Have you ever seen a fairy ring of toadstools in a pasture or a pasture with "cow pox" wherever there were urine or manure spots? Chances are that you have. Have you ever taken soil samples inside and outside those fairy rings or cow pox spots? Chances are that you have not, but I have. What I found, more often than not, were no differences in the soil test results, even though there were obvious differences in the growth of the grass in these areas. So how can we reconcile what we see in the field with what we see on the soil test report?
To answer this question, it might be helpful to have a brief refresher on how soil testing has been done for the past 60 years. Traditional soil test methods have typically involved treating a soil sample with various acids and other reagents to extract a portion of the inorganic nutrients in the soil. This can hopefully be done with as few processes as possible to be fast and cheap, and fit the industrial model. In the end, theoretically, the amount of inorganic nutrient extracted and measured is correlated with the amount of inorganic nutrient available for plant growth; this is almost always true.
Notice in my explanation of soil testing, I said inorganic nutrients. Could it be that the organic portion - which is not normally measured - is what accounts for the difference in growth we see in fairy rings and cow pox when a traditional soil test does not show a nutrient difference? Are there soil tests to measure this organic portion? I believe the answers are yes and yes. I say this because there are two relatively new soil testing methods that seem to explain the differences seen in plant growth when traditional soil tests show no difference in nutrient availability.
One of these is the Haney soil test. Basically, the Haney test measures total nitrogen (N), ammonium N and nitrate N; and extracted elemental phosphorus (P) and orthophosphate P; along with other nutrients in the soil sample. Theoretically, total N minus ammonium N minus nitrate N can be used to determine organic N, and elemental P minus orthophosphate P can be used to determine organic P when using the proper mathematical equations. While conventional soil tests typically only account for nitrate N, Haney accounts for all sources of plant-available N and similarly for P. The other test is the Solvita® test which measures carbon dioxide (CO2) released from a soil sample. To oversimplify, the CO2 released is a measure of soil microbial activity which is a measure of organic matter and organic nutrients.
Using these two tests together seems to give a better representation of the nutrients in the soil that are available to plants. They also correlate well with the differences in plant growth that are sometimes seen, but not explained, by traditional soil tests.
One potential benefit of these tests is the ability to measure changes that livestock and crop production management decisions are having on soil health. Another benefit of these tests is that they can potentially show farmers and ranchers additional opportunities to reduce the amount of fertilizer they need to purchase and use without reducing yields, even beyond what is shown on a traditional soil test. This can obviously be very good for the environment as well.