On-farm trial assessing combined organic and mineral fertilizer amendments on vegetable yields in central Uganda

Integrated Soil Fertility Management (ISFM) is a soil management approach that emphasizes combined application of organic and mineral fertilizer inputs with the goal of improving yields and fertilizer use efficiency. Combined applications have resulted in a positive interaction between organic inputs and mineral fertilizers on vegetable yields, where yields from combined treatments are greater than yields from sole fertilizer treatments. ISFM studies have been conducted with a diverse range of crops, including grains, legumes, tubers, and bananas, but not vegetable crops. Particularly lacking are ISFM studies conducted under participatory, smallholder farmer management. A researcher-designed, farmer-managed, on-farm study was conducted on highly weathered soils (Ferralsols) in the Lake Victoria Crescent of Uganda to determine the influence of combined organic and mineral fertilizer treatments on yields of a commonly grown indigenous leafy vegetable known as nakati (Solanum aethiopicum). Farmer-managed plots allowed for the effect of farmer participation and management to be analyzed in conjunction with fertilizer treatment effects. A gradient of 100% organic (sole manure) to 0% organic (sole mineral) fertilizer treatments were applied at both an upper (200kgha⁻¹) and lower (100kgha⁻¹) nitrogen (N) rate. N rates were derived from survey results on typical organic application rates used by smallholder farmers in their vegetable plots. Fertilizer treatments resulted in significantly different vegetable yields; however, combined treatments did not necessarily result in higher yields than sole treatments. Differences between organic-mineral ratios were only seen when fertilizers were applied at the higher N rate. The highest yields were obtained when fertilizer was applied at a ratio of 67% organic to 33% mineral fertilizer. Effects of soil properties on yield were also observed; after accounting for the effect of fertilizer treatment, yields significantly increased with increasing soil pH. Farmer participation level had a significant effect on yield. All treatment means were significantly increased by greater participation in the study, and the interactive effects of all treatments became less negative when participation was higher. On-farm studies are needed to demonstrate the applicability of a technology under real world conditions, but trials need to maintain farmers’ interest throughout the study period.