The “How” and “Why” of Soil Sample Fertilizer Recommendations

At Virginia Cooperative Extension, we are big into details:  plants, bugs, and especially dirt (we call it soil out of respect). We want to know why it does what it does, what we can do with it, and how the data proves its capabilities.

One of the services the Hanover Extension office offers the public is soil testing. Soil testing provides individuals with a variety of metrics on their soil’s characteristics and fertility. If your soil is deficient in a particular nutrient, the test report provides a thoughtful, research-based fertilizer recommendation to improve its fertility. These tests are invaluable–without them, gardening is just a gamble. Soil sample kits are available at all times right outside the Hanover County Extension Office’s door (13015 Taylor Complex Lane, Ashland). If you would like more information, visit VCE’s website or call Extension at (804)-752-4310.

Virginia Cooperative Extension has ample resources to help you understand the report, but we think it prudent to spend a little extra time on both the “how” and the “why” of the fertilizer recommendation. Since the “how” is rather tedious, frankly, let’s start with the “why.”

First, do no harm.

Spoiler alert: soil test reports seem complicated. At first glance, they read like a foreign language. What a riot of numbers, abbreviations, and scientific language! They leave many people daydreaming about the good old days when we just guessed how much fertilizer (or lime) our gardens needed – “Weren’t those days great? Weren’t we happy then? So why should we subject ourselves to all this extra work?!” Well, several reasons.

First of all, no matter what Bruce Springsteen says, the good old days weren’t as good as you remember. They weren’t even that long ago! Second, there is a dead zone in the Chesapeake Bay precisely the size of our collected wild guesses. And third, a little thought based on a little information will make you a better gardener. It may even save you money. The principle here is that your garden needs what it needs, and nothing more. If you’re guessing, you’re probably guessing wrong. And if you’re overdoing it, you’re polluting with fertilizer runoff. And you’re wasting money.

When confronted with this information, the great temptation is to throw the baby out with the bathwater. Fertilizer – organic and synthetic – is a tool, just like a spade or hoe. Granted, it’s an easy tool to use recklessly, which gives it a bad name. When fertilizer is applied at the correct time in the correct amounts, it does wonders without doing harm. We are all stewards of the nutrients in our gardens. Our work has immediate local consequences in that it affects the quality of the things we grow. It also has longer, broader consequences for the health and sustainability of our ecosystem. It’s important to understand that this is not a binary choice. In a manner of speaking, we can eat our cake and have it too – but only if we take care. Now, let’s talk about the details.

Wait! Nobody said there was math involved!

Your soil’s nutrient levels are charted in the test report. Each nutrient is listed in units (i.e. parts per million or pounds per acre) and given a rating based on ideal levels (i.e. low, sufficient, or high). Nitrogen, the nutrient darling, is not listed in this chart; it is too mobile in the soil and thus too fickle to warrant analysis. The other nutrients (most notably phosphorus and potassium) are more dependable. These form the basis of the fertilizer recommendation.

All fertilizers – synthetic and organic – have an analysis listed on their packaging, which gives the percentage by weight of macronutrients (Nitrogen, Phosphorus, and Potassium or N-P-K) in the product. For example, 10-10-10 synthetic fertilizer is 10% N, 10% P, and 10% K. These percentages are used to calculate the amount of fertilizer your garden needs based on the soil test recommendation.

Fertilizer recommendations are given by nutrient per area (i.e. apply 2 lbs. N/1000 sq. ft.). The area specified in the report can vary. Depending on your situation, the recommendation could be given per 100 square feet, per 1000 square feet, or per acre. Pay close attention, as your calculation will vary with the figure used in the recommendation. Also, the total amount of fertilizer used will vary by the square footage of your garden.

So how do we go about calculating the amount of fertilizer our gardens need based on the soil test report’s recommendation? First, take a good look at the recommendation. How many pounds of each nutrient does your garden need? Use your newfound knowledge of fertilizer packaging to select a fertilizer whose proportions of macronutrients are similar to the proportions of macronutrients needed by your garden. For example, 10-10-10 is a good fit for a recommendation that specifies an application of 2 lbs. N, 2 lbs. P, and 2 lbs. K, because the proportion of nutrients in the fertilizer equals the proportion of nutrients in the recommendation.

It doesn’t always work out clean and simple. Sometimes the nutrient needs are way out of proportion, in which case several incomplete fertilizers – those that lack one or two macronutrients – can be blended. For more information, see VCE Publication 424-035.

Trust me, it’s easier than it sounds.

It just takes practice. But if math isn’t your cup of tea, contact your county extension office and we will help you through it. Generally, a fertilizer recommendation can be used for two consecutive years. It is good practice to have your soil tested every three years. Keep your previous report on hand, along with a record of your fertilizer selection and calculation. This will help you understand the effect your work has on your soil. The quality of your vegetables is also good anecdotal data to track over time – keeping records has never been so tasty!

by Taylor Adams, Hanover Master Gardener 

For help interpreting the results from your report we can be reached at and by phone at (804)-752-4310.