Potassium and Magnesium in Manures and Organic By-Products

Keywords: animal manures, availability, composts, magnesium, organic by-products, potassium.

Potassium and magnesium are essential nutrients for crop development. In the context of the Nitrates Directive and the Water Framework Directive, the focus is directed more towards nitrogen and phosphorus than towards potassium and magnesium, mainly because K and Mg have no real direct effects on the environment.

In intensive agriculture the potassium and magnesium release from primary and secondary soil minerals is too low to compensate for the offtake by the crop. Besides mineral fertilisers, large amounts of potassium and magnesium can be applied to the soil via animal manures and other organic by-products. Concentrations of K and Mg in all kinds of animal manures, composts, spent mushroom composts (champosts) and some secondary materials originating from processing agricultural products are given for various countries. From this overview a large variation in K and Mg content is obvious between countries but even more in a specific country. Causes for this high variability are diverse but to a large extent due to differences in dry matter content and the original contents in the primary products.

It is generally accepted that the efficiency of K and Mg in animal manures and organic by-products is of the order of 80 to 100% compared to mineral K- and Mg-containing fertilisers. On the other hand, if applications of these organic materials are made during winter time, quite important losses, especially for potassium, can occur in light textured soils.

Knowledge of at least the order of magnitude of the potassium and magnesium contents in these organic materials is necessary for the promotion of a sustainable agriculture.

Joost Salomez, Sara De Bolle, Stefaan De Neve and Georges Hofman, Dept of Soil Management and Soil Care, Ghent University, Belgium.

Jan Bries, Soil Service of Belgium, Heverlee, Belgium.

24 pages, 2 figures, 22 tables, 17 references