Sustainable crop production in the Nigerian Sudan Savanna biome requires a good understanding of the fertility status of the soil in order to impose appropriate nutrient management strategies. Field surveys were conducted to assess the fertility status of selected soils from Shanono and Bunkure Local Government Areas (LGAs) of Kano State in the Northern Nigerian Sudan Savanna. All the surveyed fields had soil organic C, total N and ECEC within very low and low fertility classes. Very low and low available P was also found in the majority of fields with only 25.0 and 8.7% of the fields falling in the moderate P fertility range in Shanono and Bunkure LGA respectively. About 75.0% of fields in Shanono LGA and 39.1% in Bunkure LGA fell within the low exchangeable K class, with the rest of the fields having moderate level. Despite moderate to high content of exchangeable Ca and Mg in most of the fields, 4.3-8.7% of the fields had depleted status of the two basic cations. Micronutrients (Zn, Cu, Mn and Fe) levels in all the fields were in the moderate and high classes, except the subsurface soil of Shanono LGA where 4.2% of the fields had low Cu contents. Correlation analysis between northerly latitude and soil properties showed a northward increase in sand, pH and Zn levels and a decrease in silt and available Fe levels. The low status of Ca, Mg and Cu in some of the fields indicates the insufficiency of the current fertilizer recommendations which focus mainly on primary macronutrients (N, P and K). Therefore, apart from recommending all strategies that will enhance and stabilize organic matter, current fertilizer recommendations need to be reviewed and should contain other nutrients in addition to primary macronutrients.

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Site-specific nutrient management can reduce soil degradation and crop production risks related to undesirable timing, amount, and type of fertilizer application. This study was conducted to understand the spatial variability of soil properties and delineate spatially homogenous nutrient management zones (MZs) in the maize belt region of Nigeria. Soil samples (n = 3387) were collected across the area using multistage and random sampling techniques, and samples were analyzed for pH, soil organic carbon (SOC), macronutrients (N, P, K, S, Ca and Mg), micronutrients (S, B, Zn, Mn and Fe) content, and effective cation exchange capacity (ECEC). Spatial distribution and variability of these parameters were assessed using geostatistics and ordinary kriging, while principal component analysis (PCA) and multivariate K-means cluster analysis were used to delineate nutrient management zones. Results show that spatial variation of macronutrients (total N, available P, and K) was largely influenced by intrinsic factors, while that of S, Ca, ECEC, and most micronutrients was influenced by both intrinsic and extrinsic factors with moderate to high spatial variability. Four distinct management zones, namely, MZ1, MZ2, MZ3, and MZ4, were identified and delineated in the area. MZ1 and MZ4 have the highest contents of most soil fertility indicators. MZ4 has a higher content of available P, Zn, and pH than MZ1. MZ2 and MZ3, which constitute the larger part of the area, have smaller contents of the soil fertility indicators. The delineated MZs offer a more feasible option for developing and implementing site-specific nutrient management in the maize belt region of Nigeria.

The forms of iron extracted by different methods were studied in soils developed on four basement complex rocks within Northern Guinea Savanna of Nigeria namely: migmatite gneisses, older granite, quartzites and mica schists. The study shows that forms of iron generally decreased in the order of total elemental iron (Fe T) determined by XRF > double acid extractable total iron (Fe a) > dithionite extractable iron oxide (Fe d) > oxalate extractable iron oxide (Fe x) > pyrophosphate extractable iron oxide (Fe p) > 0.1M HCl extractable micronutrient available iron (Fe h). Variation in content of total irons (Fe T and Fe a) were not significant in the soils, but content of Fe h , Fe d , Fe x and Fe x/d significantly varied in the soils on the various parent materials. Therefore processes and extent of pedogenesis were considered to influence the variations resulting in recrystallization and increase in content of Fe h and Fe d in these soils. The significant correlation between various forms of Fe (Fe a , Fe d , and Fe x) with active iron ratio further affirmed that as soils on basement complexes increase in their pedogenetic age, Fe content generally increases irrespective of total forms of iron. The strong correlation between Fe T , Fe a and Fe d indicate that these forms of iron were extracted from similar sources.

This study investigates the effects of vegetation particularly fast growing exotic species like Tectona grandis on the Newer Basalt of the Jos Plateau. In particular it determines the effects exotic tree species have on the pedogenetic forms of Fe and Al and mineralogical properties of the basaltic soils age. Soil samples were determined at the depth of 0-30cm and 30-60cm under plantation of four different ages and subjected to standard laboratory analysis. The citrate bicarbonate dithionite (CBD) extractable Fe and Al slightly decreased with aging plantation, which was ascribed to the inhibitory effects of organic matter on the crystallization of Fe and Al. Similarly the active Fe value was less than unity, which implied increased crystalline forms of Fe and Al oxides are as a result of weathering intensity. Mineralogical results indicated that the soils under the fallow and 11 year old plantation had the dominance of magnetites, while those under 21 year old plantation had mixed mineralogy, consisting of magnetites, biotites and montmorillonites. The older plantations (31 and 36 year old) were dominated by kaolinites and traces of haematites and goethites. The trend suggested an increased level of pedogenesis with aging plantations. Increase in crystalline forms of Fe and Al, kaolinites, traces of haematite and goethite concludes that Tectona grandis deteriorates soil properties over time.