Notably, P appeared to have a suppressive effect when included in nutrient-mixes, as the GY gains from N×P were commensurate to those from N×S×P. Although phosphorus is generally recognized as critical for crop-productivity, wheat responded positively to applied P in only one-third of the experimental sites. This limited response is likely be due to the high P-fixing capacity of the soils studied, viz. Andosols, Nitisols, and Vertisols. Consequently, P inaccessibility appears to be a key factor limiting yield (with a maximum observed yield of only 6.1 t/ha), which is significantly below the wheat crop’s potential. Soil pH, rather than soil-type, emerged as the dominant factor influencing P availability and retention. In contrast, the wheat response to applied S was mainly determined by the adequacy of available or applied N.  These findings underscore the complex interplay among S, P, and N in wheat production systems. However, these, and the potential role of S impurities in triple-superphosphate fertilizers, particularly in relation to wheat quality and other yield-components, warrants further investigation.