From Fred Vocasek, Senior Lab Agronomist:
We are getting lab questions about a particular Soil Quality Enhancement to qualify for NRCS payments under the CSP program. It is “SQL15 – Utilize soil health nutrient tool soil nutrient pools”. This refers to what is commonly called the “Haney” test, more properly the “H3A soil extraction method.”
Several labs are offering Soil Health tests, from $50 to $75 each. (Not grid-sample friendly.) This is a package that is claimed to be an “integrated” way “to mimic nature’s approach to soil nutrient availability.” It includes a series of tests that are supposed to measure microbial respiration and carbon-to-nitrogen (C:N) ratios for the Soil Health Calculation (SHC) score.
What do the test numbers mean? Right now, we don’t know. The soil health experts will honestly state that these tests are not correlated either to yield or to productivity. An SHC score above “10” is considered to represent a “healthy” soil, but that number has no apparent relationship to yield or input costs.
Soil health tests are another attempt to account for the microbial contribution to soil fertility. The NRCS materials state that, “Studies show less than 50 percent of the nitrogen in harvested plant parts can be attributed to added fertilizers.” Some farmers are coming away from certain NRCS meetings assuming they can cut their fertilizer rates by half or eliminate fertilizer altogether.
The H3A soil test package does include NO3, P, and K tests. Nitrate is determined using the same method as any other lab, including Servi-Tech. The H3A phosphorus and potassium results are said to be highly correlated with Mehlich results. The labs that offer the test do not make nutrient recommendations from the H3A analysis, but use the current, proven test methods. Dr. Haney offers recommendations, but they are based only on crop removal, which can easily over- or underestimate the phosphorus response.
A test using a “SolVita” kit is promoted as a way to estimate microbial activity by capturing and measuring the carbon dioxide generated by microbial respiration. Measuring microbial activity is difficult. There have been many, many different methods proposed to predict such activity. They often work well under controlled laboratory conditions, but are intermittently successful when applied to the field. Microbes are heavily affected by daily changes in temperature and soil moisture.
Several laboratories have discontinued use of the SolVita kit due to lack of consistency or to excess costs. One lab found the test results were different if the soil sample was taken at 50°, 60°, or 70° soil temperature. I attended a meeting in Nebraska where a dried and ground soil sample held in storage for over a decade had a “12” score (considered “healthy”) while the undisturbed, native grass plot only had a score of “5” thus would be ranked “not healthy.” These test results may or may not be linked to field reality, so need to be viewed with caution.
Soil health discussions refer to the C:N ratio and the role of cover crops, which are supposed to provide food for the soil microbes. Simply stated, the carbon-to-nitrogen ratio for soil microbes is equivalent to the energy-to-protein ratio in livestock feeding. Most soil organic matter has a C:N ratio of 10, which would be equivalent to a beef ration with 35% to 36% crude protein. When the soil C:N ratio drops to 40 or below (equivalent to about 9% crude protein), soil microbes compete with seedling plants for the limited soil nitrate supply. This situation can be remedied by adding about 20 lb N/ac before planting or possibly by including an annual legume in the cover crop mix.
Soil health or soil quality is a large-scale, national initiative to maintain and improve our soil resource. The “Haney” test is being promoted as new and innovative, but actually employs concepts and techniques that are decades old. Soil health testing may some day have a place in soil resource management. Today there is little or no research information to validate claims being made. Today the soil health test number is just a number that is needed to qualify for CSP funds. A lot more research will be needed to determine if it is a valid tool for making better recommendations or soil management decisions.