Understanding Your Virginia Soil Test Report

By Gil Medeiros, Former Fairfax Master Gardener
“Don’t guess; soil test!” As master gardeners, we frequently exhort gardeners to do the test, but we are not so helpful in explaining what it means. The reason is that the soil test report itself is an agricultural document designed primarily for farmers and soil scientists. Backyard gardeners struggle to understand the meaning of most of it. Without going into too much chemistry, I will try to impart a working knowledge of the soil test report and what to do with it, simplifying matters for the backyard gardener. Without such adjustments, understanding what to do with your soil test report would not be possible without a degree in chemistry.

We are fortunate in Virginia to have reasonably priced, fast and accurate soil tests. In California and some other states, there is no literature to guide interpretation of state soil test reports because there are no state soil tests.
Soil test form

Purpose of the soil test report

The soil test report should help you understand three things:

1. The levels of nutrients available to plants in your soil
Nutrients are the minerals that plants require for growth. The soil test report contains availability levels of these nutrients: phosphorus, potassium, calcium, magnesium, zinc, copper, iron, manganese and boron. The units of measurement are given in either pounds per acre or parts per million.

Why should you care? Even if only one nutrient is deficient, and all the rest are OK, plants will not thrive. This is known in horticulture as Leibig’s Law of the Minimum, and it is why we pay attention to all of the nutrient levels in the report. Fairfax County soil, composed of weathered clay from the gradual erosion of the Piedmont, usually has enough of these nutrients to support plant growth – but not always. The soil test report provides some recommendations about how to correct any nutrient deficiencies that show up in the test.

Although nitrogen is the most important of all plant nutrients, critical for all plant growth, it is NOT included in Virginia soil test results, nor is it included in the reports of most other state testing labs. The reason is that nitrogen levels in soil fluctuate widely due to temperature and rainfall. The lack of consistency makes a nitrogen measurement useless to the gardener. So we handle it this way: We assume there is no nitrogen in the soil at the beginning of the growing season –- the plants used most of it the previous year and what’s left was leached out by rainfall or evaporated as ammonia. If you are an organic gardener, y