Estimation of Optimum Rate of Sulfur for Application in Soils for Wheat Production in Ethiopia –III

Six on-farm experiments were conducted in 2013/2014 seasons, in the Central Highlands (CHLs) of Ethiopia, with the main aim of estimating optimum sulfur rate for wheat. The treatments were, 4-levels of S(S0 =0, S1 =5, S2 =10, and S3 =20 kg S/ha); 2-levels of N(N0 =0, N =N1 =69 kg N/ha); and 2-levels of P(P0 =0, P =P1 =20 kg P/ha), supplied by gypsum, urea and triple super phosphate(TSP), respectively. The experimental design was RCBD, and the treatments were replicated three times. The grain and total above ground biomass (TAGB) yields, and number of tillers per plant (NTsPP) showed significant response (P<0.001) with applying S and NP. Four of the study sites: G/Silingo, Keteba, N/Suba and Bekejo showed significant response to applied fertilizers, especially S at all levels, whereas W/Gora and B/Tokofa showed marginal response, which suggested that, responses to S were varied over sites/soils. Considering the critical, soil SO4-S, estimated from the first set of (18 S response) experiments, 11.30 mg/kg, the sites like Keteba, Bekejo and N/Suba could be rated as very low; G/Silingo medium/marginal; whereas W/Gora and B/Tokofa could be rated adequate for SO4-S(sulfur). Based on this, therefore, site/soil specific S recommendations were made. In this respect, in Keteba, Bekejo and N/Suba sites with very low initial soil SO4-S values, the optimum S rate can be >20 kg/ha. But, for the moment, it is advisable to apply 20 kg S/ha with the recommended doses of NP. At G/Silingo site, whose soils tested marginal for the SO4-S, applying S at a rate of 20 kg/ha or even slightly <20 kg/ka is advisable. Whereas, at W/Gora and B/Tokofa, since the initial soils tested adequate for SO4-S, but wheat responded to S at lower levels, applying S at a rate of 5-10 kg/ha is reasonable. In general, since in all the studied sites, the maximum attainable yield of wheat grain reported by different workers was not achieved, it is important to make further investigations to identify other limiting nutrients/factors.