Pedogenesis and Fertility Capability Classification of Selected Soils Overlying Three Geological Materials of Northern Guinea Savanna in Taraba State, Nigeria.

Abstract

Sustainable use of soil resources requires extensive knowledge about the soil factors and processes. Soil data are necessary for improved land use management and conservation. Soils on the Basement Complex; pegmatite (PM), porphyritic granite (PG), and granite gneiss (GG) were studied along a toposequence in the Northern Guinea Savanna of Nigeria to evaluate the pedogenic processes in the soils, and assess the influence of geological materials on their fertility status. Four pedons were dug along each of the two toposequences in each parent material (PM, PG and GG). This resulted in twenty-four soil profile pits (3x2x4). Soil morphological, physicochemical, and mineralogical properties were determined and used to assess the soils. Dithionite and oxalate forms of iron and manganese were also determined.The influence of geologic material, topo-sequence and topo-position on the soils were studied in 3 x 2 4 factorial in a fixed factor design.Data were presented using descriptive statistics and also subjected to two-way analysis of variance using Linear Model of SPSS Version 17.0 with differences deemed significant at p<0.05. Result showed that the soils did not differ significantly in depth and structure except some restrictions due to lithic, bedrock and water table on some pedons. The soils were dominated by sub-angular blocky structure except some pedons on porphyritic granite which had platy structure in few horizons. Dark brown colour dominated the soil surface and strong brown in the subsurface horizons with greater hue variation in the subsurface horizons (10YR, 7.5YR and 5YR). Texture of surface soils was sandy loam and varied with soil depth resulting in sandy clay to sandy loam in the subsurface horizons. Sand dominated fine earth portion of the soils and ranged from 180 to 630 g kg-1 while clay and silt contents ranged from 80 to 260, 30 to 290 g kg-1 respectively. Total porosity and bulk density had ranges of 29.43 to 40.00% and 1.47 to 1.87 Mg m3, respectively. Geological materials had significant influence on exchangeable Ca, TEB, EA, CEC and base saturation statistically. The soil reaction was slightly acid to slightly alkalinic pH (H2O) (6.90 – 7.50) and moderately acid in KCl (5.20-6.80. Total exchangeable bases ranged from (Ca) 0.08 to 5.80 cmol/kg; Values of organic carbon and total N were low to moderate 3.88 to 14.39 g kg-1 and 1.44 to 2.00 g kg-1. The CEC was 5.14 to 12.0 cmol(+) kg-1 rated medium to low while base saturation was low to moderate 15.85 to 48.30%. The content of dithionite and oxalate Fe and Mn, indicating intense weathering condition of the soils as well as their combined movement with clay to endopedons. Felspar (Kaolinite, illites, and montmorillonite) Quartz, dominated the mineralogy of clay fraction.The dorminant pedogenic processes inferred include weathering, plinthization, braunification, humification, melanization, leaching, argilluviation, bioturbation and ferritization.Ochric epipedon was observed in all pedons, pedons on porphyritic granite (BDCP1, BDUSP2, and BDMSP2) were classified as Haplic Plinthustults (Haplic Acrisols), while BDCP2, BDLSP1, BDMSP1, BDLP1, and BDLSP2 were Typic Ustipsamments (Dystric Gleysols). The BDUSP1 was Typic Dystrustept (Haplic Arenosols). Fine mixed isohyperthermic and fine silty mixed isohyperthermic at family level. Pedons on pegmatite (KCP1, KUSP1, KUSP2, and KMSP2) were classified as Typic Plinthustults (Haplic Acrisols), on granite gneiss (MCP1, MCP2, and MUSP1) were classified as Typic Dystrustepts (DystricGleysols). The Fertility Capability Classification of the soils showed sandy-loam and loamy-sand texture with low nutrient reserve, low cations exchange capacity and slightly acidic soils (SlLskeh) on porphyritic granite. While on pegmatite and granite gneiss, the soils showed loamy-sand and sandyloam texture with K deficiency, low to moderate CEC, low to moderate organic carbon,with greater than 15% exchangeable sodium percentage (LsSlkedh). The fertility constraints of the soils of the three geological materials indicated acid reaction (h), low potassium reserve (K) and gleying (g) at lower slope areas. Management options xiii suggested includes construction of contour ridges and bunds, and cultivation of cover crops. Post-harvest crop residue incorporation, liming and application of gypsum to correct soil structural problems as well as use of organo-mineral fertilizers would improve the soils for crop production.