Determination of Soil Fertility Index Parameters and Speciation of Potassium and Molybdenum in Soils from Selected Farms in Kakamega County, Kenya

Abstract

Kakamega-County is a maize growing region with a high potential of being one of Kenya‟s maize baskets even though most farmers practice subsistence maize farming. However, available data show that yields have been dropping with each passing year in spite of continued application of nitrogenous fertilizers such as calcium ammonium nitrate (C.A.N) and diammonium phosphate (D.A.P). To get some insight into the possible causes of decline in maize yields in the region, it was important to conduct a systematic study to determine the fertility index parameters of the soils in the farms and speciation of some of the important elements required for plant growth and utilization of applied nitrogenous fertilizers by plants. The objective of this study was to determine the soil fertility index parameters (i.e. organic carbon, soil pH, phosphorus, total and available nitrogen) as well as the speciation of potassium and molybdenum in soils from five selected farms in Kakamega County. The study was conducted in five different maize growing farms selected randomly from five sub-counties. On each farm, soil samples were scooped from five different sampling locations in a zigzag pattern using an auger at a depth of 0 to 20 cm and mixed thoroughly to form the composite sample. Accurately weighed portions of each of the composite samples (usually ½ kg) were packed in clean polythene bags, labeled and transported to Kenyatta University for laboratory analysis. Modified Community Bureau for Reference (BCR) sequential extraction was used to obtain extractions of potassium and molybdenum in various soil fractions. The concentrations of the metals in the fractions were determined using AAS. All measurements were done in triplicate and results presented as means with standard deviations. The results revealed that the soils had a low pH ranging between 4.72±0.01 and 5.53±0.01. Soil Organic Matter ranged between 2.21±0.01 and 3.10±0.00% (critical level is 3.44%), nitrogen ranged between 0.12±0.00 and 0.17±0.01percentage (critical level is 0.2%), exchangeable potassium ranged between 0.50±0.01 and 1.50±0.01 mg/kg (critical level is 6.0 to 8.0mg/kg). Available potassium ranged between 0.30±0.01 and 1.10±0.01 mg/kg (critical level is 2.4 mg/kg). Molybdenum was below detectable levels in the bulk soil of four out of five farms. BCR results still showed molybdenum below detectable levels in all the four soil fractions of farms I, II, III and IV, (critical level is 0.043 mg/kg). The level of potassium was found to be between 5.09±0.88 and 6.85±2.00 mg/kg in the residual form. Four of the farms were found to have a low pH value of below 5.0 (critical level is 5.5). Potassium and molybdenum were not in bioavailable forms. Other nutrients found to be below critical levels were zinc, phosphorus and magnesium. It is recommended that farmers in the county lime the soils appropriately to correct pH and use fertilizers rich in micronutrients such as zinc and magnesium. They are also advised to practice organic farming, use potash as well as apply dolomite lime.