Fertilizer composition calculations Converting fertilizers A soil test recommends applying 15-15-15 at 0.8 lbs fertilizer per 100 square feet. the 15- 15-15 stands for 15 % Nitrogen 15 % Phosporus and 15% potassium Calculate the amount of 5 % Nitrogen 11% Phorporus 1% Potassium needed for 500 square feet cut flower bed