Wheat Response to Applied Nitrogen, Sulfur, and Phosphorous in three Representative Areas of the Central Highlands of Ethiopia -I

Eighteen explorative field experiments were conducted in 2012/2013 seasons in three representative areas in central highlands of Ethiopia, to evaluate the effects of sulfur (S) vis-à-vis nitrogen (N) and phosphorous (P) on wheat. Furthermore, an assessment was made on the status of sulfur in soils/plants. Two levels of S(0 and 20 kg/ha); 2 levels of P(0 and 20 kg/ha); and 2 levels N(0 and 69 kg/ha) were used as gypsum, triple-superphosphate (TSP) and urea respectively. Randomized complete block design (RCBD) was used as an experimental design in three replications. Statistical analysis using SAS showed significant response (P<0.001) in grain and other yield components of wheat. All studied fields showed full-response (100%) to applied N, as directly related to soil-test values. Likewise, 72.3% of fields/sites showed response to S. Similar as N, all fields tested low in available P, but 78% showed response to applied P. Good relationship between soil-test values and crop-response for N and S was observed. But, for P in some fields inconsistencies exist. TSP reported to contain 2-6% by weight of S, however, pair-wise comparisons didn't reveal yield response beyond 20kg S/ha. In the study, it is learnt that light textured and calcareous vertisols in the peripheries of rift-valley were found to be more deficient in S. There are strong indications that, S response/deficiency which is now observed in central highlands can be widespread, if such assessments were made across the country, especially far into out fields. Therefore, it is important to include S, in the balanced fertilizer formula. Indeed, for the smallholding farmers it is imperative to sustainably build soil organic carbon (SOC) stocks close to critical levels. It is also important to augment inorganic and organic fertilizers with local S sources (e.g., gypsum) to take advantage of integrative benefits and/or to economize fertilizer use.