Long-term addition of compost and NP fertilizer increases crop yield and improves soil quality in experiments on smallholder farms

Soil fertility decline due to low nutrient input is a constraint for sustainable agriculture in smallholder farming systems in Ethiopia. In this study, crop productivity and soil organic matter buildup were compared in soils receiving locally made compost, applied either alone or in combination with NP fertilizer. The experiments had four treatments: full dose of compost (C), full dose of fertilizer (F), half compost and half fertilizer (CF), and unfertilized control (control). The full dose of compost was equivalent to 2.4 t ha(-1) organic matter. The field study was conducted on four farm fields in the village Beseku, each representing different sub-villages. Participating farmers were selected based on their willingness and an assessment of dedication to carry out the experiment. The experiments, a randomized complete block design with three replications, were replicated across four farm fields. The treatments were repeated for six cropping seasons (years), and data on soil nutrient status and crop (maize, wheat, potato, and faba bean) harvests were collected. In the 0-10 cm soil layer, pH was (P < 0.05) lower in the F treatments than in the C and CF treatments. Compared with the F treatment, the soil organic carbon (SOC) and total nitrogen stocks increased (P < 0.05) by 4.60 and 0.42 t ha(-1) in C treatment, and by 4.74 and 0.45 t ha(-1) in CF treatment. Treatment effects on crop harvests were significant (P < 0.05) for all crops grown across the sites and seasons. The highest maize yield was obtained from CF, with relative harvest of 178% compared with the control and 126% compared with F, but was comparable to C. For wheat and potato, the yields obtained from CF, C and F were comparable. For faba bean, CF had a relative harvest of 145% over the control. Maize harvest was in the order of CF > F> C> control in the initial season, CF > C> F> control in the next three consecutive seasons, and C> CF> F> control in the final year of the experiment. The overall combined yield was in the order of CF > C> F> control for maize and faba bean, CF > F> C> control for potato, and F> CF > C> control for wheat. The addition of either compost alone or in combination with NP fertilizer improved soil properties and crop productivity, compared with control and only fertilizer addition. Therefore, compost addition can serve as a complement to fertilizer use and reduce dependence on mineral fertilizer in low-input crop production system. The apparent synergy between compost and fertilizer addition needs further research in order to be explained. (C) 2014 Elsevier B.V. All rights reserved.