SOIL FERTILITY ASSESSMENT, MAPPING AND FERTILIZER TYPE RECOMMENDATION FOR FARTA, FOGERA, AND GONDAR ZURIA DISTRICTS IN NORTHWESTERN AMHARA REGION, ETHIOPIA

Abstract

Detailed characterization of agricultural landscapes and knowledge on soil spatial variability induced soil fertility differences are crucial for developing site specific, profitable and sustainable nutrient management. In view of this, landscape characterization and soil fertility assessment studies were conducted on the agricultural lands of the Amhara National Regional State, northwestern Ethiopia in the year 2014. Semi-structured questionnaire and personal observations were used to collect the required biophysical data from the study sites. Soils (n=470), and some selected crop plant (n=89) samples were collected from spatially stratified grid points (3 km by 2 km) and were analyzed in the laboratory following standard procedures. The soil analytical results were subdivided into training set (70%) and test set (30%) just before semivariogram models that are appropriate to the soil parameters were selected and fertility mapping by ordinary kriging method was commenced. The results indicated the prevalence of five different agro-ecologies in a comparatively small (3385.17 sq. km) geographic area and related growth of diversified crop types. Slope class differences significantly (p < 0.01) affected the status of selected soil properties in all the six slope gradient classes. Six soil types, identified based on soil color, occupy the majority (72%) of the study area and showed significantly different (p<0.01) analysis results in measured soil properties. Agricultural lands in the study area are mainly characterized by three dominant textural classes (clay loam, clay, and loam), and moderately acidic soil reaction (68%), with marked deficiency of organic carbon status (43%). Soil nutrient status results revealed that potassium, calcium, magnesium, iron and manganese are sufficient and above sufficient; copper and zinc are deficient in 23 and 32% of the soils, respectively; while nitrogen, phosphorus, sulfur, and boron were identified as the most deficient nutrients in all the agricultural lands. Nevertheless, the quantity of fertilizer applied (36.5 kg N ha-1 and 13.2 kg P ha-1) and the nutrients it is composed of (only nitrogen, phosphorus and very recently sulfur) are not comparable to the prevailing nutrient deficiency problems observed in the soils. Excess concentration of magnesium in the exchange sites and the resulting low soil Ca: Mg and K: Mg ratios might create magnesium induced deficiencies of potassium and calcium to the growing plants. Plant (flag leaf) analysis of tested triticale (X -Triticosecale wittmack), rice (Oryza sativa L), and sorghum (Sorghum biocolor L) crops showed that all were sufficient in plant phosphorus concentration irrespective of the actual low to very low phosphorus status in soils. Sorghum crop was deficient in calcium and potassium as opposed to their high to very high actual soil status. Rice crop was deficient in magnesium, iron, copper, and zinc while soils collected from the rice plots showed optimum and higher concentrations of these nutrient elements. The spatial variability analysis results revealed that most soil properties were found to be best fit to the exponential model, wherebythe nugget value ranged from zero for sulfur to 2021 for manganese, and range parameter stretched from the lowest four kilometer for potassium to the highest 49 kilometer for boron. Most soil xv parameters were found to have moderate spatial dependence (25-75%) implying that the implemented agricultural and other related practices affected the spatial structure. Generally, the soil analysis results and their spatial extents as presented by soil fertility maps revealed that agricultural lands in the study area are deficient in organic matter and seven plant nutrients (N, P, K, S, B, Cu, and Zn). Therefore, appropriate soil conservation measures, application of organic fertilizer, application of four blended (NPSB, NPSBZn, NPSBCuZn, and NPSBCu) and one straight (potash) fertilizers, and lime application are recommended. In addition, foliar application of micronutrients (iron, copper, and zinc) for rice cultivation, and continuous monitoring of status of micronutrients both in soils and crops are suggested. Moreover, supporting soil analysis results with related plant analysis data should get the utmost consideration to better understand the nutrient dynamics in the agricultural soils