You may have heard that soil testing is an important part of keeping your garden healthy. However, you might be wondering exactly what a soil test tells you, and how you can act on that information.
So, what does a soil test tell you? A soil test tells you about the health of your soil, and how you can improve it. Specifically, a soil test tells you the levels of acidity (pH) and nutrients in your soil, along with levels of organic matter. If you send information about the plants you are growing, a lab can give you recommendations on how to treat your soil to improve its fertility.
You can buy a do-it-yourself soil test kit (digital or otherwise) online or at a garden store. However, these tests will not give you as much information as you would get from a soil test done by a lab. Also, you will not get specific recommendations from experts, based on the plants you are growing.
In this article, we’ll talk about what a soil test from a lab will tell you, along with ways to improve your soil. At the end, we’ll go over a few frequently asked questions about soil testing. If you want more information about the process itself, check out my article on how to do a soil test.
What Does A Soil Test Tell You?
As mentioned above, a soil test tells you about the health of your soil by indicating the levels of acidity (pH) and nutrients in your soil. The nutrients measured include phosphorus, potassium, calcium, magnesium, sulfur, and other trace elements.
A soil test may also give information about potentially harmful elements in your soil, including lead, aluminum, and salts. You may need to request some of these tests, and they may cost extra.
Generally, you will receive an indication of the level of each nutrient in your soil. For instance, the University of Maine uses four basic levels:
- Low – the amount of the nutrient is severely deficient, and may take years to correct as you gradually add amendments to the soil to offset the deficiency. Plant growth will be negatively affected.
- Medium – the amount of the nutrient is not severely deficient, but it may still limit growth later in the season, or for a plant that uses large amounts of the nutrient.
- Optimum – the amount of the nutrient is at the ideal level, and no soil amendment or treatment is needed for this nutrient.
- Above Optimum – there is too much of this nutrient in the soil. In some cases, this may interfere with a plant’s ability to absorb other nutrients from the soil (for instance, excess magnesium can prevent plants from absorbing enough calcium).
For more information, check out this resource (PDF) on soil test information from the University of Maine.
Let’s take a closer look at some of the things a soil test will measure, and how you might go about making adjustments.
Acidity or Alkalinity of Soil (Soil pH)
The soil pH tells you whether the soil is acidic (pH less than 7.0), neutral (pH equal to 7.0), or basic (pH greater than 7.0). Generally, you will want the following pH levels, depending on your application:
- Garden – a pH of 6.0 to 7.0 (slightly acidic to neutral) is appropriate for most garden plants.
- Lawn – a pH of 5.5 to 6.5 (slightly to somewhat acidic) is appropriate for your lawn.
- Potatoes – a pH of 5.5 to 6.0 (somewhat acidic) is best for growing potatoes.
- Acid-Loving Plants – a pH of below 5.0 (acidic) is best for acid-loving plants, including blueberries and azaleas. Pine trees also like acidic soil.
*Note: the actual soil pH alone is not enough to give recommendations for how to adjust your soil. The lab will also include a lime index, which gives the exchangeable or “reserve” acidity in the soil.
What If My Soil Is Too Acidic (pH Too Low)?
If your soil is too acidic for the plants you are trying to grow, then certain nutrients (including NPK, or nitrogen, phosphorus, and potassium) will not be as easily absorbed by a plant’s roots.
In this case, the lab will recommend increasing the pH by adding lime (calcium carbonate) to the soil. Depending on the actual pH and exchangeable acidity, they will tell you how much lime to add per 1000 or 100 square feet.
For instance, let’s say the lab recommends 25 pounds of lime per 1000 square feet. If your garden is 40 feet long by 50 feet wide, then the area of your garden is 40*50 = 2000 square feet.
Then you have two areas of 1000 square feet, and you would need to add 2*25 = 50 pounds of lime, spread evenly through your garden.
A good way to spread the lime evenly is to partition the garden into square areas, and add the lime a little at a time. For example, if you split the garden into 10 foot by 10 foot squares, then each square needs 2.5 pounds of lime.
For more information, check out my article on how to lower soil pH.
What If My Soil Is Too Alkaline (pH Too High)?
If your soil is too alkaline for the plants you are trying to grow, then you may also face the problem of certain nutrients becoming less available to plants. This is more likely for acid-loving plants, such as blueberries.
In this case, the lab will recommend decreasing the pH by adding sulfur to the soil. They will tell you how much sulfur to add per 1000 or 100 square feet.
According to the University of Massachusetts, no more than 15 pounds of sulfur at a time should be added to a 1000 square foot area. After adding this amount, wait 4 to 6 months, and then retest the soil, since it takes time for sulfur to lower the pH of the soil.
For more information, check out my article on how to raise soil pH.
Nutrient Levels in Soil
Even if your soil pH is within an acceptable range, you may still have problems with the levels of certain nutrients. Let’s go over some of the ones you will see on a soil test report.
The first of the “big three” nutrients, nitrogen is one of the most important nutrients for plant growth, but strangely, it is not included in soil test results. The reason is that nitrogen alters its form rapidly, moving from organic form (absorbable by plants) to inorganic form (not absorbable by plants).
This change in form is affected by many factors, including rain, temperature, and soil characteristics. For more information, check out this article on soil testing from the University of Maryland.
In some cases, you can request a nitrate nitrogen test, but as mentioned above, it may not be useful in predicting the need for nitrogen fertilizers.
If you do apply nitrogen fertilizer, remember to use a split application by adding fertilizer in different phases. The reason for this is that soil does not hold nitrogen well.
Also remember that excessive organic matter, such as that from fresh manure, can cause swings in nitrogen levels. For more information, check out my article on over fertilizing your plants and my article on low-nitrogen fertilizers.
Sometimes, you can diagnose nitrogen deficiencies visually. For more information, check out my article about diagnosing nitrogen deficiency.
The second of the “big three” nutrients, phosphorus helps plants to use the energy produced by photosynthesis. A lack of phosphorus can cause slow growth, weak roots, and low fruit yield.
To add more phosphorus to your soil, find a fertilizer that has a high 2nd (middle) number on its label (in NPK, the 2nd letter, P, stands for phosphorus). A fertilizer with the label 10-10-10 is 10% nitrogen by weight, 10% phosphorus, and 10% potassium.
Note that plants may have difficulty absorbing phosphorus when soil temperatures are low, so you may get a recommendation to use a phosphorus fertilizer in early spring.
The third of the “big three” nutrients, potassium helps plants to use nitrogen and water effectively. A lack of potassium makes plants susceptible to disease. If levels are less than 250 pounds per acre, you may get a recommendation to use a potassium fertilizer.
To add more potassium to your soil, find a fertilizer that has a high 3rd (final) number on its label (in NPK, the 3rd letter, K, stands for potassium). A fertilizer with the label 10-10-10 is 10% nitrogen by weight, 10% phosphorus, and 10% potassium.
You may be able to identify a potassium deficiency in your plants visually. For more information, check out my article on how to diagnose potassium deficiency.
Calcium is necessary for healthy plant cell walls, and is necessary for fruit and roots to form properly. Many gardeners are familiar with blossom end rot, which is caused by calcium deficiency in fruiting plants such as tomatoes and peppers.
To add more calcium to your soil, you can add lime (calcium carbonate) or gypsum (calcium sulfate). Remember that if your soil pH is too low (acidic), you will be adding lime anyway.
Even if there is plenty of calcium in your soil, your plants can still suffer from calcium deficiency. For more information, check out my article on diagnosing a calcium deficiency.
Magnesium is the central atom in a chlorophyll molecule, so you can imagine its importance for plant growth! Be careful about having too much magnesium in your soil -–it can prevent a plant from absorbing calcium, even if there is plenty in the soil.
To add more magnesium to your soil, you can use dolomitic lime (calcium magnesium carbonate) or Epsom salt (magnesium sulfate). Keep in mind that Epsom salt also supplements sulfur.
As with calcium, it is possible for plants to suffer from magnesium deficiency, even if soil levels are adequate. For more information, check out my article on magnesium deficiency.
Sulfur is necessary for photosynthesis and nitrogen fixation. There are several options to add sulfur to your soil:
- Gypsum (calcium sulfate) will supplement both sulfur and calcium in your soil.
- Potassium sulfate will supplement both sulfur and potassium in your soil.
- Sul-Po-Mag will supplement sulfur, potassium, and magnesium in your soil.
Remember that if your soil pH is too high (alkaline), you will be adding sulfur anyway.
There are other trace nutrients that are necessary for plant growth, including:
Plants do not use these nutrients as much as the nutrients mentioned earlier (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur). You are more likely to see deficiencies of these “trace” elements in sandy soils with low organic matter content.
Organic Matter Levels in Soil
Organic matter is simply material containing carbon. Most commonly, this means compost, or decomposed organic matter, which also supplies nutrients in a form that plants can absorb. Compost can be made from manure (animal waste), grass clippings, fallen leaves, sawdust, and fruit or vegetable scraps.
The optimal amount of organic material in soil will depend on the type of soil itself. An ideal range can vary from 2% all the way up to 8% organic material.
Toxic Metal Levels in Soil
Some toxic metals, such as aluminum and lead, can be harmful to plants as well as humans. If lead levels are above 300 ppm (parts per million), it is a cause for concern.
Plants such as beets, carrots, and lettuce are very sensitive to aluminum in soil. As soil pH gets lower (more acidic), aluminum becomes more soluble, and thus high levels in soil will be more harmful to plants.
Salt Levels in Soil
You can also get a test for salt levels in your soil, such as sodium. However, note that you may need to specifically request these tests, and they may cost extra.
Common Questions About Soil Testing
Let’s answer a couple of common questions about the frequency and timing of soil testing.
How Often Should I Test My Soil?
It depends on the health of your soil, as determined by your first soil test. If your soil is healthy and fertile, you might only need to test your soil every two or three years, to make sure you are still on the right track.
On the other hand, if you have severe deficiencies of any nutrient, or extremely high or low soil pH, you should test your soil one or two times per year.
For example, it can take several months for sulfur to work to lower soil pH. In that case, you might want to test pH once in the fall and once in the spring, before you decide to add more sulfur to your soil.
Once your soil is stable in terms of nutrients and pH, you can test less frequently.
When Should I Test My Soil?
You can test any time, but remember that many gardeners will wait until the last minute and test in the spring. At that point, test results may be delayed due to the flood of soil test requests.
Testing in the fall lets you avoid the rush, and also gives you time to improve the soil and let any amendments do their work.
By now, you should have a good idea of what a soil test will tell you, and how to act on that information to improve your soil.
I hope this article was helpful. If you have any questions or advice of your own about soil testing, please leave a comment below.