Nigerian black cotton soils are formed from the geochemical processes that break down basic igneous rocks under extreme seasonal weather variations. They primarily originate from the weathering of shaly and clayey sediments alongside basaltic rocks. Notably, black cotton soils are prevalent in semi-arid regions across both tropical and temperate climate zones, particularly where annual evaporation exceeds precipitation. The moniker “black cotton soil” reflects their suitability for cotton cultivation.
Two distinct categories of parent rock materials have been identified in the formation of black cotton soils: sedimentary rocks of volcanic origin, prevalent in North America, South Africa, and Israel, and basic igneous rocks found in India, Nigeria, and the southwestern United States.
Black cotton soils, characterized by their dark grey to black colouring and high clay content (>50%), are prominent across large areas of the world, covering roughly 340 million hectares or 3% of global arable land, and 43 million hectares of land in Africa. Nigeria harbours an estimated 104,000 square kilometres of black cotton soils, primarily concentrated in the northeastern region.
According to Morin (1971), the Lake Chad Basin is a unique instance of large-scale lacustrine deposits containing black cotton soil within Africa. Formed during the Tertiary and Quaternary periods, the black cotton soils in northeastern Nigeria specifically originate from the Chad Formation. Their composition reflects a layered sequence of lacustrine and fluvial clays and sands dating back to the Pleistocene age.
Montmorillonite, a clay mineral characterized by significant volume changes in response to seasonal wetness and dryness, and other minerals of the smectite group dominate the mineralogical composition of black cotton soils. Field observations of black cotton soil deposits reveal a characteristic pattern of extensive cracking during dry seasons. Crack widths of about 70 millimetres and over 1 meter in depth have been reported in literature, with the potential to extend up to 3 metres or more in thicker deposits (Adeniji, 1991).
Topography of Black Cotton Soil Deposits
As documented by Katti et al. (2002), black cotton soil formations primarily occur on terrain with slopes less than 3 degrees. The most prevalent physical setting for these soils is flat, alluvial plains, exemplified by regions such as:
- Sudan
- Texas, USA
- Darling Downs, Australia
- Accra, Ho-Keta, and Winneba plains in Ghana
- Lufina Valley, Zaire
- Kafue Flats, Zambia
- Panamalenga Plains and Springbok Flats in Botswana and South Africa, respectively
- Low-lying areas of the NorthEastern States of Gombe, Borno, Yobe, Adamawa, Taraba, and Bauchi States in Nigeria
It is important to note that, while such landscapes are most common, occurrences of black cotton soil on areas with steeper slopes have also been documented by Ahmad (1983).
Black cotton soils are unique due to several defining characteristics:
- Colour: Their appearance ranges from black or darkish grey to brown.
- Mineral Composition: They contain a high content of expansive clay minerals, particularly montmorillonite
- Moisture Sensitivity: Black cotton soils exhibit a marked tendency to shrink and swell in response to changing moisture conditions.
- Geo-environmental Phenomena: This sensitivity to moisture manifests in notable heave and crack formation, posing unique challenges for geotechnical and environmental considerations.
Chemistry of Black Cotton Soils
Black cotton soils exhibit a distinct chemical profile marked by high concentrations of silica, lime, iron, magnesia, and alumina. While trace amounts of titanium oxide are present, it is believed to be the primary contributor to the characteristic black colour (Building and Road Research Institute, 1985).
Interestingly, despite the dark colour, these soils contain relatively low organic matter, typically less than 5%. Furthermore, the silica-sesquioxide ratio exceeds 2.50, thereby classifying them as non-lateritic soils (USAID/BRRI, 1971).
The typical oxide composition of some black cotton soils from some countries is presented in Table 1.
Black Cotton Soil in Construction
Black cotton soil has been classified as an expansive soil by many researchers and its distribution and occurrence extends across various countries including Nigeria, Cameroon, Ghana, the Lake Chad Basin, Sudan, Ethiopia, Kenya, and Zimbabwe in Southern Africa. Its presence is further documented in India, Australia, the southwestern United States, South Africa, and Israel. Notably, these areas share the characteristic of being semi-arid tropical regions with annual evaporation exceeding precipitation.
These soils manifest significant volume changes in response to seasonal wetness and dryness, posing major challenges for civil engineering projects. This phenomenon is driven by the adsorption of polar molecules like water or organic matter onto clay particle surfaces within the interplanar space, causing expansion and swelling. Research suggests that the extent of swelling is primarily influenced by the type of exchangeable cations present in the aqueous solutions interacting with the clay.
Black cotton soil has long been recognized as a problematic soil in construction due to its unsuitability for both housing and road infrastructure. It undergoes high expansion and contraction, with a high level of compressibility and plasticity. Instances of building failures, particularly in Nigeria’s Adamawa state, have been attributed to differential settlements arising from the soil’s inherent shrink-swell behaviour, evident in extensive wall cracking reaching the foundation level.
Extensive reports, citing examples like road heaves around Deba and foundation cracks in buildings, highlight the detrimental effects of black cotton soils on infrastructure integrity (USAID/BRRI, 1971). Unfortunately, the associated costs of such damages within Nigeria remain undocumented. Considering the widespread presence of these soils and their intricate challenges for stable construction, mitigation strategies are necessary to ensure robust and sustainable infrastructure development in the affected regions.
The black cotton soil deposits along Gombe – Biu road in Yamatu Deba Local Government Area of Gombe State has been extensively studied by geotechnical engineering researchers in Nigeria. The location lies within latitude 10° 19’N and longitude 11° 30’E. Black cotton soil from Numan in Adamawa State, Nigeria has been extensively researched too. Numan is located at latitude 9°29’10’’N and longitude 12° 02’36’’E on the Nigerian geographical map.
A lot of methods have been proposed on the improvement of the geotechnical behaviour of black cotton soils for construction works such as stabilisation with sand, cement, lime, fly ash, quarry dust, cement kiln dust, sugarcane bagasse ash, locust bean ash, iron ore tailings, rice husk ash, etc.
Black Cotton Soil in Agriculture
Black cotton soil possesses unique characteristics that render it highly suitable for specific agricultural applications. They exhibit a high clay content and characteristic black colouration stemming from the presence of titaniferous magnetite. While rich in nutrients such as calcium, carbonate, potash, lime, iron, and magnesium, it exhibits peculiar attributes demanding tailored management practices.
Notably, black cotton soil has limitations in readily available phosphorus, nitrogen, and organic matter. Consequently, fertility may vary spatially, requiring specific attention to upland areas. Additionally, the high clay content can impede root penetration and development, potentially hindering plant growth and yields.
Despite these limitations, black cotton soil stands out for its valuable properties:
- Fertility: The inherent presence of essential nutrients fosters the growth of various crops, including cotton, sugarcane, and cereals.
- Moisture Retention: The soil’s remarkable ability to retain moisture minimizes irrigation requirements and promotes efficient water utilization.
- Organic Matter: The significant organic matter content contributes vital nutrients for plant growth and enhances soil structure.
- Aeration: The favourable soil structure facilitates optimal aeration, critical for root development and plant health.
- Erosion Control: The combination of high organic matter and good soil structure provides natural resistance against erosion, minimizing soil degradation risks.
Black cotton soil, with its unique set of advantages and challenges, requires an informed and well-adapted approach to harness its full agricultural potential. Recognizing its specific characteristics and implementing appropriate management practices are crucial for sustainable and productive agricultural endeavours.
