Evaluation of Universal Extractants for Determination of Selected Micronutrients on Soils Developed over Charnockite in Ekiti State

Abstract

Twenty six pedogenic soils developed on charnockite were used to evaluate the suitability of extractants for bioavailability of manganese, copper, zinc and iron in the soils to plants. These micronutrients cation were extracted from the soils using five different extractants, namely, Neutral Normal NH₄OAc, NH₄OAc buffered with glacial acetic acid to pH 4.8, 0.1 M HCl, 0.05 M HCl + 0.025 M H₂SO₄, 0.05 M EDTA + 0.1 M (NH₄)₂CO₃. The contents of extractable Mn, Cu and Fe ranged were 0.04-1.50, 0.39-9.01 and 14.10-639.90 mg kg^-1 respectively with Mehlich₁ having the highest mean values of 0.83, 5.45 and 341.46 mg kg^-1 across the locations. EDTA was more efficient in the extraction of the available Zn having a mean value of 0.11 mg kg^-1 and varied from 0.01-0.22 mg kg^-1 in all the soils. The 0.1 M HCl was moderately efficient in the micronutrients cation assessment, but the NH₄OAc extractants were generally less effective in assessing the micronutrients cation even though the buffered solution gave a better extractability over the neutral normal solution. All the extractants were significantly correlated (p < 0.05 and p < 0.01) with one another for available Mn and Fe in the soils except the Mehlich₁ and neutral normal NH₄OAc in the extraction of Mn. Whereas, only 0.1 M HCl correlated significantly with NH₄OAc pH 4.8 in determining the extractable Zn. Likewise, Mehlich₁ vs. NH₄OAc pH 4.8 and 0.1 M HCl vs. NH₄OAc pH 7.0 had moderate significant correlations (p < 0.05) for extractable Cu. It shows that the extractants were generally extracting the available forms of these micronutrients cation from the same source in charnockitic soils irrespective of the likely chemical, physical and mineralogical variations across the locations. The order of extractability of the extractants were: 0.05 M HCl + 0.025 M H₂SO₄ > 0.05 M EDTA + 0.1 M (NH₄)₂CO₃ > 0.1M HCl > NH₄OAc pH 4.8 > NH₄OAc pH 7.0.

Keywords: Charnockite, Correlation, Extractants, Micronutrients