What Soil Health Tests Really Tell You

07:52AM Mar 21, 2015
Despite challenges with consistency and repeatability, soil health testing is worthwhile
( Lindsey Benne )

Despite challenges with consistency and repeatability, soil health testing is worthwhile

Someday you might be able to mark a spot in a field, return to it year after year and measure your progress improving soil health. After studying the state of soil health testing for four years, Farm Journal Field Agronomist Ken Ferrie has concluded such precise measurements are not yet available. 

“However, that doesn’t mean it’s not well worth your time to conduct soil health analyses in the field and in laboratories,” Ferrie says. “It just means you have to understand what the tests can tell you and what they can’t.”

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With new tests being developed and new labs offering soil health analyses, the science of soil health testing might be where conventional soil testing was many years ago. 

“With traditional soil testing, we understand labs use different extraction methods,” Ferrie says. “Some labs report their results in pounds per acre, some in parts per million and some in the elemental form of nutrients. We know soil samples should be collected the same time every year or adjusted accordingly. If we use the same lab and collection procedure, soil test results are repeatable from year to year, and we can see trends over many years.”  

The soil test on which you base lime and fertilizer applications is one aspect of soil health testing. It analyzes the chemical component and then you, or your consultant, apply knowledge to interpret the results. 

Eventually, Ferrie believes we might develop the same kind of repeatability for the other two components of soil health—physical and biological. But we’re not there yet.

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Ferrie obtained soil health information by conducting in-field and lab tests. Using GPS, technicians went to the same field location several times. They collected soil samples and conducted in-field tests to check for consistency of results over time.

The technicians used a soil penetrometer to analyze surface and subsurface hardness. They measured bulk density, water infiltration rates and carbon dioxide respiration, which indicates how many living organisms are present in the soil. They also conducted a slake test to measure the soil’s ability to prevent crusting. 

Soil samples were collected using identical procedures, then sent to several labs. The labs conducted physical health tests, such as aggregate stability and water-holding capacity. They measured chemical aspects of soil health, such as H3A phosphorus and potassium (H3A is a weak organic acid that indicates nutrients in the soil solution), water-extractable organic nitrogen, organic carbon and the amount of carbon dioxide released in a 24-hour period. They also measured biological aspects, such as microbial diversity.

In another aspect of the study, Ferrie sent two technicians to multiple soil health test sites. Using the same procedures 10' to 15' apart, they collected duplicate samples, which were sent to the labs to check for consistency within each lab.

Conduct in-field soil tests or collect samples at the same time each year, and note environmental changes. 

In the field and in the lab, Ferrie found consistency and repeatability issues, though some tests were more consistent and repeatable than other tests.  

In the field, Ferrie found if technicians used identical testing procedures at the same time, the results for water infiltration rate, subsurface hardness and bulk density were consistent, even when different technicians conducted the tests. However, not all of the results were repeatable from month to month or year to year. 

“Subsurface hardness tests resulted in different numbers but identified the same dense layers from one year to the next,” Ferrie explains. “The bulk density tests were repeatable from one year to the next. But water infiltration rate, carbon dioxide readings (even when standardized based on temperature, moisture and bulk density) and the slake test showed a lot of variability.  

“For the most part, basic soil testing, organic matter content, aggregate stability and water-holding capacity were fairly repeatable from year to year within the same lab,” he adds. “Getting most other readings in a tight enough range to be comfortable was harder.”

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One obstacle to obtaining repeatable results from year to year is the influence of seasonal weather patterns. For example, even though all of Ferrie’s soil samples were taken at the same time of year, sampling during a drought or during a wet season seemed to vary the results. Even conditions on the day of sampling seemed to have an effect. 

“For example, the results for H3A phosphorus [P] and potassium [K] varied significantly depending on the time of year and the soil moisture when we did the testing,” Ferrie says. “The H3A P and K results are accurate on the day they are taken, but they seem to be constantly moving values. The same is true of the nitrate extraction test.”

Among the labs, the volume of soil organisms and microorganisms (obtained by measuring the amount of carbon dioxide released from soil, or the “carbon dioxide burst”) had the widest variance. 

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With one lab, results varied when an identical soil sample was submitted twice. This suggests the lab’s procedures need to be more standardized.

Despite the challenges, Ferrie’s research showed soil health tests provide valuable information to start improving your soil. Healthier soil will ultimately yield more, he emphasizes—but soil health improvements take time. “Sick soil usually didn’t get that way in just one or two years—in many cases, it takes decades,” he says.

“The good news from our research is we can look at soil health test results and pick out the healthiest and unhealthiest areas within a field,” Ferrie says. “When we look at our yield maps and history, we can confirm the correlation between higher-scoring soil and higher yield. Regardless of the lab, the healthier soil always received a higher overall score.”  

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Equally important, soil health testing can help you zero in on the most cost-effective ways to improve soil health. For example, on one very sick soil, the farmer chose to correct acidity and soil density first because they are among the easiest problems to fix. After three years, the yield gap between that farm’s unhealthy and healthy silty clay loam fell from 70 bu. per acre to 20 bu. per acre. 

“From those aspects, soil health testing is a useful tool to help create the best soil health possible,” Ferrie says. “We just might not be able to apply numerical scores as precisely as a standard soil test for nutrients and pH.” 

With soil health testing, as with traditional soil testing, there are factors to keep in mind as you interpret and apply the results:

  • It’s best to stick with one lab. Understand your lab’s testing procedures so you will know if they make a change in the future (which could affect your soil health score). If you combine results from two labs, make sure you convert their reporting methods to one scale. For example, some labs report nutrients in parts per million, some in pounds per acre and some in percentages or a combination. 

Some measure carbon dioxide respiration for a day and some for a week; some report it in pounds and some in kilograms. Labs use different scales for their overall soil health rating, so the standard for good health might be 14 on one scale and 40 on another.

  • Separate your soil into management zones based on soil type and on whether the soil is well- or poorly drained. “Drainage makes a big difference in soil health,” Ferrie says. When conducting in-field soil health tests or collecting samples for a lab, try to sample the soil at the same time each year. Be sure to note changes in environmental conditions from year to year (wet versus dry weather, for example.)     
  • Split some soil samples and submit both of them to your lab to see if their testing produces repeatable results. It will give you confidence in the lab’s procedures.
  • Think of your numerical soil health ratings as an index between good and poor health, rather than precise numbers that can be compared from year to year. “Because it’s a living system, soil is affected by many things,” Ferrie says. 
  • Rather than relying completely on numerical values, use your test results to separate each of your soil types into healthy and unhealthy zones. Just as you do with agronomic practices, use strip trials to compare soil health practices with your normal methods. 

“Look at the differences between soil health scores, rather than the actual numbers (to account for differences caused by the seasonal environment),” Ferrie says. “Try to raise the yield of your sickest soil closer to that of your healthiest soil.”

  • Stay abreast of developments in soil health testing. “More labs are getting into soil health testing every year,” Ferrie says. “This means new procedures will be developed, providing more options.” 

Building on the Systems Approach, the Soil Health 
series will detail the chemical, physical and biological components of soil and how to give your crop a fighting chance.