How to Figure Out the NPK Ratio in Plants

Take several soil samples from different areas of your garden. Use a garden shovel and dig down at least 12 to 18 inches so you access the soil where the roots of your plants will be absorbing the nutrients. Mix the samples together and place in a resealable bag. Alternatively, you can sometimes get soil test kits from your local extension office. Send the sample to the testing laboratory for analysis.

Measure the square footage of your garden to figure how much fertilizer you need. Multiply the length of one side by the length of the second side to get the total square footage.

Refer to the test results to see how much nitrogen they recommend adding to your soil, usually an amount like 0.1 lbs per 100 square feet. Divide the square footage of your garden by 100 and multiply that answer by the recommended amount. For example if you have a 400-square foot garden, you divide it by 100 square feet and get 4, which you then multiply by -- for example -- a recommended addition of 0.1 pounds of additional nitrogen to get 0.4 pounds of nitrogen for your 400-square foot garden. Do the same for phosphorus and potassium.

Refer to the numbers on the bag of fertilizer to figure how much fertilizer you need to get. Usually, test results will show a need for more nitrogen and maybe potassium and even less phosphorus. Match up the nitrogen numbers first. In the example in Step 3, you needed 0.4 pounds of nitrogen to fertilize the garden. A 10-pound bag of 25-5-5 fertilizer holds 25 percent nitrogen, 5 percent phosphorus and 5 percent potassium; 25 percent of 10 pounds is 2.5 pounds of nitrogen for the whole bag. Divide the 2.5 pounds by 0.4 and you get a little more than 6, meaning that you only need to apply one-sixth of the bag to the garden, and have plenty left for the following spring.

Recalculate if you add fertilizers together. You may have occasion to add together two different types of fertilizer (synthetic fertilizer and bone meal, for example) and need to figure the NPK ratio of the combination. Using the NPK percentages on the label and total weight, figure the combined NPK ratios by adding weighted percentages together. For example, combining a 10-pound bag of 20-5-5 fertilizer with a 5-pound bag of 10-5-5 fertilizer yields a 15-pound bag with of 2.5 pounds of nitrogen, 0.75 pounds of phosphorus and 0.75 pounds of potassium or, in NPK ratios, 16.5-5-5 fertilizer.