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How to Calculate a Fertilizer Ratio

If your soil test report recommends applying 1.5 lb of nitrogen, 0.5 lb of phosphate, and 0.5 lb of potash per 1000 sq ft, you should apply a fertilizer with a ratio of 3-1-1 since you need three times as much nitrogen as phosphate and three times as much nitrogen as potash. The simplest method of determining a ratio is to divide the weights of nitrogen, phosphate, and potash by the lowest weight of the three.

Example:

To determine a fertilizer ratio for a recommendation of 1.5 lb of nitrogen, 0.5 lb of phosphate, and 0.5 lb of potash, divide the weight of each of the three nutrients (1.5 lb, 0.5 lb, 0.5 lb) by the nutrient with the lowest weight (0.5 lb).

1.5 ÷ 0.5 = 3

0.5 ÷ 0.5 = 1

0.5 ÷ 0.5 = 1

Thus, the fertilizer ratio that best fits this recommendation is 3-1-1. You can also determine fertilizer ratios from fertilizer grades by dividing the percentage of nitrogen, phosphate, and potash by the lowest percentage of the three nutrients.

Example:

Determine ratios for the following fertilizer grades: 21-7-7. 22-6-8, and 18-5-9.
Grade = 21-7-7 Grade = 22-6-8 Grade = 18-5-9
21 ÷ 7 = 3 22 ÷ 6 = 3.7 
18 ÷ 5 = 3.6 
7 ÷ 7 = 1 6 ÷ 6 = 1.0 5 ÷ 5 = 1.0
7 ÷ 7 = 1 8 ÷ 6 = 1.3 9 ÷ 5 = 1.8
Ratio = 3-1-1 Ratio = 3.7-1-1.3 Ratio = 3.6-1-1.8