Micronutrient Soil Tests for the Semi-Arid Savannah of Nigeria: Boron and Molybdenum

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Abstract
Boron and molybdenum deficiencies have only been recently reported in the semi-arid West African savannah zone. Reliable soil test methods and analytical techniques are needed to identify dependable diagnostic indices. The present study evaluated three soil test methods each for B (hot-water, Morgan’s reagent, and double-acid) and Mo (Tamm’s ammonium oxalate extractant, 0.1 n NaOH, and anion exchange resin). Thirty surface soil samples of varying physico-chemical characteristics were cropped to peanuts (Arachis hypogaea L.) in the greenhouse in two series each lasting 35 days. The plants were harvested, ashed, and analyzed for B or Mo content. Uptake of B and Mo by peanuts was related to the soil tests through simple and multiple regression analysis which included selected soil properties as independent variables. Of the three extractants evaluated for B, the Morgan’s reagent (10% NaOAc in 3% HOAc, pH 4.8) appeared to be the most suitable for predicting the availability of boron to plants when both extractable B and the extractable B × % organic matter interaction term were included in the regression equation (R 2 = 0.90). Extractable B as the only independent variable in the equation gave a predictive value of 87% indicating that NaOAc-HOAc-extractable B would be a satisfactory index of B availability in the soils studied. Acid ammonium oxalate was the best extractant for predicting plant-available Mo when % organic matter was also included as an independent variable (R 2=0.64). Critical soil minimum values for the two nutrients could not be established in this study but a comparison of the results obtained here with the values reported in the literature, suggests that available B in the soils studied ranged from low to very low and that of available Mo from medium to low.

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