Soil Test Based Crop Response Phosphorus Calibration Study for Bread Wheat ( <i>Triticum aestivum</i> L.) Production in Agarfa District Bale Highland, Oromia, Ethiopia

Ethiopia faces a wide set of soil fertility loss issues which are declining of soil fertility due to biological and environmental factors as well as blanket fertilizer application throughout the country without considering soil types and agro-ecological are among the factors that pronounced in low yielding of food crop production like bread wheat. To tackle this problem, site specific nutrients managements and soil test-based crop response fertilizer recommendations was conducted for three consecutive years during the main cropping seasons of (2015 – 2017 E.C). The objective of the experiment was to determine economically optimum Nitrogen, Phosphorus critical (Pc) and Phosphorus requirement factor (Prf) for bread wheat production in Agarfa district. The treatments consisted of factorial combinations of three levels of TSP (0, 100, and 200) kg ha -1 with five levels of nitrogen (0, 23, 46, 69, and 92 kg ha -1 ) that gave a total of fifteen treatments. However, in the second two consecutive years, the experiment was conducted to determine phosphorus critical (Pc) and phosphorus requirement factor (Pf), and the treatments consisted of six levels of phosphorus (0, 10, 20, 30, 40, and 50) kg ha -1 combined with a single level of nitrogen (69 kg ha -1 ). Composite soil sample before plating and intensive soil samples after 21 days of sowing were taken from each plot and analyzed for selected physicochemical properties following standard laboratory procedures. Phosphorus critical level (Pc) determination was done using Cate-Nelson diagram method. Agronomic data such as above ground biomass and grain yield was collected then subjected to two-way factorial analysis of variance (ANOVA) using R software while the partial budget analysis was done using CIMMYT procedure. The results indicate that combined NP fertilizer rates significantly influenced the agronomic parameters of bread wheat. Optimum nitrogen rate (69 N kg/ha), P critical concentration (28.0 ppm) and P requirement factor (4.15) for bread wheat production were determined at Agarfa District. Therefore, uses of 69 N kg/ha fertilizer for bread wheat production at Agarfa District and areas having similar soil conditions and agro-ecology is advisable. Farther verification of the result on farm land could be a pre request before disseminating the technology to the user.