The research, led by Adebayo Jonathan Adeyemo of the Federal University of Technology, Akure, together with collaborators from institutions in Nigeria, Brazil and Belgium, was designed to test whether locally sourced soil amendments could shift the fundamental chemistry of a tropical Alfisol quickly enough to matter for the growing season that follows. The stakes are considerable. Soil acidity and weak cation-holding capacity reduce nutrient-use efficiency in highly weathered tropical soils, undermining progress toward the United Nations Sustainable Development Goals on zero hunger, responsible consumption and terrestrial ecosystem health. When a soil cannot hold onto the nutrients it receives, every kilogram of fertilizer applied is a partial loss to leaching and fixation.

The experimental design was deliberately practical. The team laid out a 3 × 3 split-plot field trial with three replications, a structure that allows the independent and combined effects of two classes of amendment to be separated statistically. Main plots received one of three fertilizer designations: no mineral input, locally sourced potassium-bearing rock dust, or plots designated for later application of conventional NPK 15-15-15 fertilizer. Within each main plot, subplots received one of three organic treatments: no organic amendment, field-stabilized poultry manure, or fresh Tithonia diversifolia biomass, a plant widely known among West African farmers as a green manure because of its unusually high nutrient content. Baseline soil samples were taken before anything was applied, and okra was sown at the time of amendment incorporation.

Crucially, the researchers took their soil measurements at two weeks after amendment, before any NPK fertilizer had been applied to the plots designated for it. That sequencing decision matters for interpretation: the NPK-designated plots are reported as NPK-pre and cannot be read as evidence of NPK effects. This careful phrasing reflects a discipline that field trials often lack, and it protects the study’s central claims from being overstated. Everything reported in the paper describes the short-term chemical response of the soil to rock dust and organic materials alone.

The headline result concerns potassium, the nutrient whose shortage most clearly defines these soils. At two weeks after amendment, plots treated with potassium-bearing rock dust recorded an average exchangeable potassium concentration of 1.67 centimoles of charge per kilogram of soil, compared with just 0.49 in untreated control plots. That is more than a threefold increase in the pool of potassium held on soil exchange sites, the fraction most immediately available for root uptake. The operational cation-exchange capacity, a measure of the soil’s total ability to hold positively charged nutrients, rose to 8.54 cmolc per kilogram under rock dust, against 4.63 in the controls, nearly doubling the soil’s buffering and storage capacity.

The organic amendments produced their own distinctive signature, particularly on acidity. Poultry manure reduced the baseline-referenced hydrogen ion ratio from 1.07 under no organic amendment to 0.45, a substantial tempering of soil acidity, while lifting the operational cation-exchange capacity to 7.48 cmolc per kilogram. This dual action is chemically coherent: manure supplies basic cations and decomposable organic matter whose charged breakdown products add exchange sites, simultaneously neutralizing acid cations and increasing the soil’s capacity to retain the nutrients that replace them. Fresh Tithonia biomass, though valued as a green manure, behaved differently in combination with rock dust, a nuance that emerges most clearly in the interaction analysis.

Statistically, the most striking finding was the significant interaction between fertilizer designation and organic amendment for exchangeable potassium, with a P value of 0.027. Within rock-dust plots, poultry manure produced the highest exchangeable potassium of any combination, 1.90 cmolc per kilogram, exceeding both rock dust alone at 1.54 and rock dust combined with Tithonia at 1.57. In other words, the best result came not from any single input but from the synergy between mineral and organic sources. The authors suggest a plausible mechanism: the organic matter may enhance the dissolution of potassium-bearing minerals in the dust while adding its own organic-associated potassium pools, and its decomposing residues help the soil retain the released potassium against leaching.

Not every measured property responded to the combined treatments. The interactions were not statistically significant for operational cation-exchange capacity, which showed a P value of 0.145, or for effective cation-exchange capacity, at 0.085. Exchangeable acidity, by contrast, responded in an interactive fashion with high significance, at P below 0.001, indicating that the amendment combinations modified the acid cation fractions of the soil in ways that depended on which mineral and organic inputs were paired. Electrical conductivity, a proxy indicator for soluble salt accumulation, remained between 0.03 and 0.04 decisiemens per meter throughout, an extremely low range that rules out any short-term salinity build-up from the amendments, a concern sometimes raised with repeated manure application.

The authors are careful about what their findings do not establish. The study captures a fourteen-day chemical snapshot; it does not demonstrate long-term potassium retention, crop uptake of the released nutrients, or yield benefits, all of which would require seasons of monitoring. Dissolution of rock dust in tropical soils is generally a slow process, and whether the early gains in exchangeable potassium persist, deepen, or wash away depends on rainfall patterns, continued mineral weathering, and crop removal over time. The team frames its contribution accordingly: as evidence of short-term modification of acidity and exchangeable-cation status without evidence of salinity problems, a foundation on which longer-term agronomic studies can now build.

Even with those caveats, the implications for smallholder agriculture in the region are hard to ignore. Nigeria imports the bulk of its potash fertilizer at considerable foreign-exchange cost, while quarries and mining operations in the country generate potassium-bearing rock waste that is largely unused. If further research confirms that such rock dust, combined with manure that farmers already produce, can durably rebuild the cation-exchange machinery of degraded Alfisols, the result would be a circular soil-fertility strategy built entirely from local materials. For the moment, the study stands as a carefully documented proof of concept: in a weathered Nigerian Alfisol, ground potassium rock and poultry manure measurably and significantly enriched the soil’s potassium and cation-holding capacity within two weeks, pointing the way toward more self-reliant and sustainable soil management across the humid tropics.

Subject of Research: Short-term effects of potassium-bearing rock dust and organic amendments on soil acidity and cation retention in a tropical Alfisol

Article Title: Soil acidity, cation balance, exchangeable potassium status and cation retention capacity under K-bearing rock dust–organic amendment systems in a tropical Alfisol of southwestern Nigeria

Article References: Adeyemo, A. J., Akinnagbe, A. E., Adeoye, M. O., Adesida, P. A., Agele, S. O., Adejoro, S. A., Oluwagbemi, I. A., Ewulo, B. S., de Freitas, D. A. F., Shittu, O. S., Awodun, M. A., Oyun, M. B., & Oliveira, D. M. D. S. (2026). Soil acidity, cation balance, exchangeable potassium status and cation retention capacity under K-bearing rock dust–organic amendment systems in a tropical Alfisol of southwestern Nigeria. Discover Sustainability. https://doi.org/10.1007/s43621-026-04688-3

Image Credits: AI Generated

DOI: 10.1007/s43621-026-04688-3

Keywords: rock dust, poultry manure, Tithonia diversifolia, soil acidity, exchangeable potassium, cation-exchange capacity, tropical Alfisol, soil fertility, Nigeria, sustainable soil management, nutrient-use efficiency, Soil