CHARACTERIZING AND MAPPING OF SOIL FERTILITY STATUS OF SIBU SIRE DISTRICT, EAST WOLLEGA ZONE OF OROMIA, ETHIOPIA

Abstract

Soil fertility characterization and mapping are the main source of information to farmers on the plant nutrient status of their soil and provides basic data for decision-making on issues such as fertilization and liming. The study was conducted to investigate the physical features and farming systems of the agricultural lands, map fertility status and recommend appropriate fertilizer(s) for the soils and lime for acidic soils of Sibu Sire District, East Wollega Zone of Oromia, Ethiopia. Field survey was conducted in 2015 and 149 soil samples were collected at a depth of 0-20cm for annual and 0-50cm for perennial crops. Wet-chemistry and MIR spectroscopy method were carried out to determine the physical and chemical characteristics of the soils. ArcGIS 10 was used to map the fertility status of the soils. The mean values of clay, silt and sand contents of the soil in the district were 49.50, 22.44 and 28.08%, respectively. Statistical values of qualitative and quantitative analysis of MIR spectra revealed that the partial least square regression (PLSR) model were calibrated and validated for each variable using reference soil data. The model performed better in predicting cation exchange capacity (CEC) (R2=0.88 and RMSE=8.98), organic carbon (OC) (R2=0.88 and RMSE=0.55) and total nitrogen (TN) (R2=0.91 and RMSE=0.04). The pH values ranged from 4.21 to 6.57 and 26.95% of the soils falls under strongly acidic soils. Average predicted OC and TN contents were 3.07 and 0.30% respectively. Organic carbon content of the soil was negatively and significantly correlated with soil pH (r=-0.30**). The mean values of CEC and percent base saturation were 22.47 cmolc kg-1 and 37.32, respectively. The mean values of exchangeable K, Ca, Mg, and Na in the district were 0.96, 5.22, 1.70 and 0.11 cmolc kg-1 respectively. Nearly, 99 and 97% of available P and SO4 2--S of the samples were below critical level. The relative abundance of the micronutrients was in the order of Fe>Mn>Zn>Cu>B. Overall analysis of lime requirement of soils of the district predictive class map showed that 70.4, 15.3, and 7.2% areas of the agricultural land in the district require 0-1, 1-2, and 2-4.7 t ha-1 of lime, respectively. To address nutrient deficiencies in the district four fertilizers blend type: blend-1 (N, P, S, B and Zn), blend-2 (N, P, B, and Zn), compound fertilizer (N, P, S and B), and blend-1 with potash (N, P, S, B, Zn and potash) were recommended. Soil test cannot go beyond the identification of nutrient levels, recommending the relevant fertilizer(s), the nutrient supplying ability of soils and demand level of specific crops need further calibration works to come up with site-soil-crop specific fertilizer recommendation. In addition, plant tissue analysis to strengthen the findings about the status of nutrients and the need for fertilizer application containing macronutrients should be done. Similarly the critical levels adopted from international literature be verified and refined to fit Ethiopian condition and specifically the study area.