Dr. Dan Talks Agronomy

Soil Health Test Results

Dan Davidson
By  Daniel Davidson , DTN Contributing Agronomist
Set some goals for your soil health and follow up by tracking the microbial activity in your fields. (DTN photo by Chris Clayton)

The buzzword in agronomy this past year has been soil health. I've written a lot about how to monitor and measure it. Still, there seem to be many questions about whether we are digging into what's going on or just scratching the surface.

In my opinion, the two best soil health tests we have today are the Solvita and Haney tests. They look at microbial activity on the soil, but these tests don't identify or measure the microbial community.

I recently listened to a webinar titled "The Haney Soil Test and Nutrient Turnover" sponsored by the Practical Farmers of Iowa (www.practicalfarmers.org/farminars). One of the presenters was Sarah Hargreaves, a Canadian soil microbial ecologist and farmer.

Hargreaves explained that the Haney test measures microbial activity and reports digestion of organic matter and mineralization of nutrients. She said plants leak carbon and nitrogen compounds that microbes readily feed upon (think soluble or active carbon). The microbes then release enzymes into the soil that further break down organic matter and release nutrients that feed crops and carbon as humic compounds that build soil.

One of the most practical indicators of soil health is soil respiration. Hargreaves told listeners that monitoring soil respiration is a good indicator of microbial activity. The Solvita test, invented by Will Brinton and Woods End Laboratory (http://solvita.com/…), measures the amount of carbon dioxide released by soil in 24 hours. This test can be done quickly by the producer and is inexpensive. It is a practical way to track the level of microbial activity in fields.

I have been using this test for a couple years and have healthy fields pushing 90 to 100 parts per million CO2 and have also seen fallow fields (after cereal harvest) drop to 12 to 15 ppm CO2 when nothing is growing. Healthy forest soils with a lot of litter recycling can hit numbers as high as 175 to 200 ppm. A good target for tillable soils, I believe, is 80 to 100 pm.


Hargreaves compared feeding microbes to feeding livestock. Both quantity and quality of the microbes are important. The measure of quantity is the amount of organic matter levels in the soil. Adopting no-till, high-residue crops and cover crops can increase organic matter and feed the herd.

In my opinion, you should work toward levels of 3% or more organic matter to have healthy soil. The greater the quantity of organic matter, the more carbon and nutrients available. Across the Corn Belt, it is reasonable to expect to be able to build organic matter with the right crop rotations and adopt production practices to sustain it.

The measure of quality is the carbon-to-nitrogen (C:N) ratio. The more ideal the quality, the better the feed value. The Haney test reports both the Solvita value and C:N ratio as well as the total amount of inorganic N and P and mineralizable N and P. Here are some things to remember about the C:N value:

-- When the C:N ratio is greater than 30:1, microbes have access to more C than N and microbes consume all available N to satiate their appetite, leading to soils deficient in N.

-- When the C:N ration is less than 20:1, microbes have more access to N than C, and that extra N will probably be lost.

-- Hargreaves said the sweet spot is 24:1 when microbes consume both C and N and also mineralize, in the form of organic biomass, N and P that is available to plants.

To make C:N corrections, Hargreaves suggested that growers need to broaden diversity in plants grown and microbes that live in the soil. Soils need greater amounts of functioning roots to both leak carbon compounds and pick up nutrients. It's recommended to have a renewed supply of fresh organic matter that is more appealing to microbes and maintaining a C:N ratio around 24:1 or slightly higher to drive microbial activity, decomposition and mineralization.

To meet these goals, growers should consider adding cover crops to keep roots active in the ground nearly all year round. Adding manures or compost also provides fresh organic matter and a fresh source of soluble carbon.

Its time to track the health of your soil. A good place to start is tracking organic matter (conventional soil test), soil respiration (Solvita) and C:N ratio (Haney). I believe goals of 3% organic matter, 80 to 90 ppm CO2 release, and a C:N ratio of 25 to 30 are achievable.

Dan Davidson can be reached at AskDrDan@dtn.com


Dan Davidson