Carbon, Clay and Iron Oxides Hold the Keys to Phosphorus in Nigeria’s Farm Soils

Phosphorus is the quiet bottleneck of tropical agriculture. Without it, crops cannot build DNA, cell membranes, or the energy-carrying molecules that power photosynthesis, yet across much of sub-Saharan Africa the element remains stubbornly locked away in forms plant roots cannot reach. A new national-scale study of Nigerian farmland, published in the journal Plant and Soil, has now mapped out, with unusual statistical rigor, exactly which soil properties decide whether phosphorus stays available to plants or vanishes into the mineral matrix. Drawing on 554 soil samples collected from every corner of the country, the research team led by Samuel Ayodele Mesele of the International Institute of Tropical Agriculture found that soil organic carbon is the single strongest predictor of plant-available phosphorus, but that its effect is constantly modified by soil texture, iron and aluminium oxides, and acidity. The findings offer the clearest picture yet of why fertiliser recommendations that work in one Nigerian field can fail spectacularly in another.

The scale of the study is what sets it apart. Nigeria spans roughly 923,770 square kilometres and seven distinct agroecological zones, from the Humid Forest in the south, where annual rainfall exceeds 2,000 millimetres, to the Arid/Sahel in the north, where precipitation drops below 500 millimetres. Between 2022 and 2023, field teams following the harmonised Soils4Africa monitoring framework collected samples from 390 primary sampling units, each a 2 by 2 kilometre grid cell, using a hierarchical probabilistic design. Within each unit, subsamples were taken at the centre and at three equidistant points along a two-metre circle in a Y-shaped configuration, capturing micro-scale variability before compositing. Samples came from two depths, 0 to 20 centimetres and 20 to 50 centimetres, and were analysed at the Agricultural Research Council Soil Testing Facility in Pretoria, South Africa, following international GLOSOLAN guidelines. Plant-available phosphorus was measured with the Mehlich-3 extraction, a standard test for tropical and subtropical soils.

To untangle the web of interacting soil properties, the researchers deployed two complementary statistical approaches. First, a Random Forest regression model ranked candidate predictors of available phosphorus, including soil organic carbon, pH, exchangeable acidity, sand, silt, clay, and oxalate-extractable iron and aluminium. The model achieved moderate predictive performance, with an out-of-bag R-squared of 0.32 and a root mean square error of 6.68 milligrams per kilogram. Second, structural equation modelling tested hypothesised direct and indirect pathways among the variables, using standardised composite indices for fine texture, oxide content, and acidity. The structural equation model explained 29 percent of the variation in available phosphorus and, crucially, allowed the team to separate direct effects from indirect ones, such as the way texture influences phosphorus by shaping organic carbon accumulation and oxide content.

The headline result is unambiguous: soil organic carbon was the most important predictor of available phosphorus in both analyses. Organic matter feeds phosphorus into the soil solution through mineralisation and microbial turnover, and decomposition products known as organic ligands compete with phosphate for binding sites on reactive mineral surfaces, effectively prising the nutrient loose. But the relationship is not linear. Partial dependence plots from the Random Forest model showed that predicted available phosphorus rose steadily with soil organic carbon and then levelled off at higher values. This plateau suggests that once organic matter is abundant, other constraints, such as sorption capacity, mineral surface reactivity, or microbial demand for the nutrient, become limiting, so each additional increment of carbon delivers diminishing returns for phosphorus availability.

Texture told a subtler and, at first glance, paradoxical story. Finer-textured soils, rich in clay and silt, were positively associated with both soil organic carbon and oxide contents, reflecting the well-documented capacity of fine particles to stabilise organic matter through aggregation and mineral association. Yet the total association between fine texture and available phosphorus was negative. The explanation lies in surface chemistry: clay-rich soils and their associated iron and aluminium oxides present vast reactive surfaces that adsorb phosphate strongly, holding it out of the soil solution where roots can access it. In other words, fine-textured soils may store larger total pools of phosphorus, but a smaller fraction of that pool remains immediately available to crops. The oxalate-extractable iron and aluminium, which represent poorly crystalline oxide phases, were positively associated with Mehlich-3 extractable phosphorus, indicating that these oxides act both as phosphorus-retaining surfaces and as reservoirs of extractable phosphorus that the Mehlich-3 reagent can partially recover.

Acidity added a further layer of control. The composite acidity index, which combined exchangeable acidity and pH, showed a negative direct association with available phosphorus, consistent with classical soil chemistry. In acidic conditions, aluminium and iron become more reactive, phosphate sorption intensifies, and insoluble phosphorus compounds precipitate out of solution. However, the study found that acidity was a secondary rather than dominant source of phosphorus variability across Nigeria. Most of the sampled soils were only moderately acid, and exchangeable acidity becomes chemically significant mainly where pH is low enough for exchangeable aluminium and hydrogen to matter. The authors caution that acidity-related constraints are therefore likely to be concentrated in the more acidic subset of Nigerian soils rather than operating uniformly across the country.

The spatial patterns revealed by the survey reinforce the mechanistic findings. Soil organic carbon and available phosphorus were greatest in the Humid Forest zone and lowest in the Arid/Sahel, while pH showed the opposite trend and exchangeable acidity peaked in the humid zone. Clay content and oxalate-extractable iron and aluminium generally increased toward more humid and mid-altitude environments, revealing a coordinated gradient in organic matter, soil reaction, texture, and oxide properties. K-means clustering identified three broad soil groups: a low-fertility sandy group with low organic carbon, few oxides, and the least available phosphorus; a moderate-fertility group; and a high-fertility clay-rich group with higher organic carbon, oxides, and phosphorus but also greater acidity. Projected into principal component space, these groups formed a continuum rather than sharply discrete classes, underscoring how smoothly soil properties grade into one another across the landscape.

Land use emerged as a powerful influence as well, affecting every measured soil property, whereas depth effects were confined mainly to soil organic carbon, available phosphorus, sand, and clay, with topsoil richer in carbon and phosphorus than the subsoil. Farming system classifications, from maize-mixed to agropastoral and irrigated systems, were included to represent management intensity, and the absence of significant depth-by-land-use interactions suggests that management effects penetrate the profile in a consistent way. The authors note that the structural equation model is best viewed as a parsimonious pathway model linking measured properties rather than proof of causal mechanisms, and that the remaining 71 percent of unexplained variation likely reflects fertiliser history, crop management, parent material, mineral phosphorus fractions, microbial processes, and short-term moisture dynamics.

The practical implications are considerable. The study argues that soil fertility management in Nigeria should be matched to the dominant local constraint rather than treating tropical soils as a uniform category. Sandy, low-organic-carbon soils, typical of the drier north, need practices that build organic matter and enhance nutrient retention, since their weak holding capacity also raises the risk of phosphorus losses through leaching. Finer-textured, oxide-rich, or more acidic soils require a different prescription: organic inputs paired with appropriate phosphorus fertilisation and, where exchangeable acidity is elevated, liming to reduce aluminium- and iron-associated constraints. None of these practices is new, the researchers emphasise, but their relevance varies systematically with the dominant soil constraint, and the national dataset now provides the evidence base for targeting them across contrasting agroecological zones.

For a country that must feed more than 200 million people amid population growth, intensive agrochemical use, and mounting concerns about nutrient depletion, the message is timely. Phosphorus deficiency constrains crop production across the tropics even where fertiliser is applied, because a large proportion of soil phosphorus occurs in forms inaccessible to plants. By quantifying how organic carbon, texture, oxides, and acidity jointly regulate availability at national scale, the study transforms a set of site-specific observations into a coherent framework for action. It also charts a path for future research: testing whether the same pathways operate with equal strength in topsoil and subsoil, and integrating fertiliser history and microbial processes into the models. What emerges is a picture of phosphorus availability not as the product of any single soil property, but as the outcome of a coordinated dance between biology, physics, and chemistry, one that farmers and policymakers can now follow step by step.

Subject of Research: Controls on plant-available phosphorus by soil organic carbon, texture, iron and aluminium oxides, and acidity across Nigerian agroecosystems

Article Title: Soil organic carbon, texture and oxide-associated processes regulate phosphorus availability across Nigerian agroecosystems

Article References: Mesele, S. A., Nguemezi, C., Jibrin, J. M., Ande, O., Jayeoba, J. O., Steverink-Mosugu, M., & Huising, E. J. (2026). Soil organic carbon, texture and oxide-associated processes regulate phosphorus availability across Nigerian agroecosystems. Plant and Soil. https://doi.org/10.1007/s11104-026-09145-8

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09145-8

Keywords: soil organic carbon, phosphorus availability, Nigeria, tropical soils, iron and aluminium oxides, soil texture, soil acidity, Mehlich-3, Random Forest, structural equation modelling, agroecological zones, soil fertility management