Assessment of the Metallic Micronutrient Elements in the Benchmark Soils of Savanna Regions, North - East Nigeria

The status of metallic micronutrient elements extractable in 0.005M DTPA was assessed in 31 locations across the North-east Nigeria which comprises of six states (Adamawa, Bauchi, Borno, Gombe, Taraba and Yobe). Composite surface (0-20cm) and sub-surface (20-50cm) soil samples were collected from areas having different parent materials, vegetation zones and soil texture. The soil samples were analyzed for available Cu, Fe, Mn and Zn following discrete procedures. Few important soil properties were also analyzed (particle size analyzes, soil pH, soil organic carbon, available phosphorus-Bray-1 P and total nitrogen). The soils were sandy to clayey in texture. Soil pH ranged from strongly acidic (pH 4.44) to slightly alkaline (pH 7.47) in both surface and subsurface soils. Soil organic carbon ranged from very high (3.18 mg/kg) to very low (0.09 mg/kg). The content decreased from Southern guinea savanna to Sahel savanna respectively. Available P in dilute acid fluoride (Bray-1 P) ranged from very high (24.5 mg/kg) to low (2.1 mg/kg) in both surface and sub-surface soils, while total nitrogen was generally low in most of the samples analyzed except in few. DTPA Cu was higher in clayey surface soils (0.143 mg/kg) and subsurface soils (0.36 mg/kg) than in the sandy surface (0.119 mg/kg) and subsurface soils (0.189 mg/kg) and the content increased with depth. The content was also highest in basalt and alluvium and lowest in Aeolian sand. Available Fe and Mn extractable in DTPA were higher in surface than in subsurface soils. Generally, both metals were abundant in the soils regardless of parent materials, vegetation, and texture and soil depth. Extractable Fe correlated negatively with available P (r = - 0.972**) and total N (r = - 0.700**), while Mn was related positively with pH (r = 0.382**) and negatively with Bray-1 P (r = - 0.852**). Available Zn extractable in DTPA was generally deficient in all the samples irrespective of soil parent materials, vegetation zones, and soil texture and soil depth. Available Zn correlated positively with organic carbon (r = 0.560**) but negatively correlated with percentage sand (r = - 0.613**). Results from the analysis indicated that areas represented by the majority of the soil samples were deficient in Zn extractable in DTPA. Available Cu was also probably deficient in the soils. However, Fe and Mn were found to be sufficient in the soils of the study area regardless of parent materials, vegetation zones or soil texture. Available micronutrients were generally low in soils developed on Aeolian sand deposits. DTPA extractable micronutrients also showed decreasing trend from the Southern Guinea savanna northward to the Sahel savanna.