The study, conducted by soil scientists Michael Akaninyene Okon and Angelica Ngozichukwu Udi of the Federal University of Technology Owerri and published in BMC Agriculture, focused on three communities — Isingwa, Ngwaukwu and Ntigha — where cassava cultivation dominates the landscape. The team collected fifteen surface soil samples from a depth of 0 to 20 centimetres in March 2023, timed deliberately at the onset of the rainy season, using a free survey approach across randomly chosen points. Each location was geo-referenced with a handheld GPS, and field observations of colour, structure, drainage, stoniness and topsoil depth were paired with laboratory measurements of bulk density, porosity, hydraulic conductivity, water retention and aggregate stability.

The setting matters. Isialangwa North lies in the humid tropics, where annual rainfall of roughly 2,500 to 3,000 millimetres arrives in intense bursts peaking in July and September, and temperatures range from 27 to 35 degrees Celsius. The soils there formed from coastal plain sand of the Benin formation, a parent material that gives the region its characteristically sandy texture. Farming is the principal economic activity, sustained by bush fallowing supplemented with inorganic fertiliser, and practised almost entirely as rain-fed agriculture on ridges and mounds. In such an environment, the physical fabric of the soil — how it stores and transmits water, how it withstands the pounding of tropical storms — largely determines whether a farm thrives or slides toward erosion.

To translate raw measurements into a health verdict, the researchers adopted a rating scheme aligned with the Food and Agriculture Organization’s list of physical soil health indicators, scoring each attribute from 0 to 3, where 0 denotes healthy, 1 sub-healthy, 2 weak and 3 degraded. The framework rests on a simple premise: a healthy soil should hold water well, drain freely, resist compaction, provide sufficient depth for roots and maintain a stable structure. Any significant deterioration in these properties pushes the soil down the scale, with consequences that cascade into nutrient cycling, root development and ultimately crop yield.

On several counts, the news was genuinely encouraging. Bulk density, a measure of how tightly soil particles are packed, ranged from 1.18 to 1.29 megagrams per cubic metre across the three sites — values low enough to indicate the soils are not compacted and will not physically restrict root growth. Total porosity, calculated from bulk density and a standard particle density of 2.65, ranged from 51.09 to 55.62 percent, comfortably in the healthy class. Saturated hydraulic conductivity, measured with the constant-head method, fell between 0.009 and 0.011 centimetres per second, within the range considered ideal for crop production in Nigeria. The soils were also well drained, essentially free of stones, and — in Isingwa and Ngwaukwu — possessed healthy topsoil depths, with only Ntigha slipping to sub-healthy on that measure.

But two indicators told a darker story. Aggregate stability, assessed through water-stable aggregates larger than 0.25 millimetres, ranged from a mere 6.96 to 8.79 percent, earning the soils a degraded rating of 3. In practical terms, when rain hits these soils, the clumps of particles that give soil its sponge-like architecture fall apart — a process known as slaking — leaving fine particles free to disperse, crust and wash away. The mean weight diameter of the aggregates, at roughly 0.012 millimetres for wet sieving, was far below the 0.4-millimetre threshold that marks even the ‘unstable’ category, placing the soils firmly in the very unstable class. Soils with such low values, the literature warns, erode far more readily than those with robust aggregation.

Water retention fared no better. Using Saxton’s hydraulic calculator, the team found the soils could hold only 7.3 to 8.1 percent water — squarely in the ‘low’ band of the standard rating, and thus degraded. Ntigha retained slightly more water than the other sites, a difference the researchers attribute to its higher clay content, but even that advantage was marginal. The culprit is almost certainly the region’s sandy texture combined with chronically low organic matter: sandy soils have few fine pores to capillary-bind water, and without organic matter to act as a sponge, rainfall simply percolates beyond the reach of roots. For rain-fed farmers, that means crops face water stress within days of every dry spell between storms.

The correlation analysis added a revealing layer to the story. Water-stable aggregates correlated positively with clay content, total porosity, organic carbon, total nitrogen, organic matter, microbial biomass carbon, calcium and total exchangeable bases — and negatively with bulk density. Bulk density, in turn, was strongly and negatively linked to organic carbon, nitrogen, organic matter and microbial biomass, and showed a perfect inverse relationship with total porosity. Water retention rose sharply with clay (a correlation of 0.910) and total exchangeable acidity, but fell with sand, silt, pH, organic carbon, nitrogen, calcium, magnesium and base saturation. The pattern is coherent: the same organic matter that feeds soil microbes also glues particles into stable aggregates, and anything that depletes it — continuous cultivation, erosion, removal of residues — simultaneously undermines structure, water storage and fertility.

Soil colour and structure filled in the picture. All three sites showed a fine granular structure, the most desirable arrangement for aeration and root penetration, but its grade was weak, with aggregates barely observable in the field — a rating of 2, or weak. Isingwa and Ngwaukwu soils were brown, a hue associated with humification and the decomposition of plant and animal remains, while Ntigha’s reddish tone pointed to abundant iron oxides inherited from the coastal plain sand parent material and intensified by the region’s climate. Colour, the researchers note, is a quick visual proxy for organic matter content and drainage conditions, and the brown-to-reddish palette here ranked as weak rather than the dark, humus-rich colours of a truly thriving soil.

Averaged across all ten physical indicators, each of the three communities scored 1 — sub-healthy — and the authors are clear about what should happen next. They recommend cover cropping, crop rotation, organic mulching and organic manuring to rebuild aggregation and water-holding capacity, alongside reduced or no-till approaches and integrated nutrient management to protect the structure that remains. They also call for follow-up studies that fold chemical and biological indicators into the assessment, since physical health alone cannot capture the full functional state of a soil. For a region where farming is the economic backbone and the rains arrive with erosive force, the message is urgent: the soils of Isialangwa North are not yet broken, but the glue that holds them together is thinning, and only deliberate conservation can keep them on the right side of the health scale.

Subject of Research: Physical soil health assessment of tropical agricultural soils in Isialangwa North, southeastern Nigeria

Article Title: Assessment of the physical health of tropical agricultural soils in Isialangwa North, Southeastern Nigeria

Article References: Okon, M. A., & Udi, A. N. (2025). Assessment of the physical health of tropical agricultural soils in Isialangwa North, Southeastern Nigeria. BMC Agriculture, 1(1), Article 13. https://doi.org/10.1186/s44399-025-00011-z

Image Credits: AI Generated

DOI: 10.1186/s44399-025-00011-z

Keywords: soil health, tropical soils, aggregate stability, water retention, bulk density, hydraulic conductivity, soil porosity, Nigeria, cassava farming, soil degradation, soil conservation, soil structure