Magnesium Fertilisers in Soil and Plants: Comparisons and Usage

Keywords: Magnesium, Sugar beet, Potatoes, Fertiliser, Crop nutrition, Magnesium deficiency.

Over the past 50 years, magnesium has been increasingly recognised as a major plant nutrient which is frequently deficient in light textured soils in intensive cropping. Consequently, it has become an important constituent of fertilisers for arable land.

The paper outlines the occurrence of the element in the lithosphere, in soils and its essential nature in plant nutrition. Most early attempts to correct deficiency in plants were with foliar sprays (or even injections!) of magnesium salts, particularly Epsom salts (MgSO₄.7H₂O). It was discovered in the early sugar beet experiments that these treatments, whilst giving temporary and partial relief, had little effect on final yield. Real progress was only made when soil applications of magnesium were used. This paper reports a summary of responses by sugar beet to magnesium on more than 100 fields, most of which contained less than 50 mg/l soil exchangeable Mg.

Details of the range of magnesium fertilisers in common use are described, particularly in relation to their total and plant available magnesium concentration. Kieserite is the preferred product for quickly-available magnesium needed on very deficient soils. However, few fields remain in this category due to regular additions of magnesium, and calcined magnesite may be a better product for the maintenance applications needed on most fields. Work on the importance of calcining conditions on magnesium availability to plants is reviewed.

Compared to sugar beet there are few data dealing with magnesium usage on the potato crop. However, application rates appear to be in accordance with current recommendations and the forms used are probably similar to those used on the sugar beet crop. Recent studies at Cambridge University Farm have emphasised that other nutrients, particularly nitrogen and potassium, have a profound effect on magnesium uptake. Work is continuing to investigate whether current recommendations need to be made more cultivar-specific, so that yield potential is not lost in some varieties.

Dr Philip Draycott, Ashfield Green Farm, Wickhambrook, Newmarket, UK.

Dr Marc Allison, Cambridge University Farm, Girton, Cambridge, UK.

28 pages, 9 figures, 11 tables, 29 refs.