Soil testing is necessary for most farmers and gardeners to get a clear picture of how to amend their land for optimum plant health and yield. While you can conduct simple soil tests to check the pH and moisture levels, a more thorough analysis warrants sending soil samples to a laboratory.
Soil samples for nutrient analysis last up to 15 days when stored in a dark place at 4°C (39°F). For microbial analysis, storing your soil samples at -80°C (-112°F) will keep them viable for testing for 6 months. However, refrigerating samples overnight is enough for routine soil sample testing.
The rest of the article will explain the factors that affect how long soil samples last. I will also discuss some tips about collecting soil samples and preparing them for long-term storage. If you want to learn more, keep reading until the end.
Factors That Affect How Long Soil Samples Last
Although research has shown that it’s possible to store soil samples for about six months to over a decade, most farmers, gardeners, and researchers don’t need to keep the soil that long.
Some soil testing laboratories don’t recommend drying or freezing soil samples, especially when you’re submitting them for microbial analysis. Instead, they recommend refrigerating the sample or keeping it in a cooler maintained at about 4°C (39°F) before and during shipping.
Soil samples can last a long time when stored under optimum conditions. Paying attention to them can help you improve the viability of the samples and the accuracy of the analysis. Exposing the soil to unsuitable conditions will significantly reduce the reliability of the results.
Here are some factors that can affect the shelf life of the soil sample for testing:
Moisture
Excess moisture in soil samples can facilitate chemical reactions that aid in the mineralization or decomposition of organic matter, releasing various forms of nutrients. This process, although naturally occurring in the soil, is not representative of the actual nutrient content of the land.
Note that the sample consists of a small amount of soil and can undergo mineralization more quickly than your farm or garden. Consequently, the sample may yield a higher nitrogen or nutrient content in the analysis than is actually available for plants on your land.
If you can deliver the sample to the lab in less than two hours, it’s usually okay if the soil sample is wet or moist during collection, especially if the lab has the equipment to dry it appropriately before testing.
Otherwise, you must dry the sample adequately before storage. When properly dried and stored at recommended temperatures, the soil sample can remain viable for testing for at least six months up to several years.
Heat
Like moisture, heat can also initiate chemical reactions that degrade organic matter and release nutrients into the soil or the air. Ideally, you’d want to keep your soil stable and halt all chemical reactions and microbial processes to get accurate results.
Soil Microbes
Soil microbes are sensitive to temperature changes, which can activate, deactivate, or even kill them. That’s why soil samples for microbial analysis must be sent to the laboratory or testing center right away.
Optimum temperatures for soil microbes to perform biochemical activities range from 15 to 30°C (59 – 86°F). Temperatures outside that range will slow down their activity.
High temperatures can either activate or kill microorganisms responsible for decomposing the organic matter in your soil, affecting the nutrient analysis values.
On the other hand, extremely low temperatures can prevent microbial activities. Most microorganisms become dormant at temperatures below 5°C (41°F). This is one of the reasons why gardeners don’t fertilize the soil in winter when the ground freezes.
Soil samples intended for microbial analysis are ideally tested within 24 hours.
Gardeners and farmers usually request such a test to properly diagnose the causative agents of plant diseases. The procedure is necessary to ensure they use the appropriate treatment and preventative measures to avoid future reinfection.
Nitrogen Content
As mentioned, low temperatures will halt all microbial activities and the decomposition of organic matter, keeping the levels of nitrogen and other nutrients stable in storage. However, while refrigeration can prevent mineralization for a few days, the soil sample will likely resume the process after a month.
Refrigerating your samples at 4°C (39°F) or freezing them between 0 and -20°C (32 and -4°F) will ensure you get accurate readings for up to 15 days. Any longer and the low temperatures will initiate mineralization and alter the results.
How To Collect and Prepare Soil Samples for Long-Term Storage
Most cooperative extensions and laboratories recommend sending your soil samples to them for testing in less than 24 hours after collection. They also recommend specific collection, preparation, packaging, and storage methods.
There are some things you need to remember when collecting the soil samples:
- Collect soil samples in the fall or early winter. Having nutrient analysis tests in the fall will give you enough time to amend the soil for the spring planting season. On the other hand, if you plan to store the samples for a long time, you wouldn’t need a freezer during winter. An unheated garage or basement will often suffice.
- Use sterile tools for soil sample collection. This is easily the most important rule in any laboratory setting that you also need to follow when sending samples over to one. Using contaminated equipment will compromise the accuracy of the analysis.
- Collect samples from different parts of your land. The number of samples depends on the size of your farm or garden. You can also decide how many samples to collect to compare well-performing and poorly-performing areas of your garden.
- Dig out soil samples at the appropriate depth. You must adjust the depth of soil sample extraction depending on the type of plants you grow in your garden and the length of their roots.
- Remove plant debris and stones from the samples. Plant debris can affect the analysis as it decomposes. Meanwhile, the stones will reduce the amount of soil available for testing.
- Seal the samples securely. This is especially important if you’re sending the sample for microbial analysis. Sealing the containers will prevent the spread of disease and the risk of contaminating other samples in the lab.
- Label the samples accordingly. Some laboratories may have specific instructions for labeling your samples. However, some basic details you must include are your name, the date of collection, the location the sample was collected, and the tests you want the laboratory to run (i.e., comprehensive nutrient analysis, pH, etc.).
If you can’t send the soil sample to the nearest laboratory within the recommended time, here’s how to prepare your soil for long-term storage:
- Choose a non-windy, non-sunny place to dry your soil samples. Ideally, the location should have a roof and temperatures ranging from 20 to 25°C (68-77°F). The sun’s light and heat can affect the soil’s composition, while the wind can introduce contaminants to your samples.
- Air-dry your samples. Spread the soil on a dry piece of paper. It can take at least two days to dry the samples, considering there’s no direct sunlight or wind.
- Place the soil in a sealed bag or container. The seal will protect the soil from moisture and contamination.
- Store the samples in a dark room. As mentioned, direct sunlight and heat can affect soil composition. If you don’t want to refrigerate your samples, you can store them in a box in an unheated garage in winter.
- Use a separate cooler for the samples. For long-term storage, you wouldn’t want soil samples to stay in the same refrigerator where you store your food. Moreover, regular home freezers aren’t cold enough for soil samples intended for microbial analysis.
- Secure a backup power supply in case of power outages. Freeze-thaw cycles can affect bacterial or fungal communities in the soil sample. Keeping the temperature stable is crucial when storing the samples for a long time.
Long-term storage of soil samples is impractical, as it can be costly and challenging to find a freezer that you can set to as low as -80°C (-112°F).
It’s always best to have your soil tested as soon as possible and keep a log of the results, so you can compare them to the results next time you have another soil test.
Final Thoughts
Soil samples can last up to 15 days when refrigerated or frozen without affecting the nutrient analysis. Freezing the samples at -80°C (-112°F) will keep your soil suitable for testing for up to six months.
For regular nutrient analysis, submit your soil samples to the laboratory as soon as possible – ideally in less than 24 hours. Otherwise, chemical reactions might occur in the soil and affect the results.
