Soil health lab test quick guide: What can they tell us?

By: Gabi Bolwerk and Bailey Tangen, Extension educators

Fall is typically known as harvest season, but it is also a great time of year to test your soils. Many folks get a standard suite of tests that report soil organic matter, soil texture, and nutrient levels. But, for those interested in soil health, there are lots of lab test options to consider.

Should you use lab soil health tests?

With all of the available options, it can be hard to distinguish which lab tests, if any, should be used to measure soil health. See the pros and cons below to help determine if you should get lab soil health tests done.

Pros of lab soil health tests:

• They provide quantifiable information you can refer back to in future years.
• They can help determine if a management change has impacted your soil health and function.

Cons of lab soil health tests:

• There is no one test that can determine the health of your soil, due to soil’s dynamic nature. Soils are constantly in flux, with aspects that can change based on the time of year, weather, and management. Biological aspects of soil are especially dynamic, making them hard to evaluate accurately.
• Soils have microenvironments, meaning conditions and properties of soil can differ on a very small scale. For example, soil microbes occur more densely around plant roots than in bare soil. Test results may look different based on where you sampled within the field.
• It is hard to make broad interpretations of lab soil tests. Lab soil tests need to be interpreted in the context of your soil type, management, climate, and landscape position. For example, a sandy soil won’t have the same level of aggregate stability as a soil with high clay content.

Science-backed soil health tests

If you choose to use lab tests, be sure that the tests you choose align with your soil health goals and can provide you with useful information. Align the information you are curious about with the test that best investigates that aspect of your soil. Starting with the end in mind is a valuable way to set yourself up to accurately interpret and use the test results.

No matter which tests you choose, it’s crucial that you use a certified soil testing lab. Certified labs use proper techniques and procedures so they produce valid, replicable results. Visit this website for a list of Minnesota Department of Agriculture certified soil testing labs across the United States.

Useful soil health tests should be sensitive to management changes, provide useful information, and have reliable, repeatable testing methods. Below are some lab tests that fit these standards.

• Soil organic matter: Soil organic matter is the living, dead, and decomposing biological material in soils. It is a common soil health measurement, with a reliable and repeatable testing method. Higher levels of soil organic matter are known to improve soil structure, support soil biology, provide nutrients, and increase resilience to precipitation extremes. Soil organic matter levels vary by soil texture and management, so it’s important to compare your own measurements over time to see trends.
• Aggregate stability: Aggregate stability is a measure of how well soil aggregates hold together when wet. It is typically reported as percent aggregate stability, with a higher number indicating healthier and more stable soil. Stable aggregates are important for creating and maintaining pore space so air and water can move properly in the soil.
• POXC: Permanganate oxidizable carbon (POXC) tests measure a fraction of carbon in the soil that is considered to be biologically available, or “active”. This means soil biology is able to access those carbon sources as food. POXC results are reported in parts per million (ppm), which is equivalent to the milligrams of oxidized carbon per kilogram of soil. POXC is most useful when comparing values over time rather than interpreting a single test value. An upward trend in POXC indicates increasing soil health.
• ACE protein: Autoclaved-citrate extractable protein (ACE protein) tests measure the fraction of organic nitrogen bound in the soil. Soil microorganisms can mineralize organic nitrogen, which transforms the nitrogen into a plant-usable form. ACE protein has a positive correlation with grain yield and has been shown to be responsive to tillage practices and crop rotational diversity. An upward trend in ACE protein indicates an increased pool of organic nitrogen for microorganisms to transform to a plant-usable form.
• PLFA: Phospholipid fatty acid (PLFA) tests indicate the type of microbes and the microbial biomass present in a soil sample. An upward trend in microbial biomass, and a fungi to bacteria ratio nearing 1:1 indicates an increase in soil health. Microbes are a crucial component of a functioning soil by playing a role in nutrient cycling and plant health. Different microbes have different functions, making microbial diversity important. Microbes are sensitive to environmental changes, so it’s important to sample at the same time of year and in similar conditions if possible.

For information on the Haney test, see this blog post.

How to soil sample

The foundation of a good soil lab test is a proper soil sample. Be sure to use the sampling techniques recommended by your lab of choice to ensure the most accurate results. This video from University of Minnesota Extension demonstrates how to take and prepare soil samples.

It is important to sample to the same depth each time you take soil samples. This will help to ensure the most accurate and comparable results as possible.

It’s also important to sample during the same time of year each time you sample. For example, if you took samples in late October one year, you should take samples in late October every year that you sample. Consistent timing leads to the most accurate test results, as some measurements can vary based on the time of year.

It is not necessary to take soil health tests every year. Soil health is slow to change, so testing every 3-5 years is sufficient. Additionally, soil health test results should be compared among years on the same field. It is not useful to compare different fields to each other, as there are too many factors that can make the results differ, such as landscape position and precipitation.

Final thoughts

Soil health lab testing gives you a snapshot in time of your soil condition and can be a valuable way to track changes across time. If you choose to lab test your soil, know your goals and what information you want the tests to provide. Although there are some benefits of soil health lab testing, our favorite and most reliable method for assessing soil is to go out in the field and make your own observations. These in-field methods engage your senses to get a full picture of your soil's health. When you are familiar with your soils, you can see changes in condition and function over time based on management. For information on how to perform a visual soil health assessment, see this blog post.

Resources

• A minimum suite of soil health indicators for North American agriculture - Bagnall et al. 2023
• An Ace in the Hole for Soil Health and Nitrogen Tracking - Sever 2024
• Indicators of Soil Health - Minnesota Office for Soil Health
• Organic matter: The backbone of soil health - Young 2022
• Soil health testing: Can you manage what you measure? - John S. Breker, AGVISE Laboratories

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