Soil Sulfur Deficiency Indices Assessment for Wheat Production in Ethiopia

Sulfur (S) deficiency is becoming one of the soil health challenges in the Ethiopian crop production systems. However, visual identification of its deficiency, especially in cereals is difficult, because the symptoms are nearly identical with those of nitrogen. Hence, deficiency indicators are necessary for balancing fertilizer use. For this purpose, 18 sulfur response experiments conducted in 2012-14 were considered. Major aim was identifying more suitable indices of S supply and setting their critical thresholds. The treatments were: absolute control (CK); nitrogen (N); nitrogen and sulfur (NS); and nitrogen, phosphorus and sulfur (NPS). The levels of nutrients tested were: S (0 and 20 kg S/ha), P (0 and 20 kg P/ha) and N (0 and 69 kg N/ha) in the form of gypsum, triple-super phosphate (TSP) and urea, respectively. Treatments were arranged in randomized complete block (RCB) design and replicated 3 times. In the study, from the selected indices: N/S-ratio and S concentration in wheat at booting showed better sensitivity as indicators of S deficiency than the organic carbon (OC) in native soils. Critical levels (CLs) were set at 90% relative yield (RY), using the Cate and Nelson model, and estimated to be 16.5:1(N/S-ratio), and 0.16% (S concentration); and 2.07% (for the soil OC). Therefore, sulfur responsive soils/treatments in wheat at booting can be separated from un-responsive ones, in which case much sulfur response is expected for sites/treatments with N/S-ratio >16.5:1; TS <0.16%; and the soil OC <2.07%. This study further affirmed that, plant analysis could be used as a better tool for assessing sulfur deficiency in wheat than soil analysis. Thus, the results could be used as provisional recommendations for wheat growing and as the basis for further sulfur research in Ethiopia. However, differences between the estimated values and those reported in literature have been observed. Therefore, the follow-up research should focus in identifying/standardizing a more reliable index of S deficiency and CLs, through a more reliable research condition.