Compost and fertilizer - alternatives or complementary? : management feasibility and long-term effects on soil fertility in an Ethiopian village

Abstract: Decline in soil fertility due to nutrient depletion is a concern for low-input crop production in the highlands of Ethiopia. Fertilizer addition is insufficient due to infrastructural and socioeconomic constraints. Effects of compost addition, alone or in combination with NP fertilizer, on crop productivity and soil fertility were studied in long-term on-farm experiments in Beseku, Ethiopia. The combined treatment resulted in an added benefit (synergy), i.e., a higher yield than when compost or fertilizer was added alone. The highest yield increase was found for maize where the combined treatment had 78% and 26% higher yields compared to the control and fertilizer treatment, respectively. Plant available concentrations of B, P, S, and Zn increased in the compost and/or the combined treatment compared to the control. Soil organic carbon and total nitrogen stocks increased in the combined treatment compared with the fertilizer treatment. Substrate-induced respiration from the combined treatment was lower compared to the compost treatment, but catabolic versatility was higher in the combined treatment compared with the compost and the control. This suggests that a combination of compost and fertilizer induces a wider microbial catabolic capability which might lead to higher nutrient mobilization. The apparent yield synergy in the combined treatment likely attributed to; (1) alleviation of micro- and macronutrient limitations allowing for a more efficient use of fertilizer N and P and/or (2) improvement of the soil microbial catabolic capability. However, the indirect effects of compost on soil physical properties leading to improved nutrient use efficiency are also a possible explanation. The plot level N balance was strongly negative for the fertilizer treatment and the control, whereas it was close to steady-state in the combined and compost treatments. All treatments except the control had positive P balances. Therefore, the addition of compost, alone or in combination with fertilizer, improves the nutrient status of the soil and serves as a complement to fertilizer use reducing the dependence on mineral fertilizer in low-input crop production systems. The major factor limiting the adoption of compost by farmers was lack of knowledge. Practical and theoretical training had a positive effect on adoption.