Comparative effects of foliar potassium nitrate, thiosulfate, and phosphite on nutrient concentration in lettuce

Foliar potassium sources differ in their accompanying anions, which may influence the uptake of other nutrients; however, their comparative effects on lettuce mineral composition are poorly documented. This study evaluated the effects of different foliar potassium sources on the mineral nutrition of iceberg lettuce (Lactuca sativa L.) grown commercially in the Ödemiş district of İzmir Province, Türkiye. The experiment was arranged in a randomized complete block design with four treatments and five replications. Foliar treatments included potassium nitrate (KNO3; 46% K2O; 3 kg/1000 L/ha), potassium thiosulfate (25% K2O; 5.5 L/1000 L/ha), and potassium phosphite (25% K2O; 5.5 L/1000 L/ha), applied three times at 15-day intervals starting 15 days after transplanting. Control plots were sprayed with water only. Soil and plant samples were analyzed for N, P, K, Ca, Mg, Na, Fe, Cu, Mn, and Zn concentrations. Foliar potassium applications increased nutrient concentrations compared with the control. Potassium nitrate resulted in the highest leaf N (37.3–37.8 g kg−1) and K (52.1–52.3 g kg−1) concentrations. Potassium phosphite produced the highest leaf P concentrations (7.1–7.9 g kg−1). Calcium and magnesium accumulation increased under potassium thiosulfate and potassium phosphite treatments. Iron concentrations increased under all potassium treatments, with the highest value (133 mg kg−1) recorded under potassium thiosulfate in the second year. Zinc and copper concentrations were highest with potassium nitrate. Potassium nitrate provided the most balanced mineral profile, whereas potassium phosphite and potassium thiosulfate showed complementary potential for targeted P management and enhanced Ca, Mg, and Fe accumulation, respectively.