Abstract:
The morphological, physical, chemical and mineralogical characteristics of ancient dune soils of the Sokoto sedimentary basin of northwestern Nigeria were studied with a view to elucidating their pedogenesis. The phosphorus forms and adsorption characteristics of the soils were also examined. The results of the studies show that the dune soils presently are characterized by a Sudano-Sahelian climate bordering on semi-aridity towards the northern part, that is around lllela town. The dune soils have similar properties especially in the surface horizons but expectedly differ mainly in their physical properties since they are mainly Entisols. The soils are mainly deep to very deep and have a colour with strong chroma, > 4 throughout their profiles. They are generally sandy except in the Sokoto dunes where sandy loam textures are encountered. Structurally, the surface horizons exhibit weak development and are, therefore susceptible to some amount of erosion hazard. All the soils are characterized by high bulk densities, low total poaisities and high macroporosities of more than 30 percent. The soils therefore have very low water retention capacities. Organic carbon contents are extremely low, being less than 2 g kg-1 in all the soils. The available phosphorus levels are low to moderate in Sangiwa and Sokoto dunes and moderate to very high in the lllela dunes. Soil pH indicates an extremely acid to moderately alkaline soil reaction while the total exchangeable bases are very low, often less than 1.% cmol(+) kg-1, in all the soils. Exchange acidity is also low and less than 2.0 cmol(+) kg-1 soil. The soils have very low CEC, which range from 1.0 to 4.6 cmol(+) kg-1 soil in all the pedons and low to moderate base saturation reflecting the rather siliceous nature of the parent material. Electrical conductivity (EC), exchangeable sodium percent (ESP) and free carbonate values are all very low, with EC values of less than 0.24 dS m-1, ESP of less than 4 percent and free carbonate contents of less than 10 g CaC03 Kg-1 soil in all the soils. The values of DTPA cationic micronutrients, Cu, Zn, Mn and Fe are all well above the critical limits of 0.2, 0.8, 1.0 and 4.5 µg g-1 respectively. Phosphorus fractions and adsorption capacities are similar in all the dunes. The soils have a high total-P content, ranging from 404 to 1881 µg g-1, but the occluded forms account for proportionately higher percentages than the other forms of P. Adsorb-P followed by Al-P are the dominant extraclable inorganic-P forms. The soils have very low P sorption capacities which average about 100 mg kg-1 soil. The P-sorption capacities at standard P solution concentration of 0.3 mg/1 are low and average about 20 mg kg-1 soil. The soils also have very low bond energy. The mineralogy of the soils are similar. Magnetite is the dominant magnetic mineral while quartz is the dominant non-magnetic mineral in the fine sand fractions. In the silt fractions, quartz is dominant followed by kaolinite in all the soils. Kaolinite is the dominant mineral in the clay fraction of all the soils. Parent material, topo-drainage and paleoclimatology were presumed to have had the most profound effect on the dune soils. Organic matter mineralization, limited clay eluviation-illuviation, horizonation, textural and structural differentiation are the main significant pedogenic processes occurring in the soils but these processes are better expressed in the Sokoto dunes and in pedon LDS 2 of Illela dunes, thereby producing Bt horizons. According to the criteria of the USDA Soil Taxonomy, all the soils on the Sangiwa and Mela dunes are classified as Ustic Quartzipsamments except pedon LDS 2 on Illela dunes which is classified as Arenic Haplustult. Soils on the Sokoto dunes are classified as Kanhaplic Haplustults except for pedons SDS 3 and 4 which are classified as Lithic Haplustult and Ustic Quartzipsamment respectively. In the FAO/UNFSCO soil map of the world legend, the soils on Sangiwa dunes are placed as Ferralic Arenosols while those on Sokoto dunes are placed as Haplic Alisols except for pedon SDS 4 which is placed as Ferralic Arenosol. Pedons LDS 1, 3 and 4 on the Mela dunes are all placed as Ferralic Arenosols, while pedons LDS 2 is placed as Haplic Acrisols. Organic matter and related properties, exchangeable cationic bases, especially K. low water and nutrient holding capacities are the major constraints to productivity in all the soils. In addition, phosphorus is also a constraint to productivity in the Sangiwa dunes. Micronutrients are however, adequate for most crops
PEDOGENESIS OF ANCIENT DUNE SOILS IN THE SOKOTO SEDIMENTARY BASIN, NORTHWESTERN NIGERIA
