Evaluation of Different Indices of Sulfur Availability in Soils for Wheat (Triticum aestivum L.) Production in Ethiopia –II

Using high-analysis fertilizers lacking adventitious sulfur (S), coupled with traditional cropping-systems that mine S from native soil, leads to S deficiency. However, under field conditions, S deficiency symptoms are not easily identifiable in cereals, because they are mistaken with those of Nitrogen (N). Hence, S availability indicators are necessary for rational fertilizer use. Eighteen explorative field experiments were conducted in 2012/2013 seasons in Central Highlands (HLs) of Ethiopia, with the purpose of evaluating S deficiency indicators in wheat, with the ultimate aim of setting critical-thresholds. In the study organic carbon (OC), and SO4-S in soils; total S and N/S ratio in grains were considered. Two levels of S (0 and 20 kg/ha); 2-levels of P (0 and 20 kg/ha); and 2-levels of N (0 and 69 kg/ha) as gypsum, triple-superphosphate (TSP) and urea, respectively were used. The experimental was laid out in randomized complete design (RCBD) in three replications. In the study, N/S ratio and S content in grain showed better association with S-uptake, with the degree of correlation, -0.83767 and 0.85547, respectively both significant at P<0.001. However, based on the minimum criteria set in literature, the total S in wheat grain showed better sensitivity, whereas N/S ratio was marginal. The critical-thresholds set at 90% Relative Yield (RY), were approximated to about 0.118% for total S, 14.7:1 for N/S ratio in grains; and 11.3 mg/kg for SO4-S in native soil. Therefore, wheat grains from S responsive sites and/or treatments can be distinguished from un-responsive ones, in which case much S response is expected for sites and/or treatments with total S content of <0.118%, N/S ratio >14.7:1 and SO4-S <11.3 mg/kg. In general, the results suggest that plant analysis, (in this case, grain), might be taken as a better tool for assessing S supply of soils or wheat crop than the soil analysis, and therefore, this preliminary result could be used as the basis for S research and as a provisional recommendation for wheat growers in Ethiopia.