Fertilizer bag numbers reveal nutrient content

Published 8:44 am Monday, February 27, 2023

By Keri Jones
MSU Extension Service

Go down the garden section of any home improvement store, and you will find a dizzying array of fertilizer options available to help you reach your garden goals. But which one should you choose?

The numbers on each bag of fertilizer mean something different, so let’s take some of the confusion out of this common problem.

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Every gardener knows plants need sunlight and water. There are also several nutrients plants need to obtain from soil. If the soil does not contain enough of these required nutrients, you can add them with commercial fertilizers or compost.

Some necessary nutrients are needed in higher quantiles than others.

Three soil-obtained nutrients needed in highest quantities are nitrogen, phosphorus and potassium, also known as N, P and K. These nutrients are readily available in various amounts in most fertilizer bags.

On a fertilizer bag, there typically are three numbers listed in bold lettering separated by dashes. These numbers represent the percent by weight of the three nutrients inside the bag: nitrogen first, phosphorus expressed as phosphate second and potassium expressed as potash third.

One example of a fertilizer blend catered to vegetable growers is 4-18-38. This blend contains 4% nitrogen, 18% phosphate and 38% potash. Note that the three percentages do not add up to 100. It is normal for commercial fertilizer blends to contain filler material that make it easier for gardeners to apply.

Because each nutrient is listed by weight, you have to use twice as much of the blend 10-10-10 as the blend 20-20-20 to meet the same nutrient quantity levels.

The decision of which blend best suits your situation relies on the nutrient requirements of the plants you want to grow and the amounts of those nutrients present in your soil.

How do you know how much fertilizer to add to promote plant growth? The easiest way is to get a soil test. Visit the Mississippi State University Soil Testing Lab at http://extension.msstate.edu/content/soil-testing for more information on how to collect, submit and interpret your soil test.

Soil test results will provide recommendations for the quantity of N, P and K required and the best time to apply that fertilizer.

When you are armed with that information, choosing the right fertilizer blend becomes much easier. If your soil test results call for twice as much nitrogen as potassium, then look for a blend with a larger first number and lower third number, such as 30-0-10. If you can’t find exactly the right ratio, prioritize satisfying the nitrogen recommendation first.

If you have a large area to fertilize, choosing a fertilizer with a higher percentage of active ingredient will be more efficient and more cost-effective. Following soil testing recommendations also limits excess application of fertilizer while satisfying the nutrient requirements of your favorite plants, and this saves money.

In addition to the big three of nitrogen, phosphorous and potassium, there are other nutrients plants need to get from the soil that are just as important.

Though they are not needed in the quantity that plants require N, P and K, these micronutrients are vital. If any of them are missing or not found in sufficient quantities in the soil, plant growth will be impacted. Soil testing can provide information on some of these remaining necessary nutrients.

Fortunately, many native Mississippi soils have the necessary nutrients in sufficient quantities for the plants you are growing.