The objective of this research was to evaluate the effectiveness of soil and foliar application of potassium (K) on leaf and seed mineral concentration levels, and seed composition (protein, oil, fatty acids, and minerals). Soybean cultivar (Pioneer 95470) of maturity group 5.7 was grown in a repeated greenhouse experiment in a randomized complete block design. Treatment consisted of two concentrations of foliar K application (T1, rate of 1.75% and T2, rate of 2.5%) and soil application (T3, rate of 190 mg/kg and T4, rate of 380 mg/kg). Potassium was applied for each type at V3 (vegetative) and R3 (beginning of seed pod initiation) stages. The results showed higher K and S concentrations in leaves in T1 and T2. The concentrations of B and Zn decreased in all treatments, whereas Fe concentration increased in T1 and T3. In seeds, most mineral concentrations were stable, except for Fe which increased in both T1 and T3. Seed protein percentage increased 3.0% in T3 compared with the control (no K application). Seed oil percentage showed a general decrease in all the treatments, except for 3.2% increase in T4. Palmitic acid percentages showed significant increase in all concentrations, the highest percentage increase of 16.9% was observed in T4. Stearic acid increased in T2 and T3. Linoleic acid percentages increased in both foliar treatments, but linolenic acid percentage increased in high soil treatment T4 alone, with an increase of 12.2% in comparison to the control. Significant decrease (15.8%) in linoleic acid was found in foliar application, T2. Oleic acid decreased uniformly in all treatments, where the highest decrease (19.2%) was observed in soil application, T4. Our research demonstrated that both foliar and soil application of K were found to selectively alter seed composition. Further research is needed to be conducted under field conditions before conclusions can be made.