Abstract:
Knowledge of spatial variability of soil properties assists in decision making towards selection of appropriate management options pertinent to specific contexts. Liming acid soils is one of the available intervention options to amend soil acidity which can increase soil pH and alleviate Al toxicity to plants and thus maintain a suitable condition for growth of a variety of crops. Appropriate rate of lime and P fertilizer are therefore an important strategy on improving crop growth in acid soils. There is however, scarcity of information on interactive effects of lime and P fertilizer application for malt barley and bread wheat performance in the study area. Experiments were conducted under laboratory conditions and in the field to evaluate; spatial variability of selected soil properties along toposequence and depth, compare lime requirement determination methods, and investigate response of malt barley and bread wheat to lime and P fertilizer in Welmera District Central Highlands of Ethiopia. For the spatial variability study, soil samples were collected from cultivated land of two mini watersheds namely at Robgebeya (RG) and Gefersa (Gf) following toposequence and depth. The collected soil samples were analyzed for selected properties following standard procedures. The lime requirement method comparison study was conducted using 24 soil samples collected from cultivated acidic Nitisols of the study area. This study utilized the buffers of modified Mehlich (MMB) and Adams-Evans (MAEB) and titration with 0.022 M Ca(OH)2. The field experiments were conducted in Welmera District at RG and Watabacha Minjaro (WM) study sites and Holeta Agricultural Research Center (HARC). The results of the spatial variability study indicated that the soil was clay in texture at both mini watersheds. The highest bulk density (1.2 g cm-3 ) was recorded in the deepest (70-120 cm) layer at both mini watersheds. The pH varied from 4.63 (very strongly acidic) in 0-20 cm soil depth at middle slope position to 5.76 (moderately medium acidic) at 70-120 cm soil depth and toe slope position of RG mini watershed. Low soil AP (2.74 ppm) and high available P (23.77 ppm) was recorded in 20-40 cm at upper slope position of RG. The highest macro nutrient concentration was recorded in the deepest soil layer (70-120 cm) at upper slope position of RG. The soils of RG had low Ca (6.73 cmolckg-1 soil) in 0-20 cm and the higher Ca (17.6 cmolckg-1 soil), lower Mg (2 cmolckg-1 soil) recorded in 0-20 cm soil depth at upper slope position of Gf, the higher Mg (14 cmolckg-1 soil) recorded in 70-120 cm at upper slope position of RG mini watershed. Content of Fe vary from lowest, 9.2 ppm in 70-120 cm to 589.5 ppm in 0-20 cm soil depth at RG mini watershed. Zinc vary from nil in 0-20 cm soil depth to 2.1 ppm at lower slope position of Gf mini watershed. Cu in the soil vary from 0.12 ppm in 0-20 cm soil depth at toe slope position to 5.17 ppm in 0-20 cm soil depth at upper slope position of RG mini watershed. Comparison of LR study indicated that MAEB, MMB and titration with 0.022 M Ca(OH)2 estimated the lime requirement at the rate of 24.7, 4.0 and 3.12 t ha-1 , xviii respectively. Hence, lime rates estimated using MMB and titration with 0.022 M Ca(OH)2 raised soil pH near to the target value while the MAEB method overestimated LR. The result of the field experiment indicated that liming increased soil pH while it reduced exchangeable acidity as expected. Interaction of lime (t ha-1 ) by P (kg ha-1 ) at the rate of 3.12/16.5 at HARC and WM and 3.12/33 at RG were good results compared to the other higher yield out puts when seen from the higher amount of inputs applied for both crops. From results of spatial variability study, the slope of the landscape has contributed to dislodge most of the important soil nutrients such as Ca, Mg, P and N. Soil pH and exchangeable acidity of the soil improved with increased soil depth. There was variation in soil depth than in slope position in soil properties. Soil of the study area needs more attention to improve the soil acidity and fertility status since top soils are affected by Al and Mn toxicity and deficient in most useful macronutrients this necessitate improvement of soil fertility and acidity. This might be possible through cautious use of materials (lime) that minimizes soil acidity and use of P fertilizer to improve the P uptake of the soil. As observed from comparison the Modified Mehlich Buffer or titration showed the promising results to amend the acidic soils. However, field calibrations are required using crops soil pH requirements before these tested methods are taken as the best to amend the acidic Nitisols to pH 6.0. Furthermore, this type of experiment should be conducted using large number of samples collected from acidic soils across the country and including other LR determination methods. From the result of field experiment to fertilize the soil for better yield and yield components of malt barley and bread wheat it is provisionally recommended that 3.12 t ha-1 lime by 16.5 kg ha-1 P be can be used at Welmera District in central highlands of Ethiopia. It would be good to verify in similar areas and soil types; and investigate the economic feasibility of the use of the respective rates of the inputs (lime and P fertilizer).