Abstract

Cotton (Gossypium hirsutum L.) is an important cash crop in Turkey, which supports a long tradition of textile industry and trade. Aiming at enhancing cotton production and quality, the Turkish authorities are promoting a transition from traditional to modern agriculture practices, including the revision of advice regarding mineral nutrition. 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 of sulfur’s (S) significance as an essential macronutrient for cotton crop growth.

Muriate of potash (KCl) and sulfate of potash (K₂SO₄) 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% potassium oxide (K₂O), 48% sulfur trioxide (SO₃), 6% magnesium oxide (MgO), and 17% calcium oxide (CaO).

The objective of this study was to compare the effects of three different K sources – K₂SO₄, polyhalite, and KCl – on cotton yield, quality, and nutrient content and uptake. Standard N and phosphorus pentoxide (P₂O₅) rates of 250 and 184 kg ha^–1, respectively, were employed throughout all four treatments. The control treatment was applied with N and P only, while the other four treatments received an equal dose of 210 kg K₂O ha^–1 in the forms of K₂SO₄, polyhalite, or KCl. Unequivocally, the results of the present study demonstrate the pivotal role of K in cotton production, highlighting the need to enhance its application practices. Results indicate KCl as the preferable K donor to obtain high cotton yields, with 6.3 Mg ha^–1, compared to 2.7, 5.6, and 4.8 Mg ha^–1 for the control, K₂SO₄, and polyhalite, respectively. However, some of the quality properties, such as fiber fineness, length, and elongation performed better under polyhalite and K₂SO₄ fertilizers. The direct and indirect influences of nutrients such as S and Ca on lint development and quality remains to be revealed. Nevertheless, compromising between yield and quality, S containing fertilizers must be considered. While polyhalite appears too slow to stand alone as K donor, this fertilizer shows significant potential, on less calcareous soils, as a slow-release donor of S, Ca, Mg, and K. Basal polyhalite application, in combination with other NPK fertilizers and included in practices suited to meet crop requirements during the season, can provide the Turkish cotton industry with a significant step towards enhancing yields and quality.

Keywords: Gossypium hirsutum L.; MOP; SOP; polyhalite; potassium.