The research team, led by Olamide Omolafe Ogunremi of the University of Ilorin and North-West University, set out to answer a deceptively simple question: does adding biochar to oil-polluted soil help or hinder a plant’s natural capacity to deal with heavy metals? Spent engine oil is a widespread pollutant in many developing regions, released from garages, workshops and informal dumping sites. Beyond the hydrocarbons themselves, used oil carries a cocktail of toxic metals including copper, zinc, lead, cadmium and chromium, all of which can persist in soil for decades and find their way into food crops and groundwater.

Tithonia diversifolia, a shrubby member of the daisy family native to Mexico but now naturalized in more than 60 countries, was an intriguing candidate for the experiment. In Nigeria it grows abundantly on farms, roadsides and waste grounds, often on soils too poor for crops. Previous studies had flagged the species as a promising phytoextractor, meaning it can pull metals out of the soil and store them in its tissues. Its invasive vigor, rapid biomass production and extensive root system all suggested it could thrive where other plants would simply die. Crucially, because it is not a food crop, any metals it absorbs pose no direct risk to the human food chain.

The team also suspected that the plant’s medicinal chemistry might play a role. Tithonia diversifolia produces distinctive secondary metabolites known as targitinins, and other medicinal plants have been shown to tolerate heavy metals by chelating them, forming stable complexes that neutralize the metal ions. That built-in tolerance, the researchers reasoned, could make the species especially well suited to surviving the harsh chemistry of an oil-spiked soil.

To test the idea, the researchers ran a greenhouse experiment at the University of Ilorin’s Botanical Garden. Sandy loam soil was sieved, homogenized and spiked with spent engine oil at a rate of 50 milliliters per kilogram, roughly 5 percent by volume, then left to equilibrate for two weeks. Three biochars were then mixed in at four application rates: zero, 1, 2 and 3 percent by weight. Two of the biochars were produced locally, one from rice straw and one from sorghum straw, while the third was a standard reference biochar made from wheat straw at 700 degrees Celsius, supplied by the UK Biochar Research Centre. After a month of incubation, Tithonia seeds were sown in 30 pots and the plants were grown for six weeks under a completely randomized design with three replicates.

The growth results were striking. Rice straw biochar in particular acted as a growth booster. Plants receiving 3 percent rice biochar reached the greatest height recorded in the study, 13.36 centimeters, and also showed the thickest stems. Leaf counts and leaf areas were significantly higher in rice biochar treatments than in untreated controls, and plants given rice biochar produced significantly greater shoot and root fresh weights and shoot dry weight. Sorghum biochar at 2 percent produced the longest roots, measuring 9.73 centimeters on average. By contrast, the standard wheat straw biochar tended to suppress height and stem girth as its application rate increased, a reminder that biochars are not interchangeable commodities but chemically distinct materials shaped by what they are made from and how they are burned.

The heart of the study, however, lay in how the biochars changed the plant’s relationship with metals. The researchers quantified this using two standard indices. The bioconcentration factor, or BCF, compares the metal concentration in the plant’s roots with that in the soil; a value above 1 signals a hyperaccumulator. The translocation factor, or TF, compares metal levels in shoots with those in roots; values above 1 indicate the plant moves metals upward into its above-ground parts, which is the hallmark of phytoextraction, while values below 1 indicate the plant traps metals below ground, the signature of phytostabilization.

Those indices revealed a remarkable plasticity. In untreated contaminated soil, Tithonia behaved as a phytostabilizer for copper, keeping it locked in roots, but it readily extracted lead, cadmium and chromium, with translocation factors well above 1 for all three. When sorghum biochar was added, however, the plant’s copper behavior flipped entirely: at every sorghum biochar rate, translocation factors for copper exceeded 1, converting a copper-storing plant into a copper-extracting one. Meanwhile, the standard biochar at 1 and 2 percent achieved the opposite effect for zinc, producing bioconcentration factors of 1.23 and 1.30 alongside translocation factors of essentially zero, meaning zinc was absorbed by the roots but firmly prevented from reaching the shoots. The same standard biochar at 2 percent also curbed the plant’s uptake and transport of cadmium, lead and chromium, damping down what had been vigorous extraction in the untreated soil.

The mechanisms behind these shifts are rooted in the physical chemistry of biochar itself. Earlier characterization work by the same team showed that the rice and sorghum biochars possess high pH, high porosity, abundant fixed carbon and rich surface functional groups. Biochar immobilizes metals through several parallel processes: negatively charged surface sites attract positively charged metal ions electrostatically; oxygen-containing functional groups can reduce chromium to less mobile forms that then bind to the surface; alkaline biochar raises soil pH, causing metals such as lead to precipitate, often reacting with phosphate and carbonate groups in the char; and ion exchange and complexation sequester additional metal ions. The porous structure also creates habitat for soil microbes, whose metabolic activity and organic acid secretions further interact with metals, and the dark color of biochar alters soil thermal dynamics, aiding germination and early growth.

The authors conclude that the phytoextraction and phytostabilization capacities of Tithonia diversifolia depend on the specific features of the biochar applied, which are in turn governed by the feedstock, the pollutant and the soil type. That dependency cuts both ways: biochar can be chosen to suppress metal movement into edible vegetation, or to supercharge a plant’s extraction of a particular contaminant. The team calls for long-term field trials and monitoring strategies across varied biochar types and plant species, and stresses that producing standardized, quality-controlled biochar should be a priority if the approach is to move from greenhouse pots to real-world remediation. If those trials succeed, a weed that farmers spend seasons eradicating could become a deliberately planted partner in restoring some of the world’s most damaged soils.

Subject of Research: Biochar-assisted phytoremediation of heavy metals in spent oil-contaminated soil using Tithonia diversifolia

Article Title: Efficacy of biochar on the phytoremediation potential of Tithonia diversifolia on spent oil-contaminated soil

Article References: Ogunremi, O. O., Amubieya, O. F., Ogunkunle, C. O., & Fatoba, P. O. (2025). Efficacy of biochar on the phytoremediation potential of Tithonia diversifolia on spent oil-contaminated soil. BMC Environmental Science, 2(1), Article 12. https://doi.org/10.1186/s44329-025-00026-6

Image Credits: AI Generated

DOI: 10.1186/s44329-025-00026-6

Keywords: biochar, phytoremediation, Tithonia diversifolia, heavy metals, spent engine oil, soil contamination, phytoextraction, phytostabilization, rice straw biochar, sorghum straw biochar, soil remediation, greenhouse experiment

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