It's all about the compound

Waste products from the chemical industry or metal processing are often used in agriculture - especially when the availability of established fertilizers is limited or their price has risen. Particular attention must be paid to the form of magnesium. Although these by-products contain magnesium, this can usually only be used by plants to a limited extent.

It is particularly important for magnesium foliar fertilization that the nutrient is quickly available to plants. After all, acute peaks in demand should be covered or a deficiency rectified immediately. For this reason, the nutrients contained in the fertilizers should be water-soluble and immediately available to plants without prior conversion. ll this applies to magnesium sulfate, as found in the soil fertilizer ESTA Kieserite and the foliar fertilizer EPSO Top. Magnesium oxide, on the other hand, is significantly less available, as the following experiment shows. Furthermore, attention should be paid to the solubility of the various products (see table below).

The different solubility of the magnesium forms can be demonstrated in experiments. In the following laboratory experiment, wheat plants were supplied with a good (plant on the left) or low (plant in the middle) supply of magnesium in the form of ESTA Kieserite via the roots. Subsequently, foliar fertilization with magnesium sulfate was applied to some plants with a low magnesium supply. In other plants, magnesium oxide was applied via the leaf to remedy the deficiency. The rapid effectiveness of magnesium sulphate is visible to the naked eye in the following photos. 

Foliar fertilizers achieve rapid effectiveness if they are quickly soluble in water and can, therefore, be made available to plants in the shortest possible time. There are major differences in the magnesium compounds, which cannot be overcome even by very fine grinding.

As the table shows, magnesium hydroxide or magnesium oxide and magnesium carbonate only have very low solubilities of far less than one gram per liter (g/l) of water, whereas the various forms of magnesium sulfate are soluble at 400 to 700 g per liter of water and can, therefore, be absorbed very well during foliar fertilization.

The product details of the fertilizers offered contain information on the nutrient form in which the magnesium is present. In the EC declaration on nutrient content, all nutrient information is given in oxide form. This is merely a conversion for better comparability of the absolute contents.

Additionally, information on the chemical composition and the actual nutrient form is provided on the packaging or in the product data sheet. For rapid effectiveness in foliar fertilization, magnesium in the form of sulfate should be preferred.