Soil Health Discussion

Adding to the Soil Health Discussion

Erica Olson,

If you can’t take your soil to the doctor, how do you know if it’s healthy? There is no simple answer to this question. How do YOU define what makes a soil healthy? Is it organic matter? Is it structure? Is it chemistry? Is it biological activity? All of these pieces play a role, and the value of each varies from field to field, crop to crop, year to year.

Soil health is such a broad topic and often lacks specifics. However, it is clear that there is no one size fits all answer, especially with the wide variety of soil types and landscapes in Wisconsin. As part of the Nitrogen Use Efficiency Project, Discovery Farms hopes to add some necessary pieces to the soil health puzzle to equip farmers with the information they need to put soil health concepts to work on their farms. 

We are building on work currently conducted at Cornell University. Cornell uses a soil health assessment that breaks soil health measurements into three categories: biological, chemical, and physical. This approach attempts to identify limiting factors in a soil; although it is not always clear a yield boost will occur when the limiting factor is improved. We have selected a few tests from this assessment that we feel may be useful indicators of soil biology, soil chemistry, and soil physical properties. We will compare these across farm fields in various regions around the state.

The tests we have chosen target a select few biological and physical soil health indicators (see below for description of tests). Our study is still in its infancy but by adding more fields and years of data, we hope to better understand what these assessments can tell us and how theytranslate into management suggestions. Because, as Dr. Matt Ruark, UW-Madison Soil Scientist and UW Discovery Farms Faculty Advisor, explains, “at the end of the day there is money to be made through investment in soil improvement. Our vision is that boosting soil health can benefit yield and should not be thought of as something to do in spite of yield.”

Although there is a lot more to learn about soil health, there are soil science basics that provide the foundation for refining our understanding. Dr. Ruark will highlight the science behind soil health at the upcoming Discovery Farms Annual Conference on December 13th. §

Biological Measurements

Soils are living systems containing a vast amount of biological activity. Living organisms in the soil are why plants are able to grow and absorb vital nutrients. This year we performed three biological measurements defined below.

Potentially Mineralizable Nitrogen (PMN)

A measure of the amount of nitrogen that can be converted by soil microbes from an organic form (i.e. in the soil organic matter) to plant available forms (ammonium and nitrate). We are currently working to better understand what this measurement may mean for nitrogen fertilizer guidelines.

Potentially Mineralizable Carbon (PMC),

also known as soil respiration, is a measure of carbon dioxide produced by microbial respiration in soil. This measurement can be used to estimate microbial biomass and thus the microbial activity of the soil.

Active Carbon 

A measure of the carbon in the soil that is available to microbes as an energy source. It is a small but important part of the total soil organic matter.

Physical Measurements

Physical components of a soil influence hydrologic characteristics, nutrient retention, erosion control, and habitat for organisms. This year we added two physical measurements of soil health (below) to our assessment.


A measurement of downward movement of water into the soil. It is important for soil to have a good infiltration rate to decrease runoff and increase the amount of water that can be plant available later in the growing season.

Aggregate Stability 

A measurement of the ability of soil aggregates to resist falling apart when struck by rainfall. Aggregate stability is also an important factor of soil structure. Unstable soil aggregates may result in soil crusting leading to poor infiltration, and difficult crop management.