Critical Level of Mg in Western Nigerian Soils as Estimated under Greenhouse Conditions

Widespread Mg deficiencies have not been reported in western Nigeria. But with continuous intensive cultivation, heavier fertilization especially with K, and the introduction of high yielding, responsive crop varieties Mg deficiency is likely to become a serious problem in the near future. Knowledge of the critical available Mg levels can be useful in conjunction with routine soil analysis data to monitor the onset of these deficiencies. There is no previously published work on Nigerian soils that estimated the available Mg level below which response to Mg fertilization can be expected. The objective of this investigation therefore was to approximate this critical level under greenhouse conditions. Twenty-three Nigerian soils collected from various ecological zones and soil types were cropped to maize (Zea mays L.) in the greenhouse with and without Mg application. The maize tops were cut after 35 days, dried, weighed, milled, and analyzed for Mg. Using the dry matter weights relative yields were worked out for all the soils and the new Cate-Nelson (6) statistical procedure used to estimate the critical level. This procedure involved a step-wise splitting of the data into two groups using successive tentative critical levels to determine the level which maximized the overall predictive ability (R²) with the means of the two groups serving as the predictor values. An approximate critical level of 0.28 to 0.35 meq/100 g was obtained by this technique. Tests on individual plot yields showed that 0.28 meg/100 g neutral N NH₄OAc, extractable Mg was low enough for statistically signficant response to Mg application. All soils (except no. 10) that had less than 5% saturation of CEC responded to Mg application; those with more than 7% Mg saturation did not respond to Mg. Mg deficiency symptoms were associated with soil Mg level of 0.17 meq/100 g or lower prior to planting and with plant Mg content of less than 0.1%. Although the critical level of available Mg approximated in this study (0.28 to 0.35 meq/100 g) is much lower than the average available Mg content (1.2 to 1.5 meq/100 g) of western Nigerian soils. Caution is nonetheless needed in evaluating the findings of the present study as greenhouse results may not necessarily reflect the true situations in the field.