The following research hypotheses were established in this study: the applied urea and ammonium nitrate solution with potassium thiosulfate (UAN-KTS) has a positive effect on the chemical composition of spring wheat, spring rape and maize; fertilization with nitrogen, potassium and sulfur increases their contents in the usable parts of plants; the forms of applied fertilizers reduce the antagonistic effect of nitrogen and potassium on the content of other elements in plants. Two doses of nitrogen (N 1—optimal dose of nitrogen for a plant species; N 2—25% lower dose of nitrogen) and different ratios of N:K:S elements (N:K:S—without K and S; N:K1:S1—a narrowed ratio; N:K2:S2—the optimal ratio; N:K3:S3—an expanded ratio) were applied. The experiment was carried out on two soils of different quality. The improved soil quality resulted in a significant increase in the calcium (as opposed to magnesium) content of the three crops, in the nitrogen and sulfate–sulfur (VI) content of spring wheat grain and spring rapeseed, and in the phosphorus content of spring rapeseed and maize aerial parts. Reducing the dose (N 2) had a negative effect on the nitrogen content of spring wheat and on the sulfate–sulfur (VI) content of spring rape, maize and especially spring wheat. Increasing the N:K:S ratio had a small but usually antagonistic effect on the nitrogen content of all plant species, but a synergistic one on the potassium content of spring wheat, maize and partly of spring rape, and especially on the content of sulfate–sulfur (VI) in the crops. The effect of type of soil and fertilizers with different N:K:S ratios on the content of other macronutrients was related to plant species. The new fertilizer with the N:K2:S2 ratio had the greatest effect on the content of the tested macronutrients in spring wheat, spring rape and maize. In order to confirm the obtained relationships, it seems justified to carry out field experiments and studies with other plant species.