Evaluation of the magnesium supplying powers of the Nigerian savannah soils

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Abstract
Greenhouse and laboratory studies carried out to evaluate the Mg supplying potential of the savannah soils of Nigeria revealed that the Mg status is a function of parent material, age of the soil (and thus its degree of weathering and leaching) organic matter, and silt+clay fractions. The availability is a product of an interplay of secondary soil factors such as cation balance, soil reaction and CEC (largely governed by organic matter levels) acting on the exchangeable form. The soils are generally low in Mg (averaging between 435 and 543 ppm total and 0.43 and 0.74 meq/loo g exchangeable Mg respectively). In 50% of the samples the available Mg values fell between the critical range of 0.2-0.4 meq/100 g while the Mg/K ratios in 60% of the samples were <2.0, the critical value. Many of these soils are therefore considered deficient in Mg. Those derived from sandy parent materials and developed under semi-arid conditions, low in organic matter and CEC (the Sahel and Sudan savannah in the extreme north of the country) contain the lowest amounts of available Mg (varying between 0.20 and 0.50 meq/100 g). This is where the problem is likely to be prevalent in the immediate future. Mg deficiency symptoms which appeared on 33 of the soils during the greenhouse cropping, were associated with 0.17% Mg or less in plant tissue and 0.36-0.38 meq/100 g or less exchangeable Mg. The southern Guinea savannha soils with relatively high organic matter levels and CEC, contain the highest amounts of both total and exchangeable Mg (averaging about 536 ppm and 0.7 meqf100 g respectively). Crop response to Mg in this zone is unlikely in the immediate future but when K fertilization is introduced on a wide scale (as is likely to happen in the area fairly soon) deficiency resulting from K/Mg imbalance may come up sooner than is apparent.