Urea lifts African nightshade yield, while poultry manure leads on vitamin C in Cameroon trial

Urea produced the largest harvest of African nightshade in a field experiment in southwestern Cameroon, while poultry manure produced leaves with the highest reported vitamin C, carbohydrate and fat concentrations. The results suggest that fertiliser choices can affect both crop quantity and composition, although important reporting limitations make firm recommendations premature.

The study, posted on bioRxiv on November 3, 2026, is a preprint and has not been certified by peer review. It concerns Solanum scabrum, a leafy vegetable also called huckleberry or country njama njama in Cameroon—not a study of berry fruit.

Four treatments, different outcomes

Researchers conducted the experiment from February to May 2026 in Lysoka village, Buea. They grew the Foumbot variety under four treatments: urea, poultry manure, pig manure and an unfertilised control. A randomised complete block design provided four replicate plots per treatment, with plot averages used for analysis.

The methods describe urea applied at 100 kilograms of product per hectare, equivalent to 46 kilograms of nitrogen, divided between two applications after transplanting. Each manure treatment was described as eight tonnes per hectare on an oven-dry basis, incorporated into the soil two weeks before transplanting.

Across four harvests, the reported cumulative fresh yield was 20.72 tonnes per hectare with urea, 14.9 with poultry manure, 7.14 with pig manure and 6.05 without fertiliser. Urea therefore produced more than three times the control yield. Poultry manure also substantially increased harvest weight, but did not match urea.

Growth measurements followed a similar pattern. At the final growth assessment described in the results, urea-treated plants averaged 25.92 centimetres tall and 29.38 leaves, compared with 19.89 centimetres and 21.92 leaves in the control. The researchers report statistically significant treatment differences using analysis of variance and Tukey comparisons.

Nutrient concentrations tell another story

Urea also gave the highest reported protein concentration: 290.3 milligrams per 100 grams, compared with 260.4 under poultry manure and 185 in the control. Those are the units supplied by the authors.

Poultry manure led on the other three measured nutritional components. Its reported carbohydrate concentration was 6.52%, compared with 5.25% for urea and 4.25% for the control. Fat concentrations were 7.8%, 6.57% and 3.43%, respectively.

The vitamin C contrast was particularly clear in the reported averages. Poultry-manure samples contained 42.57 milligrams per 100 grams, versus 24.20 with urea and 27.53 in the control. Pig manure produced an intermediate value of 36.53 milligrams per 100 grams.

These measurements concern concentrations in sampled material, not demonstrated benefits to people eating the vegetable. Nor does a higher concentration necessarily establish which treatment supplies the most of a nutrient across an entire harvest. Nutritional samples were collected at a single early sampling point, whereas cumulative yield combined later harvests.

What the experiment cannot settle

Random allocation makes this more informative about treatment effects than an observational comparison of farms. Nevertheless, it tests particular fertiliser regimes, not fertiliser source independently of nutrient dose, application timing or composition. The treatments were not matched for nitrogen input, and the manuscript's manure nitrogen totals do not agree with its stated application rates and nitrogen percentages.

Other reporting inconsistencies warrant clarification. Experimental plots are described as nine square metres, but manure quantities are also specified for 16-square-metre plots. Growth timings differ between the abstract and methods. The nutritional methods identify fresh harvested material, but do not clearly explain whether all final concentrations are expressed on a fresh- or dry-weight basis.

The authors propose that readily available nitrogen helped urea drive growth, while slower or more balanced nutrient delivery helped manure influence composition. These are possible explanations, not mechanisms directly demonstrated by measurements of nutrient release or plant metabolism.

With four plots per treatment, one variety and one site-season, the findings remain local and preliminary. The experiment did not test combined manure–urea treatments, long-term soil effects or consumer health outcomes. Its useful message is narrower: yield and individual nutrient concentrations need not respond identically to fertilisation. Testing integrated treatments across seasons, with consistent dose reporting and repeated nutritional sampling, would help establish whether those goals can be reconciled.