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.

Key Takeaways

  • Validity of Soil Tests: Soil tests are typically good for 2 to 3 years, but may vary with soil use and environmental conditions.
  • Storage and Handling: Store soil test kits in airtight containers, away from moisture and direct sunlight to maintain their effectiveness.
  • Soil Sample Expiration: Freshly collected soil samples are ideal, but if stored correctly, they can remain valid for testing up to 4 years. Longer storage may require freezing.
  • Factors Affecting Soil: Weather, soil amendments, and types of crops grown can significantly alter soil composition over time.
  • Testing Frequency: In busy gardens or farms with multiple annual crops, consider annual testing to monitor changing soil conditions.

Longevity of Soil Tests

Soil tests come in several different ways. You can purchase a soil test kit at a local gardening store or online that contains can test for a few soil characteristics, such as pH and NPK values. You can also collect soil samples and send them to labs that will conduct more thorough tests, such as testing for microbial populations and micronutrient content.

Regardless of the type of soil test you use, the results will not remain valid forever. Soil changes over time, eventually causing the results you receive in a soil test to become inaccurate.

Soil test kits can grow old and no longer be good for testing. Most home soil test kits must be used within a few months, as the product label indicates.

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, meaning that the strips will not work after the chemicals expire. 

Soil test kits not kept in an airtight container or 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. 

Handling and Storage 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 (15-20 cm) deep from multiple locations around a testing plot — can themselves go bad or expire if not used or stored correctly. Soil samples are tested almost immediately after collection, meaning you don’t need to worry about the expiring samples.

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 they are too old or not stored correctly. When kept correctly, most samples 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.

For optimum soil quality for testing, keep the following in mind:

  • When 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 to store your sample for a long time, allow it to dry out completely. Place it in an airtight container 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.

Factors Influencing Soil Test Validity

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.

You might be amazed at how quickly the soil quality can change in a busy garden or farmland. Within just two years, soil can be different enough from how it was due to plant nutrient consumption and decay.

Soil changes can vary so greatly even within a year that you’d likely find a meaningful difference in the amounts of certain soil components, such as nutrient levels, microbial activity, pathogenicity, and organic matter content.

Here are some factors that affect how long the soil test results remain valid:

Weather and Climate

Natural phenomena like rain or wind erosion can slowly alter soil qualities, even for unfarmed or untilled soil. As the soil gets exposed to the elements, the chemicals contained in the soil change.

Think about every time it rains. Moisture and chemicals from high up in the atmosphere fall onto the ground, mixing the contents and chemical properties. If the rain was acidic due to pollution and it rained every day for a week, the soil would likely be more acidic afterward.

Soil Amendments

If you amend the soil regularly to improve texture or pH, these amendments must be replenished regularly to maintain desired levels. The soil will naturally revert to its original pH, which is influenced by environmental factors, such as the amount and frequency of rain.

Using organic mulches that decay over time can also contribute to changes in the soil components and quality. For instance, carbon-rich mulches like wood chips and sawdust will force soil microbes to consume nitrogen within the soil to break them down.

Crops Grown in the Area

The type of crops and gardening or farming behavior can also influence how quickly the soil quality changes.

For instance, peas and broccoli can return nitrogen to the soil after harvest, making it available to the next batch of crops after four weeks or so. On the other hand, some plants like tomatoes and peppers feed heavily on soil nutrients, leaving the soil with little nutrients for the next batch of crops.

Plant pathogens like Phytophthora and pests like nematodes can disappear from the soil after a few years of growing resistant cultivars. That’s why it’s best to rotate your crops and avoid growing the same plant in the same spot for 2-3 years.

That’s why the recommended and average validity of soil test results is 2-3 years. If you have a busy garden where you plant multiple crops yearly, it’s best to test your soil once a year.

Maintaining Soil Test Validity

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 validity for longer amounts of time. 

One such way is to keep your soil in a controlled environment. Try growing plants in a greenhouse or a pot indoors 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 the materials your plant uses 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 on the same plot of land under the same conditions whenever possible. For example, if you typically test soil in April after a week of no rain, do the same when taking your next soil test. Doing so will give you 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

Many factors can influence how long a soil test is good for. While soil test kits and soil samples can both expire, over time, soil undergoes significant change such that, after about two years, the result will no longer represent the soil’s current conditions.

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 kit is not expired and testing 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)

Dr. Moritz Picot

Dr. Moritz Picot is a horticulture enthusiast and the founder of TheGrowingLeaf.com, where he serves as the lead content writer. He established the website in 2022 as a valuable resource for both gardening aficionados and beginners, compiling all the gardening tips he has accumulated over the past 25 years. Alex has a passion for nurturing plants, transforming backyards into inviting spaces, and sharing his knowledge with the world.

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