The experiment was established in the spring 1999 on lessive soil developed from light boulder clay. Apple trees of Golden Delicious cv. on M.26 rootstock were planted in 3.5 × 1.2 m (2381 trees·ha^-1) spacing. Research involved the effect of diversified levels of potassium fertilization: 12, 16 and 20 mg K 100 g^-1 d.m. of soil and three types of potassium fertilizers: potassium chloride (KCl), potassium sulfate (K₂SO₄) and potassium nitrate (KNO₃) on trees yielding and nutrition, as well as nutrient contents in the soil. Potassium fertilization using potassium chloride, potassium sulfate and potassium nitrate resulted in increased content of available potassium both in arable (0–20 cm) and in subarable (20–40 cm) layers of an apple orchard. Increase of available potassium content from 12 to 20 mg K∙100 g^-1 f. m of soil, due to fertilization with potassium chloride, potassium sulfate and potassium nitrate did increase soil pH (in H₂O) in arable and subarable layers as a result of higher contents of available calcium. Alkalizing effect of potassium sulfate and potassium nitrate, generally assumed as physiologically acidifying, occurred on neutral and alkaline soils [pH (in H₂O) over 6.90]. Fertilization with potassium chloride, potassium sulfate and potassium nitrate increased the content of sulfates in arable and subarable layers. The same result was observed in the case of chlorides in subarable layer. Application of potassium fertilization with potassium nitrate brought about decreased values of available magnesium content only in subarable layer. Due to the use of high rates of potassium nitrate, on the level 20 mg K·100 g^-1 of soil, there was observed increased soil salinity (EC) both in arable and subarable layers. Introduction of potassium fertilization and increasingly higher rates of potassium fertilizers did not influence on the content of nitrogen, phosphorus, potassium, calcium, magnesium, sulfur and chlorine in apple tree leaves. Increasing potassium levels in the soil and potassium fertilizers did not affect the yield of Golden Delicious apple trees after coming them into full fruition period.

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