Background <p>Apples are important cultivated fruit crops grown worldwide and represent one of the most nutritious foods in a healthy diet. They are rich in nutrients, such as sugars, organic acids, vitamins, and amino acids, and possess a pleasant aroma. Spraying different types of potash fertilizers on apples can affect the fruit quality at maturity. However, the underlying molecular mechanisms of this effect remain unclear. In this study, two types of potassium (K) fertilizers (K₂SO₄ and KH₂PO₄) were applied to the leaves of Haruka apples to elucidate the regulatory mechanisms of different K fertilizer types of on fruit nutritional quality and aroma, construct ceRNA network, and identify the most effective K fertilizer type.</p> Results <p>Seventy-seven and 45 differentially altered compounds (DCCs) were identified in the nutritional and volatile compounds of fruit flesh of apples under K₂SO₄ treatment, whereas 25 and 8 DCCs were identified under the KH₂PO₄ treatment, respectively. Both K₂SO₄ and KH₂PO₄ increased the content of tyrosine, vitamin B2, and lipids in mature apples. Additionally, K₂SO₄ enhanced the nicotinamide content. KH₂PO₄ improved the aroma of apples during ripening only through the lipoxygenase (LOX) pathway, whereas K₂SO₄ improved the aroma during ripening through the LOX, MEP, and shikimate pathways. K fertilizer treatment increased the expression of <i>LOX</i> and alcohol dehydrogenase (<i>ADH</i>) genes by regulating the LOX pathway, whereas the expression of <i>LOX</i> and <i>ADH</i> genes was downregulated in the KH₂PO₄ treatment compared with that in the K₂SO₄ treatment. ceRNA networks associated with apples sprayed with potash fertilizer were also established, showing that mdm-miR159a and PC-3p-45634_116 are involved in apple aroma and lipid synthesis.</p> Conclusions <p>The application of K₂SO₄ was more effective than that of KH₂PO₄ at improving fruit nutritional quality and aroma, offering valuable guidance for production in orchards.</p> Graphical Abstract <p></p>