Soil Test

Reliable soil tests are the only way to really know what is in your soil.


Before adding fertilizer to a garden, it is really important to know what is already there. Whether you are growing grapes, tomatoes, or herbs, the nutrients and the microbes in the soil dictate how well those plants will grow. As much as we would all love a convenient, reliable, over-the-counter soil test, it doesn’t exist (yet).

Soil test (Idaho Extension)

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Feeding the soil

Plants use 17 elements to grow. Oxygen (O), hydrogen (H) and carbon (C) are taken from air and water. The other elements (minerals) are absorbed from the soil with the help of microorganisms. There are three primary nutrients: nitrogen (N), phosphorus (P) and potassium (K); three secondary nutrients: calcium (Ca), magnesium (Mg) and sulfur (S); and eight micronutrients: boron (B), chlorine (Cl), cobalt, (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn).


In the 1940’s and 50’s, the idea of “better living through chemistry” led to tons of chemicals being added to agricultural soil. Some of it worked and some of it backfired. The current view is “better living through biology”, which means learning about natural plant life cycles, identifying which nutrients are currently available to plants, and finding natural ways to add whatever is needed.


Time soil tests for the best results

The results obtained from a soil test are not written in stone. Conditions in March will be far different from those in July. Variables such as temperature, moisture, and the molecular exchanges (cation exchange capacity) that are occurring all the time underground can change which nutrients are available to your plants. This is especially true for nitrogen. Signs that a soil test is needed:

1. It’s never been done before.
2. Plants are exhibiting chlorosis (yellowing leaves).
3. Plants are not thriving.
4. It has been 3 years since the last soil test.

My 2015 soil test results from UMass (Kate Russell)

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How to red soil test results

The funny thing about soil chemistry is that the absolute measurements are not nearly as important as the ratios between certain elements. For example, my soil test showed an abundance of every nutrient needed by plants except one: iron. Unfortunately, iron is needed by plants to help them absorb practically everything else. In effect, my plants were sitting at a banquet with their mouths taped shut! By spraying the leaves with foliar iron, which plants can absorb through their leaves, they then had the iron they needed to make everything else available.


​Laboratory soil test results will show two figures for each element reported. One figure is the recommended range and one is what is in your soil. Hopefully, they will be relatively close. If your garden has plenty of a nutrient, it is a waste of time and money to add more. In fact, adding more can compound ratio problems that make nutrients unavailable to plants. And don’t be surprised if your soil test lab does charges extra for nitrogen testing.


Nitrogen is a very fickle, fleeting element that is here and gone before you know it. It is worth the extra cost to find out what current nitrogen levels are, but keep in mind that those figures are only relevant for a few days, since nitrogen responds quickly to changes in temperature and moisture. Nitrogen is the single most limiting factor in plant growth. Regularly adding aged compost and treating with blood meal, cottonseed meal, alfalfa meal, feather meal or fish emulsion can help maintain nitrogen levels.


Do regional differences matter?

Before sending a soil sample to the cheapest lab, keep in mind that regional differences may impact your results. As quoted from the University of Massachusetts (my go-to lab):

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“The UMass Soil Lab accepts soils from all over the continental U.S. However, we use the Modified Morgan extracting solution for nutrient analysis. This extracting solution was developed for New England's sandy, acidic soils, and climate. Additionally, lime and fertilizer recommendations are intended for soil conditions and climate in New England. While useful information can be obtained by getting a soil test at UMass, differences in soil types and growing conditions need to be taken into account when interpreting test results and recommendations.” That being said, the UMass soil testing lab is highly respected. Simply use the results with a grain of salt, or find a local lab. 

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​Rather than wasting time, money and effort by gardening blindly, a good soil test can help you decide which elements are needed and protect the environment and ground water supplies from excess chemicals and nutrients.