For a long time, many producers and researchers have looked at soil as a sterile environment we could control just by adding all the nutrients we wanted, often as synthetic fertilizers. The reality is: Soil is a dynamic, living biological environment that can supply a lot of those nutrients itself if we nurture it and allow it to function the way nature intended. That’s why we believe effective management of plant nutrition in a regenerative system, or even a conventional system, needs to look beyond the standard, prescriptive yardsticks of physical and chemical soil tests by measuring biological factors as well.
Whether you’re growing hay or silage or managing pastures, forage plants rely on a whole host of living organisms below the surface to break down organic materials to release nutrients, help transfer those nutrients into and between plants, and even become “plant food” when they die. The soil ecosystem has a direct impact on fertility and nutrient availability.
At Noble Research Institute, our research and outreach is focused on helping producers implement long-term, regenerative ranching practices that nurture natural soil fertility. Often, producers ask us how they can tell what progress they’re making in regenerating their soils. While our current research is addressing that very question in more detail, there are both chemical and biological soil health and fertility indicators we know we can monitor and track over time with the tests listed below.
Soil should be more than a sterile media; it should be a living ecosystem full of nutrients and biological life. Photos courtesy of Noble Research Institute.
These tests are not necessarily used to recommend soil nutrients and amendments in this case, but to measure and monitor progress (or setbacks) in nurturing the ecosystem whose health has a tremendous effect on plant nutrient availability and uptake.
Chemical properties of soil to monitor
Pull a representative 6-inch-deep soil sample in the same season and environmental conditions each year to reveal trends. Collect with a soil probe and transfer to a plastic or other inert sample bag (not cloth) to submit to a soil lab for a conventional soil test. For trends to be accurate, use the same lab and request the same test methodologies be used each year.
Recommended chemical tests, methodology and target values for evaluating regenerating soils include:
1. Soil pH, measured using a 1-to-1 ratio of soil to water. A pH value of 6.0 to 7.0 is preferred to be suitable to most plants and microbes. The trend should be toward this range and not away from it.
2. Organic matter (OM) expressed as a percent. The dry combustion method is preferred, but loss on ignition (LOI) is acceptable. OM percentage should be trending up over time in regenerating soil.
3. Phosphorus (P), measured with the Mehlich-3 (M3) test. The critical value is 40 pounds per acre, or 20 parts per million (ppm) on the M3 test. P should not be excessively low or high.
4. Potassium (K), extracted with ammonium acetate. Critical value is 220 pounds of K per acre, or 110 ppm of K per the ammonium acetate test. Soils trending below these levels are going in the wrong direction.
Biological properties of soil to monitor
Use same sampling procedure as for chemical properties. The Haney test and a phospholipid fatty acid (PLFA) test will capture many of the biological properties of soil essential to assessing and monitoring overall soil health. Again, the need to sample in the same season and environmental conditions each year cannot be overemphasized, in order to reveal trends accurately.
Haney test – priority values to monitor when trying to manage plant nutrients in the soil:
1. Soil respiration: The carbon dioxide (CO2) released by soil microbes. Sample is dried, rewetted, and CO2 is measured ideally after four days, but the more common 24-hour test is acceptable. Prefer a result greater than 140 ppm CO2-C.
2. Total organic carbon: Readily available carbon that is driving soil microbes. Measures water-extractable organic carbon (WEOC). Prefer levels greater than 200 ppm.
3. Nitrogen: Three measures to track. Inorganic N is the nitrate-N plus the ammonium-N extracted using H3A. Water-extractable organic N (WEON) is nitrogen being released through microbial activity from the organic N pool. Available N is the total of the two measurements (inorganic N and WEON) added together, representing the pounds of plant-available N per acre.
4. C to N: the ratio of organic carbon to organic nitrogen in the soil (WEOC to WEON). Healthy soil that is most conducive to soil microbe activity has a C-to-N value between 8 and 15.
5. Soil health calculation (SHC): Summarizes the overall health of your soil based on several indicators in the Haney Test. Prefer a score greater than 17. The higher the score, the better, and you want to see it increase over time.
PLFA test – analyzes phospholipid fatty acids found in the cell membranes of living organisms. The test measures not only the total amount of PLFA, reflecting the total soil microbial biomass, but also looks for signature fatty acids found in specific groups of bacteria, fungi, etc., of interest.
- PLFA: Total biomass. Prefer a value greater than 2000 ng per g (nanogram per gram). This value should trend up as soils regenerate.
- PLFA: Arbuscular mycorrhizal fungi (AMF). These fungi are natural biofertilizers that exchange water, nutrients and pathogen protection with plants for carbon made during photosynthesis and then secreted through plant roots. Test results are in ng per g and should trend up.
- PLFA: Functional Group Diversity Index. This represents the number of different microbial species present as well as the relative abundance of each species. Prefer a value greater than 1.2 and should trend up.
One other biological test producers may want to do on their land is to count earthworms. Earthworms can contribute to the natural fertility of soil, and therefore their numbers can be monitored as an indicative measure of natural soil fertility. They also help soil’s aggregate stability and water infiltration.
Earthworms: Measure out 1 square foot on the ground and use a spade or trowel to dig down 1 foot deep for 1-cubic-foot sample. Turn the soil over into a bucket and count the number of earthworms found in the hand-dug surface soil. Prefer more than 10 per cubic foot. The number should trend up as the soil gets healthier.
