EFFECTS OF CROPPING SYSTEMS, NUTRIENT LEVELS AND CROP RESIDUE APPLICATION ON SOIL ORGANIC CARBON CONTENT UNDER MINIMUM TILLAGE EXPERIMENT IN TESO DISTRICT, KENYA

Abstract

Minimum tillage practice and the use of cropping systems that maximize crop residue addition to the soil have been efficient agricultural practices in maintaining or increasing soil organic carbon (SOC) which, though central to the sustainability of soil fertility on smallholder farms in the tropics, has significantly declined to very low levels. An on-farm experiment was carried out at Asinge (0º 36' N; 34º 20' E and 1420 m above sea level) in Teso district to test the effects of cropping systems, nitrogen levels and crop residue (maize stover) addition on the changes in soil organic matter and overall maize-soybean production in western Kenya. A 3 x 2 x 2 factorial experiment arranged in a split-split plot design with three cropping systems (maize-soybean rotation, maize-soybean intercropping and continuous maize) as main plots; nutrient N levels (0 and 60 kg N/ha) as subplots and crop residue management (with and without crop residue) as sub-subplots was initiated during the 2005 long rain season. Main plots of 12 m x 12 m were split into subplots of 5.75 m x 12 m each (separated by 0.5 m paths) to accommodate different fertilizer N combinations and hence possible N response. Each subplot was split into sub-subplots of 5.75 m by 5.75 m with inter sub-sub plot spacing of 0.5 m to test with and without crop residue treatments. N fertilizer (Urea) was applied at 0 and 60 kg N/ha with a blanket application of P fertilizer triple superphosphate (TSP) at 60 kg P/ha and also a blanket application of K fertilizer muriate of potash (MOP) at 60 kg K20 /ha to eliminate possible deficiencies for these two nutrients. Crop residue was applied at 0 and 2 t/ha. Maize and soybean were planted as the test crops. Harvesting of crops was done at maturity to determine yields and nutrient uptakes. Soils and plant tissues were sampled after harvesting the crop each v season for chemical analysis. Statistical analysis was done using Genstat Discovery Edition 3 for all the data obtained to determine treatment effects. Results for the 2005 LR, 2005 SR and 2006 LR indicated significant differences (p<0.05) on soil organic carbon and soil total nitrogen with treatments under crop residue application having higher contents of both soil organic carbon and soil total nitrogen compared to no-residue treatments. In all cropping systems yields were significantly different (p<0.05) as a result of fertilizer addition; higher yields were obtained in treatments receiving 60 kg N/ha compared to treatments receiving 0 kg N/ha. Rotation cropping system outperformed other cropping systems by having higher mean yields of 5.23 t/ha of maize in 2006 LR season with continuous and intercropping systems having maize yields of 3.96 and 2.54 t/ha respectively. Economic analysis showed that treatments receiving fertilizer and crop residue in all cropping systems were profitable. However, all treatments under intercropping (maize + soybean) gave gross margins of above Ksh16, 000 hence an attractive alternative and farmers would get better yields when soybean is integrated into the cropping system. There is need for more minimum tillage research especially on the effects of cropping systems and nutrient inputs on different soil types and climate, crop residue management (especially in crop/ livestock systems) and equipment development to determine which of the practices is suitable in sustaining SOC and crop productivity in the nutrient depleted soils of western Kenya region