Low nutrient input and low soil fertility are limiting agricultural productivity in Ethiopia. The main objectives were therefore to evaluate the effects of combined compost and mineral fertilizer (MF) application on soil properties, yield, agronomic nitrogen use efficiency, and adoption of compost application in maize (Zey mays L.)—potato (Solanum tuberosum L.) cropping systems. Yield data were collected from 20 smallholders applying (i) compost and MF at a total rate of 110 kg N ha⁻¹, with 6 Mg compost ha⁻¹ + MF (6CF), 12 Mg compost ha⁻¹ + MF (12CF), and 16 Mg compost ha⁻¹ + MF (16CF; compost on a fresh weight basis), (ii) MF application of 108 kg N ha⁻¹ (F), and (iii) zero fertilization. Soil from 0–20 and 20–40 cm depths was collected from 16 farms using compost and MF. Compost + MF treatments showed significantly lower soil bulk density and iron contents, while pH, electrical conductivity, and cation exchange capacity were higher compared to F treatments. The 6CF, 12CF, and 16CF showed 22, 43, and 54% higher maize grain yield and 8, 16, and 18% higher potato tuber yield compared to F, respectively. The scarcity of organic material was a major socioeconomic constraint for smallholders for producing and applying compost.

Composting organic waste and human excreta could significantly reduce the amount of waste dumped and increase soil fertility and agricultural yields. However, studies focusing on the replacement of mineral fertilizer with compost from these resources are rare. The presented study quantifies the potential of human excreta and other organic waste for compost production. During wet and dry seasons, the generation and composition of household solid waste (HSW) was measured from three wealth categories: poor, medium, and rich, as well as the organic waste generated from 20 commercial facilities. Furthermore, the amount of human excreta, when converting unimproved into ecological sanitation facilities, was assessed. The HSW generation was significantly higher in the wet (0.77 ± 0.07 kg fresh weight (FW) cap⁻¹ day⁻¹) compared to the dry season (0.54 ± 0.04 kg FW cap⁻¹ day⁻¹). Organic waste was the main component of HSW in the dry and wet seasons, accounting for 84% and 76% of the total HSW, respectively. Annually, about 6824 Mg of organic dry matter could be collected from households, 212 Mg from commercial units, and 12,472 Mg from ecological sanitation. With these resources, 11,732 Mg of compost could be produced annually and used for fertilizing 470 ha of farmland, completely replacing mineral fertilizer.