The uptake of magnesium under exhaustive cropping

1. A range of soils were cropped exhaustively by perennial rye-grass for up to eleven months in the glasshouse; some were also cropped with Dutch white clover.2. At least 100 lb./acre of magnesium was taken up from most soils, and some soils supplied more than 200 lb./acre. The amounts of magnesium taken up decreased with successive harvests, suggesting that the available magnesium was progressively depleted. Although the ‘exhaustion’ magnesium (Mg taken up by crops+residual exchangeable magnesium) was greater than the initial exchangeable magnesium, these two measurements were closely correlated (r = 0·99). If non-exchangeable magnesium was released during cropping, the releases were proportional to the initial exchangeable magnesium contents. However, the exchangeable magnesium measurements may have extracted only a proportion of the magnesium available naturally.3. Any releases of magnesium were small compared with the amounts available from the outset, and the crops obtained magnesium mainly from the exchangeable form. Rye-grass and white clover gave similar results.4. The exchangeable magnesium in some exhausted soils was increased by wetting and drying. This effect may occur in the field, where the magnesium lost in cropping could be replenished by only small releases of non-exchangeable magnesium.