How to Tailor Fertilizer Recommendations to the Fertilizers You Have

Most soil test reports include science-based recommendations for applying fertilizers to help avoid overapplying nutrients. Overapplying nutrients not only costs more, but can harm the environment. As the saying goes, "Don't guess, soil test."

We know that soil testing isn't always possible. In the absence of a soil test, a general recommendation is to apply 1.75 pounds of 10-10-10 per 100 square feet of your vegetable garden for the upcoming growing season. While this guideline supports healthy plants, we still strongly encourage soil testing in the future.

Understanding Your Soil Test Report

Let's walk through the example soil test report below. Near the top, you'll see the blue bars representing the pH and the amount of phosphate, potash, magnesium, and calcium in the soil (within the circle at the upper part of the report). In this report, the soil pH is within the optimum range, and the nutrient levels are above optimum. This means that pH adjustments are not necessary, and phosphorus and potassium fertilizers should not be used, as more than enough of these nutrients are already present in the soil.

Standard soil tests do not measure nitrogen because nitrogen levels can change rapidly due to factors such as flushing out of the root zone, loss to the atmosphere, soil microorganism activity, and plant uptake. Instead, nitrogen recommendations are generally based on crop needs and historical data, not direct measurement.

The recommendation from this soil test report is to apply 0.5 lbs of urea per 100 square feet of area to meet the plant's nitrogen needs (within the circle at the bottom of the report). Adding phosphate and potash is not recommended because more than the plants can use in a single growing season is already in the soil.

Another Example, When Other Nutrients are Needed

Let's look at another example. In this report, the pH and the calcium level are within the optimal range, while phosphate and potash levels are below optimum, and the magnesium level is above optimum. In this case, the plants will benefit from adding phosphate and potash to the soil.

The report's recommendation is to apply 1.75 pounds per 100 square feet of 10-10-10 fertilizer.

If you don't have 10-10-10 fertilizer, you can adjust the amount applied based on the type of fertilizer you do have.

How to Adjust Fertilizer Amounts

Start by focusing on nitrogen, then phosphate, as an excess of these nutrients can cause environmental problems.

For example, you have a 20-10-20 fertilizer instead of a 10-10-10 fertilizer. 10-10-10 fertilizer has half the nitrogen of the 20-10-20 fertilizer. You can reduce the recommended amount by half.

1.75 lbs. per 100 square feet of 10-10-10 → apply half → 0.88 lbs of 20-10-20 per 100 square feet

A Simple Calculation Approach

Here's another soil test report. The recommendation is to add 2.5 lbs per 100 square feet of 5-10-10 and 1.0 lbs per 100 square feet of 0-46-0 (triple phosphate).

Step 1: Calculate the Nutrient Amounts

To calculate the amounts of nitrogen, phosphate, and potash to apply, multiply the recommended amount of fertilizer by the decimal form of each nutrient percentage in the fertilizer. If you're using more than one fertilizer, add the amounts together. As you do this, remember to convert the percentages to decimals before multiplying. For example, 10% becomes 0.10.

For our example, we're applying nutrients at the following rates per 100 square feet: 0.13 lb nitrogen (2.5 x 0.05), 0.71 lb phosphate (2.5 x 0.10 for 5-10-10 and 1.0 x 0.46 for 0-46-0 added together), and 0.25 lb potash (2.5 X 0.1).

Step 2: Find the N:P:K Ratio

Determine the ratio between nitrogen, phosphate, and potash (N:P:K) by dividing the application rates by the nitrogen rate.

Nitrogen → 0.13 ÷ 0.13 = 1

Phosphate → 0.71 ÷ 0.13 = 5.4

Potash → 0.25 ÷ 0.13 = 1.9

After rounding, the N:P:K ratio is 1:5:2. 

Using a Different Fertilizer

Let's say you have 15-15-15 fertilizer. This fertilizer has an N:P:K ratio of 1:1:1, which does not exactly match the recommended ratio. To avoid overapplying nitrogen, base your application on the nitrogen recommendation.

To determine how much fertilizer to apply, divide the recommended amount of each nutrient by the decimal form of its percentage in the fertilizer.

0.13 lbs per 100 square feet ÷ 0.15 percent nitrogen = 0.87 lbs of 15-15-15 per 100 square feet. This means applying 0.87 lbs of 15-15-15 fertilizer per 100 square feet will give you 0.13 lbs of nitrogen.

You can also calculate the amounts of phosphate and potash in 0.87 lbs of 15-15-15 fertilizer by reusing the Step 1 calculation. Since 15-15-15 has 15% phosphate and 15% potash, multiply 0.87 lbs by 0.15 for each nutrient.

Phosphate → 0.87 lbs x 0.15 = 0.13 lbs

Potash → 0.87 lbs x 0.15 = 0.13 lbs

When applying 0.87 lbs of 15-15-15 fertilizer to 100 square feet, you're also applying 0.13 lb of phosphate and 0.13 lb of potash.

Subtracting the amounts already supplied from the target application rates, we find that we need 0.58 more lbs of phosphate (0.71 lbs – 0.13 lbs = 0.58) and 0.12 lbs of potash (0.25 lbs – 0.13 lbs = 0.12 lbs) per 100 square feet.

Step 3: Use Single-Nutrient Fertilizers to Complete the Recommendation

To complete the recommendation, use fertilizers that only supply phosphate or potash, such as triple phosphate (0-45-0) or muriate of potash (0-0-60). Divide the remaining amount needed by the decimal form of its percentage in the fertilizer.

Phosphate → 0.58 lbs phosphate per 100 square feet ÷ 0.45 percent phosphate in 0-45-0  fertilizer = 1.3.

Apply 1.3 lbs of triple phosphate (0-45-0) per 100 square feet.

Potash → 0.12 lbs potash per 100 square feet ÷ 0.60 percent potash in 0-0-60 fertilizer = 0.20.

Apply 0.20 lbs of muriate of potash (0-0-60) per 100 square feet.

Every Expert was once a Beginner

Like any new skill, adjusting fertilizer recommendations takes practice. However, once you get the hang of it, you'll have the flexibility to use the fertilizers you already have.

This article benefited from editorial assistance provided by Microsoft Copilot.