How Long Are Soil Tests Good For?

When it comes to testing your soil, there are multiple reasons why you may want to carry out this procedure and different methods to use. You may even decide to test your soil when it is frozen. However, regardless of how or why you may decide to test your soil, you might wonder how long soil tests are good for.

Soil tests are typically good for 2 to 3 years. After that time frame, the soil is likely to have changed in its composition, making it such that the last test you took is likely not representative of the current soil. If your soil undergoes intensive farming, you may need to test it more often.

Understanding soil tests and how soil changes over time can be difficult but it’s extremely beneficial to the health of your soil. Read on to learn more about how long soil tests last, how long soil samples last, and how soil changes over time to make previous results invalid. You will also find out if soil test results can be made to last longer.

Longevity of Soil Tests

Soil tests come in a number of different ways. For most people, you can purchase a soil test kit at a local gardening store or online that contains a number of components that are typically tested for. In other instances, you can collect soil samples and send them to labs that will test for certain components you might want to isolate in your soil.

Regardless of the type of soil test you use, there are multiple pieces of information that inform how long a soil test will last. Soil changes over time, causing the results you receive in a soil test to grow inaccurate with the passing of time. Additionally, actual soil tests can also grow old and become no longer good for testing.

For soil tests that are completed at home, most soil test kits need to be used within a few months. The test kits often contain pH strips to test the acidity of your soil. These tests work by creating a miniature chemical reaction that causes the strip to turn a different color depending on the soil’s pH. These strips rely on the chemicals on the strip to remain active, which means that after the chemicals expire, the strips will not work. 

Additionally, soil test kits that are not kept in an airtight container or are exposed to fluids are likely to expire, and the test will no longer work. It’s especially important that in summer, these tests are kept out of direct sunlight as condensation that might build in the test kit can damage or destroy it, making any results invalid. 

Longevity of Soil Samples

In addition to the actual test, the other major component of soil tests is the soil samples themselves. 

Soil samples —  the collection of dirt 6 to 8 inches (15.25 cm to 20.3 cm) deep from multiple locations around a testing plot — can themselves go bad or expire if not used or stored correctly. For most home gardeners, soil samples are tested almost immediately after collection, meaning that you don’t need to worry about the samples expiring.

However, if you find yourself in a situation where you cannot immediately test the soil after completing the sampling, note that soil samples can expire if too old or not stored correctly. Most samples, when kept correctly, can be stored for up to 4 years. In some cases, soil can even be preserved for as long as ten years.

While professional researchers are normally the only ones who might find a need to store a soil sample for that long, the implications of these results can impact others as well.

Bearing this in mind:

  • If you are testing your soil in the winter, allow it to thaw and dry for 24 hours before conducting your test. 
  • If you plan to test your soil soon after collection but not immediately, allow the soil to sit in a dark, cool place.
  • If you plan on storing your sample for a long period of time, allow it to dry out completely. Additionally, place it in a dark, cold location that is not exposed to other elements like air or water. As long as these soils are kept largely dry and airtight, they are usable for testing within 1 month. 
  • If you plan on storing the soil for longer than 1 month, consider freezing the sample.

Test Validity Over Time

One of the tricky aspects of testing soil is that, over time, soil tests will no longer be representative of the soil’s actual contents. While this might seem confusing, from a common-sense standpoint, it’s understandable. 

It would be a little extreme to assume that the contents of the ground you are standing upon are exactly the same as when the T-Rex roamed the Earth millions of years ago. This just doesn’t make sense.

What is somewhat surprising, however, is just how quickly soil changes over time. Within just two years, soil can be different enough from how it was prior to impact how plants grow. Even on an annual basis, soil changes are ample that, if measured, you would likely find a meaningful difference in the amounts of certain soil components on a year-to-year basis. 

This is because, as the soil gets exposed to the elements, the chemicals contained in the soil change rapidly. Think about every time it rains; moisture and chemicals from high up in the atmosphere fall onto the ground far below, mixing the contents and chemical properties. Imagine, for example, if the rain was acidic due to pollution. If it rained every day for a week, the soil would likely be more acidic afterward.

While the simple passage of time will average out these extreme scenarios, it is still more likely than not that the soil will change slightly after two years of exposure to the elements. Granted, it isn’t necessarily certain that the soil will change, but most likely it will. Meaning that after two years, soil tests from the past will likely no longer be representative of the current soil.

Maintaining Test Validity Over Time

Eventually, every soil test will stop being representative of the soil it was taken from, regardless of the sample and test quality. Fortunately, there are steps you can take to maintain test representativeness for longer amounts of time. 

One such way is to keep your soil in a controlled environment. Try growing plants in a greenhouse or in a plot inside in order to limit exposure to the elements. 

Another way to help maintain test validity is to plant the same type of plant in the same soil. By doing this year after year, you can use a guide to measure how much of what materials your plant is using from the soil. You can then provide the soil with additives to account for changes in the soil caused by your plant.

Furthermore, when conducting soil tests, be sure to do it in the same plot of land under the same conditions whenever possible. Thus, if you typically test soil in April after a week of no rain, do likewise when taking your next soil test. Doing so will allow you to get a better picture of how much your soil has changed over time, allowing you to determine if you need to test your soil as frequently.

Final Thoughts

When it comes to the validity of soil tests over time, there are a number of factors that can influence how long a soil test is good for. While soil tests and soil samples themselves can both expire, over time, soil undergoes significant change such that, after about two years, soil samples done on that soil should be retested.

As such, if your plant requires very specific conditions to survive, try testing your soil every year. Otherwise, most soil test results will be valid for about two years if the test isn’t expired and takes place immediately after sampling.

If you want to learn more about improving soil quality, you can read my other article here: How to Improve Soil Quality (The Ultimate Guide)

Alexander Picot

Alexander Picot is the founder of and its lead content writer. He created the website in 2022 as a resource for horticulture lovers and beginners alike, compiling all the gardening tips he discovered over the years. Alex has a passion for caring for plants, turning backyards into feel-good places, and sharing his knowledge with the rest of the world.

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