Abstract

Cabbage (Brassica oleracea var. capitata f. alba) is one of the most important vegetable crops in Turkey. The transition from traditional to modern agriculture includes revision of mineral nutrition practices. Beyond the common basal nitrogen (N) and phosphorus (P) application, potassium (K), which is usually ignored by Turkish farmers, is required to enhance yield and quality. There is also increasing awareness that sulfur (S) is an essential macronutrient, particularly for Brassicaceae crop species that produce highly appreciated secondary metabolites such as glucosinolates and antioxidants. Muriate of potash (MOP) and sulfate of potash (SOP) are very common fertilizers. Both are donors of soluble K, and the latter also supplies S. Polyhalite is a natural mineral, which occurs in sedimentary marine evaporates, and consists of a hydrated sulfate of K, calcium (Ca) and magnesium (Mg) with the composition of 14% K₂O, 48% SO₃, 6% MgO, and 17% CaO. The objective of this study was to compare the effects of three different K sources - polyhalite, SOP, and MOP - on cabbage yield, quality, and nutrient content and uptake. Standard N and P₂O₅ rates of 250 and 100 kg ha^–1, respectively, were employed throughout all five treatments included in the experiment. The control treatment was applied with N and P only, while the other four treatments received an equal dose of 300 kg K₂O ha^–1 in the forms of MOP, SOP, polyhalite, or polyhalite+SOP. While MOP application significantly enhanced cabbage crop performance, the additional S, provided through SOP or polyhalite, obtained much higher yields and better quality. The best treatment was polyhalite+SOP, with a marketable yield of 81 Mg ha^–1, 60% higher than the control. While leaf S content and uptake did not differ among SOP, polyhalite, and polyhalite+SOP, polyhalite significantly raised leaf Ca and Mg contents, which may explain its advantage over SOP. Whereas polyhalite application as the sole K source is impractical, a suitable combination of polyhalite with SOP provides a promising solution for Turkish cabbage growers. The optimum rates for the two fertilizers should be determined considering crop requirements, soil nutrient status, and fertilizer cost vs. the expected benefits.

Keywords: Brassica oleracea var. capitata f. alba; MOP; polyhalite; SOP.