Bio-slurry and mineral fertilizer raise pepper yields in a southern Ethiopian field trial

Hot pepper is an important cash and food crop in Ethiopia, where inadequate nutrient management can constrain production. A bioRxiv preprint by Abrham Shumbulo and Yitbarek Abrham Biramo examines one possible response: combining liquid residue from biogas digesters with a blended fertilizer supplying nitrogen, phosphorus and sulfur, known as NPS. The study has not been certified by peer review.

Testing two nutrient sources together

The experiment took place in Kucha District, Gamo Zone, during the 2024/25 cropping season. Researchers grew the Mareko Fana pepper variety on clay-loam soil with a pH of 6.2. Initial soil tests reported low organic matter and low capacity to retain nutrient cations, providing a local context for testing an organic amendment alongside mineral fertilizer.

The design crossed four bio-slurry rates—0, 20, 40 and 60 cubic metres per hectare—with four NPS rates—0, 75, 150 and 225 kilograms per hectare. Each of the 16 combinations appeared in three randomized blocks, giving 48 plots. The slurry came from digesters primarily supplied with cow dung and locally available grasses.

An important qualification concerns the comparison treatment. The methods describe a uniform application of 100 kilograms of urea per hectare, split between transplanting and early vegetative growth. Thus, the treatment called the control had no added NPS or bio-slurry, but was not a completely fertilizer-free treatment. Plants also received supplementary irrigation and pest and disease management.

Researchers measured plant development, growth, fruit dimensions and dry fruit yields. Marketable fruits had to meet appearance and condition criteria, including freedom from damage and disease. Yields were measured from the central plot area and converted to a per-hectare basis. The team used analysis of variance and least significant difference tests to compare treatments.

A strong yield response, but not a unique winner

The combination of 150 kilograms of NPS and 60 cubic metres of slurry per hectare produced the highest reported marketable dry fruit yield: 2.94 tonnes per hectare, compared with 1.28 tonnes for the no-NPS, no-slurry comparison. That is approximately a 130% increase, or about 2.3 times the comparison yield.

However, 150 kilograms of NPS with 40 cubic metres of slurry produced 2.85 tonnes per hectare of marketable dry fruit. The authors report that these two treatments were not statistically distinguishable for this outcome. The highest numerical yield therefore does not demonstrate that the additional slurry was necessary to achieve a comparable marketable harvest.

The 150-kilogram NPS and 60-cubic-metre slurry treatment also produced roughly 34 fruits per plant, compared with about 13 in the comparison treatment, and larger fruit dimensions. Increasing both inputs to their maximum tested rates did not maximize yield: the authors report a decline at 225 kilograms of NPS combined with 60 cubic metres of slurry.

Higher nutrient inputs also delayed crop development. The paper reports later flowering and maturity at higher rates, showing that greater growth and earlier harvest were not interchangeable benefits.

What the experiment cannot establish

The authors suggest that improved nutrient supply, soil conditions and microbial activity explain the responses. These are proposed mechanisms, not processes directly demonstrated by the reported measurements. Initial soil and slurry analyses do not establish long-term improvements in soil health.

Likewise, “quality” needs a narrow interpretation. The study assessed fruit dimensions, marketability and soluble solids measured with a refractometer. It did not report measurements of capsaicinoids, vitamin content or taste, so the results do not establish hotter, more nutritious or better-tasting peppers.

Some numerical reporting also requires caution. The manuscript reports a highest total dry yield of 3.16 tonnes per hectare, but its marketable, unmarketable and total yield figures do not consistently reconcile, and the unmarketable-yield discussion uses inconsistent units. This article is based on the complete PDF text extraction supplied; figures and table layouts may be degraded, limiting independent checking of plotted values.

Finally, this was one site, one season and one pepper variety. No economic analysis established whether transporting and applying the slurry paid off, and long-term soil effects were not assessed. The findings support further testing of combined nutrient sources under local conditions, rather than a blanket recommendation for the highest-yielding treatment.