As a first step, we can use the previous formula: Fertilizer rate = Required nutrient application rate X 100 / % Nutrient in the fertilizer Fertilizer rate = 15 x 100 / 30 = 50 lbs. Since 1 gallon of the fertilizer weights 10.86 lbs: 50 lbs. / (10.86 lbs./gallon) = 4.6 gallons.