Testing polyhalite as a tool to overcome nutrient deficiencies in organic cabbage culture

Summary

In addition to regular nitrogen (N), phosphorus (P), and potassium (K) requirements, Brassica crops need significant amounts of calcium (Ca), magnesium (Mg), and sulfur (S). The organic farming approach restricts the use of chemical fertilizers, considerably challenging balanced mineral nutrition of cole crops such as cabbage (Brassica oleracea var. oleracea). Polyhalite, a natural mineral, is an authorized fertilizer (Polysulphate®) for producers of organic crops in many countries. Consisting of 14% K2O, 48% SO3, 6% MgO, and 17% CaO, polyhalite can be considered a useful supplementary fertilizer of four essential nutrients in organic farming. The objectives of the present study were to evaluate the effects of polyhalite application on cabbage performance and compare it with equivalent commercial organic fertilizers. When used on fertile soil, rich with K, Ca, and Mg, the effects of supplementary nutrition on crop performance were absent. Sulfur uptake tended to be greater with polyhalite than in the non-fertilized control (p = 0.071), driven by a combination of increased marketable yield as well as S concentration in leaves. Sulfur nutrition appeared key to enhancing cabbage crop performance. Polyhalite displayed a strong tendency to enhance and stabilize yields, compared to alternatives. It tended to be better as a Ca donor compared to foliar Ca application, and was at least equivalent to gypsum. Harbouring four essential nutrients, polyhalite may be a suitable fertilizer, particularly for the organic market. It is advantageous being natural and easy to spread. However, in order to fully demonstrate the advantages of polyhalite, it should also be tested at sites with much weaker soil fertility.