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Evaluating Soil Health

Soil Health Goals
Figure 1.
  • Healthy soils are important for cropping systems and there are ways to measure soil health improvement using on-farm testing methods.

  • Soil health assessments through a lab are also valuable to measure certain soil characteristics and keep quantitative records over time.

Healthy soils function efficiently to support resilient life. In corn and soybean cropping systems, this means optimum cycling and retention of both water and nutrients. There is a lot of interest in shifting farming practices to create more stable and productive systems, but how do we measure progress as a healthier soil is built? There are many methods for measuring changes in soil health and the measurement chosen should be consistent with the prioritized goal.

Soil Health Goals

Many short-term soil health goals are biological and chemical in nature. For example, increasing microbial activity or adjusting soil pH may lead to the release of more nutrients available for the plants during the growing season of application. Reaching these goals often involves the addition of soil amendments, but maintaining these goals requires the optimum physical environment. Creating an optimum physical environment is a long-term goal that is achieved as biological and chemical factors come together to affect soil structure and soil function. Changes in cropping practices such as tillage, cover crops, or rotation are often required to create an optimum physical environment.

Soil Health Tests

There are two ways to compare treatment effects in agricultural soils. Side-by-side comparisons such as test strips or test blocks within a field work well for short-term and in-season measurements. If areas with and without treatment share the same soil type and weather conditions, this reduces environmental variability and gives a better idea of how much change is attributed to the treatment. Before and after comparisons are useful for treatments that take a longer time to take effect and that use measurements that are not sensitive to seasonal variation.

On-Farm Testing Methods

There are a multitude of ways to track soil health progress and a number of these are relatively easy to do on your own.

Slake test
Figure 2. Comparison of two soil samples using the “Slake Test”. Less soil moved into solution with the healthier sample on the right. Soil with less structure on the left, more easily dissolved into solution and settled out to bottom of jar.

Slake test

The slake test is an excellent measure of soil health because it ties together chemical, biological, and physical measurements into a single indicator. In the ideal chemical environment, abundant root and microbial activity thrive and produce materials for strong aggregation while roots and macrobiota also create tiny channels for water infiltration. Strong aggregation and channels for water keep a clump of soil together when it is suspended in water. Without strong aggregation and water channels, a clump of soil will slake apart when suspended in water (or under a strong rain).

To use the slake test for a side-by-side comparison, you will need the following:

  • Two glass jars large enough to hold a chunk of soil

  • Hardware mesh, chicken wire, or mesh onion bags

  • Hardware mesh, chicken wire, or mesh onion bags

Position the mesh to hang inside the glass jars and fill the jars with water. Place the chunks of soil on the mesh and watch to see the difference in how the chunks slake apart. Take a picture of the amount of soil at the bottom of the jars after 10-15 minutes to compare to future years (Figure 2).

Root digs

The shape, color, and smell of your roots can tell you a lot about your soil. Roots growing in a healthy environment are white with fine strands and a natural shape. If roots are stunted or bent, it is a sign they have encountered a zone of compaction, such as a side-wall or plow pan. Poor drainage can cause roots to be brown, mushy, or smell bad. Roots that are cut off indicate undesirable soil organisms.

Soil your undies

While this underwear test is best known as an audience-shocking field day activity, it also does a good job of demonstrating microbial activity in the soil and provides an easy and consistent comparison across sites and years. Simply bury a pair of 100% cotton underwear 4-6 inches deep, leaving the waistband sticking out at the surface. Mark it with a flag and dig it up 60 days later. Compare how much material is left between different fields and take a picture to compare across years. Healthy soils that are teeming with decomposers and quickly cycling nutrients may have only the waistband remaining while dormant soils may keep most of the underwear intact.

Tea bag index

Tea bag index
Figure 3. Burying tea bags in soil to monitor rate of breakdown as indicator of soil biological activity.

Similar to the “soil your undies” approach, you can also use household items (in this case, tea bags) to approximate microbial activity. Tea, like crop residues, can break down at different rates depending on their carbon to nitrogen ratio (C:N). Green tea bags, which have a lower C:N ratio similar to alfalfa, decay fairly quickly and are used to quantify short term organic matter decomposition. Red/rooibos tea bags have a higher C:N ratio similar to corn stalks and will break down slower. Burying the two types of tea bags for different periods of time in soil can be used to measure mass and carbon loss to quantify decomposition rates. Bury some tea bags (and mark/flag them well) and get an approximation of how quickly your crop residues will decompose and provide nutrients to the crops (Figure 3).

Lab Testing Methods

Laboratory tests are more convenient if you have a large number of samples and wish to keep a clear quantitative record of changes in your soil.

Soil health assessments

Many commercial soil testing laboratories now offer soil health tests that include a suite of analyses that are used in combination to create a soil health score. These tests aim to characterize the quality of the environment for microbial life and simulate nutrient availability for plants. Through measuring microbial activity and various forms of carbon and nitrogen, these assessments give a good idea of how dependent the crop will be on additional fertilizers and soil amendments or how capable the system will be to support itself.

Soil health assessments provide a more complete picture than a single test, but care should be taken when comparing results across time and against other fields. Many of the components that are measured change throughout the growing season. Comparing spring soil samples from a cold and rootless period in one year to summer soil samples taken during a time of high temperatures and root activity the following year could give the illusion that microbial activity has changed due to practices when it has only increased due to seasonal conditions.

Many soil testing labs will provide an overall soil health score based on the combination of test results in the assessment. They will also provide interpretations and recommendations. However, many of these overall soil health scores are not well-calibrated to crop performance and the lab-provided interpretations should be taken with a grain of salt. Due to texture and environment, not all soils have the capacity to reach a high soil health score relative to other soils. Soil health scores are most useful when comparing a soil type (or texture) to the same soil type without changes in farming practices or the same soil from previous years.

Organic matter fractions

Soil organic matter is material in the soil made up of plant, microbe, and animal material. It is present in many forms. Some forms are easily decomposable and quickly contribute to nutrient cycling, while other forms do little beyond existing in the soil. Total organic matter may be a useful measurement for carbon sequestration. However, it is more useful to measure active forms of soil organic matter when trying to determine a soil’s potential to support more plant and microbial life. Many commercial labs offer tests for “active carbon” or POXC (permanganate oxidizable carbon), which represents a soil carbon that has been processed by microbes and is likely building soil carbon.

PLFA and enzymes

Phospholipid fatty acids (PLFA) are membranes found in organisms and have a different composition for bacteria, fungi, and other soil microbes. PLFA analysis can determine the abundance of microbial life and provide a snapshot of microbial community composition. Healthier soils will have a greater abundance of microbial life. Higher ratios of fungi to bacteria are also considered an indicator of a healthier soil. Both measurements will vary over the season as temperature and moisture vary. Different laboratories also use different PLFA markers and may sometimes change markers from year to year. It is important to collect samples under similar conditions and submit samples to a consistent laboratory.

Enzymes are produced by microbes to serve specific functions in nutrient cycling. Looking at microbial activity through enzymes instead of PLFA means asking, “What are they doing?” instead of “Who is there?” Most laboratories measure enzymes involved in carbon, nitrogen, and phosphorus cycling. An increase in the presence of these enzymes indicates the community driving these cycles is not only present, but also active. Like all biological measurements, enzyme analysis is also sensitive to changes in temperature, moisture, and root activity. Care should be taken when comparing analysis results over time.

Overall, soil health analysis along with soil health improvement is complex, takes time, and requires some form of consistent testing. If you have questions or want help setting goals for your soil health, reach out to your local Golden Harvest Agronomist, Sales Representative, or Seed Advisor.

All photos are either the property of Syngenta or are used with permission.


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