Comparison of Soil Health Tests
April 18, 2023 | By Zach Larson, ForGround by Bayer Sustainable Systems Agronomist
What is soil health and why is it important?
Soil health is “the continued capacity of the soil to function as a vital living ecosystem that sustains plants, animals and humans”. (1) Currently, there are over five different carbon farming programs available to producers, including the Bayer Carbon Program. These programs compensate farmers for adopting farming practices that help increase soil carbon stores, which in turn, helps improve soil health. With all that is continually changing in agriculture and with the pressure to grow more on less acres, the interest in soil heath testing is greater now than ever before.
What do soil health tests measure and what are the variables caused by changing conditions?
A soil health test measures biological, chemical, and physical soil properties. While measuring the physical and chemical properties of a soil has its challenges, measuring and interpreting soil biological properties can be very challenging. The soil microbial community can vary greatly depending on where the sample is taken in the field (slope, crop rotation history, low-lying areas, soil type, and other landscape factors) and the timing of when the sample is taken. Additionally, Large differences in soil microbial activity can result from differences in soil moisture at the time of sampling, the time of year a sample is taken, and the cash crop grown.
Today, there are a wide variety of commercially available soil health tests that use soil samples collected from the field, and many focus on different aspects and indicators of soil health. There are also in-field soil assessments that use various soil health test kits, sensory observations, or field soil property measurements. (2) A producer will need to determine which test best measures the soil biological, chemical, and physical constraints associated with soil health that are important to their operation.
What types of soil health tests are currently available?
Comprehensive Assessment of Soil Health (CASH) Test
One of the popular soil health tests available is the Comprehensive Assessment of Soil Health (CASH) test available from the Cornell Soil Health Testing Lab. It was designed for producers who want to go beyond simply testing the nutrient levels of their soils. The assessment provides standardized information on important biological, physical, and chemical soil properties in addition to standard nutrient analysis. This test measures and rates 12 different soil attributes and then combines them to form an overall quality score (out of 100). (1)
The cost of this basic test is around $100 per sample, while additional Soil Health Analysis Packages can increase the cost to around $165 per sample.
Haney Soil Health Nutrient Tool Test
The Haney Soil Health Nutrient Tool Test is a soil health test that provides several different measurements of nitrogen (N), phosphorus (P), and potassium (K) to estimate their availability over the course of the growing season. The Solvita CO2 burst test is part of this tool that measures biological activity or respiration. This test utilizes lower acidity reagents that more closely mimic the pH of naturally occurring biological activity in the soil to determine the quantity of soil nutrients available compared to traditional soil tests that use stronger acids to measure the total soil nutrients that have been determined essential for crop development. One soil health parameter is unique to the Haney test, which uses water-extractable organic carbon and N and their ratio to understand the level of nutrients that will become available during the growing season. This test measures the portion of organic matter that dissolves in water (and therefore is more accessible to soil microbes). The Haney Soil Health Nutrient Tool also provides a soil health score. While there is no threshold that indicates high soil health, an increase in the number over time indicates improving soil health. (2)
The cost of this basic soil health assessment starts around $55 per test. This same test with the addition of a complete soil analysis will add an additional $10 per sample.
Phospholipid Fatty Acid (PLFA) Tests
PLFAs are structural molecules found in the membranes of all living organisms. The levels of certain fatty acids in the soil are used to indicate the activity and the size of bacterial, fungal, or other types of microbial populations, with higher levels associated with better soil health. However, this test does not identify which organisms make up these active species. It’s important to note that PLFA test results may differ depending on the lab. Producers and consultants using this test should be sure the lab has not changed their PLFA markers.
The cost of a PLFA test can range from $55 to $85 or more, depending on the laboratory used. If a soil health assessment is requested, it can add an additional $50 to $60 dollars per sample.
Soil Food Web Biology Test
For the most part, the Soil Food Web Biology test focuses on the same groups of microbes as the PLFA test, but this test uses microscopy to determine the relative abundance of these microbes and their activity, which significantly increases the cost of this type of testing. It also distinguishes active bacteria and fungi from totals of both groups.
The cost for this type of test can start around $225 to $300 per sample.
Active Carbon (Permanganate Oxidizable Carbon – POXC) Test
This is an affordable soil health test developed by The Ohio State University for farmers to measure the biologically active organic matter in the soil. Active organic matter is a small fraction of total organic matter (5–20%), but its role is very important to crop nutrition as nutrients from active organic matter are rapidly cycled and are easily taken up by crops. (3) POXC is often referred to as active carbon. This test is especially important in high-organic-matter soils as this active pool of organic matter largely contributes to the fertility needs of a growing crop year to year. While testing for total soil organic matter (SOM) is still an important soil test, tracking POXC levels, which can change more rapidly, just measures the active carbon fraction present in the soil. This test can be used to measure the microbial biomass and particulate organic matter as it changes over a brief period.
The cost for this type of test can start around $25 per sample.
What conclusions can be made concerning soil health tests?
Soil health tests are a tool to better understand the complex interactions between soil organisms, soil chemistry, and the physical condition of the soil to enable the soil to sustain all plants and animals. Agricultural practices can influence soil health, soil erosion, nutrient leaching, and soil productivity. The current soil health tests available take a “snapshot” of several aspects of soil activity to assess if management changes have changed the soil’s ability to sustain plants. These tests do not evaluate all aspects of soil health but measure some important values that are known to change as soil health changes.
As management practices change, these measured values can change rapidly. Soil heath values can drastically differ from soil to soil, even as soils change across a field, so the concept of good or poor soil health values depends on how that soil was formed over time and other important factors. As was apparent in the reviewed soil health tests, they correlate to SOM, which has been identified as critical to soil function, structure, and health. That is why many soil health discussions focus on carbon.
The Soil Heath Institute has recently recommended that a minimum suite of three measurements to be widely recommended for all producers across North America that are practical and affordable when assessing soil health.
- Soil organic carbon concentration (SOM).
- Carbon mineralization potential (microbial nutrient availability).
- Aggregate stability (humus levels). (4)
All three tests are directly correlated back to carbon levels in all soils.
How to take a good soil health sample:
- It is important to take a representative sample from a uniform soil type, slope, and tillage management area in a field.
- Do not include any soil samples from areas on the field border, or from areas that flood often.
- Take the original soil health test and any follow-up tests at the same time each year, usually prior to the spring planting season.
- It is important to understand that soil organisms cycle rapidly in response to weather, organic additions, plant growth, and other factors, which makes it difficult to determine the best sampling time.
- A large change in soil moisture can drastically change the soil health number that a soil health test may record.
- When following the initial test with additional tests, 2–3 years later, take these samples as close as possible to the location of the original soil samples.
- Use the same soil health test and the same laboratory for the soil heath analysis to track changes in soil health (such as the effect of management changes) over the years.
- Take a uniform 6- to 12-inch-deep soil sample with a soil probe if possible, following the sampling directions of the test that is used.
- Note that there is no universally accepted way to evaluate soil health.
- Be patient. Many changes that can be documented with a soil health test may be slow to show up.
With the opportunities now available to row crop producers to profit from carbon captured in their fields (carbon farming), it may be more important for producers to measure the amount of organic carbon sequestered in their soils. Not all carbon programs document this increase (or decrease) of carbon stored in the soil over time. Understanding the complex carbon soil dynamics can be complicated and challenging, so testing requirements may change over time. A soil health test can be an important tool to help producers understand their changing soil health.
Resources: 1 Moebius-Clune, B.N., Moebius-Clune, D.J., Gugino, B.K., Idowu, O.J., Schindelbeck, R.R., Ristow, A.J., van Es, H.M., Thies, J.E., Shayler, H.A., McBride, M.B., Kurtz, K.S.M., Wolfe, D.W. and Abawi, G.S. 2017. Cornel Soil Health. College of Agriculture and Life Sciences. Manual. Cornell University. https://soilhealth.cals.cornell.edu/manual/. 2 Zuber, S. and Kladivko, E. 2018. How to Understand and Interpret Soil Health Tests. AY-366-W. Purdue University. https://www.purdue.edu/purdue/search.php?q=%202018.%20How%20to%20Understand%20and%20Interpret%20Soil%20Health%20Tests.%20AY-366-W%20. 3 2022. Soil Health Testing. Soil Fertility College of Food, Agriculture and Environmental Sciences. The Ohio State University. https://soilfertility.osu.edu/our-research/soil-health-testing. 4 2022. Soil Health Institute Issues Recommendations for Evaluating Soil Health. Cotton Grower. https://www.cottongrower.com/cotton-production/soil-health-institute-issues-recommendations-for-evaluating-soil-health/.
Legal Statement: This Bayer Carbon Program described in this material is subject to the current version of the Bayer Carbon-Smart Practices Master Agreement. The information is to aid in the understanding of the Bayer Carbon Program and does not change or modify the Bayer Carbon-Smart Practices Master Agreement in any way. Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.
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