Magnesium fixation—a possible cause of negative yield responses to lime applications

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Abstract
Negative yield responses are common when highly leached soils are limed to neutrality. A number of explanations have been proposed, but very few have been experimentally verified or are entirely acceptable. In this paper the problem is re‐examined and a new possible causal mechanism is proposed. Data assembled from the literature are used to demonstrate that a consistently observed reduction in Mg uptake by plants when soils are limed to neutrality is often accompanied by a generally ignored reduction in exchangeable soil Mg. Using data for soils of North American and African origin, it is demonstrated that when the soils were limed with pure Ca sources, the level of exchangeable soil Mg was reduced between 36 and 93%. Furthermore, after treatment with a Mg containing liming material, initial increases in the level of exchangeable Mg were followed by similarly large decreases when the soil pH approaches neutrality. The applicability of known Mg fixation mechanisms is briefly discussed and the suggestion is made that Mg fixation may offer an acceptable explanation for some of the yield depressions observed on liming to neutrality. Research is currently in progress to gather sufficient data to answer the many questions raised.