In the last decade or so, there has been an overwhelming interest in soil health amongst many progressive farmers, agricultural businesses and agricultural enthusiasts.
Farmers have intuitively understood the importance of soil health for generations and recent efforts have focused on how to better measure and quantify soil health.
So. What exactly is soil health?
According to the Ohio State University soil health team, soil health is:
• Holistic. Soil health considers chemical, physical, and biological properties, which are interrelated and equally important.
• Feeding the soil, not the plant. Sustainable agricultural practices harness biological processes to maintain and improve soil health for multiple seasons.
• Good stewardship. Healthy soils perform essential ecosystem services – some of which we are only beginning to understand.
• Cost-effective. Long term improvements in water management, nutrient cycles, and soil food webs can minimize inputs and maximize productivity.
It is understood that measuring soil health properties represents a great opportunity to better understand how farm management practices impact key soil functions.
Soil health testing builds on our current methods of soil testing to provide additional information to farmers. Rather than focusing solely on soil chemistry, additional biological and physical components of soil are incorporated into a common framework.
Building and maintaining high functioning soils require that all three key components (chemistry, biology and physical structure) to be consciously managed and optimized.
In 2020, Ohio State University Extension began measuring soil health across Ohio through eFields on-farm research efforts.
Although there are many potential soil health measurements or indicators to choose from, Ohio State University Extension, selected the following indicators to measure soil health:
1. Total organic matter. A very important soil property that is has long been recognized as a master variable in soil. This is commonly measured in routine soil testing.
2. POXC (permanganate oxidizable carbon). POXC (aka “Active C”) is a biologically active pool that represents a small fraction (<5%) of total organic matter. POXC is a more sensitive indicator of changes in management practices compared with total organic matter, which changes slowly over time.
In contrast POXC represents a more dynamic pool of organic matter and nutrients that are more rapidly cycled and plant available.
3. Aggregate stability. Soil aggregation is the essence of a soil’s structure. Soil minerals (sand, silt, clay) are bound together by organic matter to form micro-aggregates which, in turn assemble to form macro-aggregates. Aggregate stability is measured as the ability of a soil to hold together and not slake in water.
This relates to greater pore space for gas exchange, faster water infiltration, resistance to water and wind erosion and compaction by traffic.
Soils were sampled from 88 fields across 26 counties in Ohio during May-July 2020. Soil cores (10-15 cores per sample) were taken from 3 different depths 0 – 4 inches, 0 – 6 inches, and 0 – 8 inches. (Clinton County had 5 fields represented in this study).
Soil cores from each depth were pulled from the same locations. Fields represented different soil types and management histories (ex., long-term no-till vs. recently tilled, history of cover crops vs. no recent cover crops). The Ohio State Soil Fertility Lab handled all soil samples for analyses. Analysis included: Routine nutrient analysis: pH, total organic matter, cation exchange capacity (CEC), Mehlich-3 extractable nutrients, Permanganate oxidizable carbon (POXC), and Aggregate stability.
The following are some of the key results:
Influence of cation exchange capacity
A primary challenge of quantifying soil health is to know how soil type influences your values and what represents a “good” vs. “bad” value. Cation exchange capacity (CEC) is a reasonable predictor of soil type, with sandy soils having low CECs and clay soils having higher CECs. Ohio State University examined how CEC was related to total soil organic matter, POXC (a biologically pool of organic matter), and aggregate stability. Overall, all three measurements increased as CEC increased.
Influence of depth
As expected, most soil properties differed according to the depth the soil was sampled. The shallower depth (0-4”) yielded greater values than the deeper sampled soils (0-6” or 0-8”). Nutrients and organic matter are naturally stratified in soil (enriched at the surface) relative to deeper depths. This underscores the importance of keeping sampling depth consistent between samplings and over time to be able to evaluate trends in soil test values.
Influence of no-tillage
Years in no-tillage had mixed effects on soil health in farmers’ fields. Overall, no-till did not show a clear trend in soil health, likely indicating that differences in soil type and other management practices need to be examined when considered effects of long-term no-till practices.
Influence of cover crops
The number of years a field was in continuous cover crops was associated with positive increases in all three soil health measurements. These relationships suggest that long-term cover cropping is associated with positive increases in soil health.
In summary, soil type matters. The type of soil has a major influence on soil health properties. It is agreed there is a need to adjust what a “good” soil health value is based on the type of soil. As CEC increases, total organic matter, POXC and aggregate stability increase.
Depth of soil sampling matters. As sampling depth increases, soil values typically decrease. We need to consider the depth sampled when we examine soil health values and be consistent with depth the soil is sampled over the years.
Management matters. The way soils are managed over time have large impacts on soil health. Across 88 fields, OSU found that:
- Years in no-till was associated with both increase and decreasing soil health values.
- Years in cover crops was associated with increases in soil health values.
Although there were 261 soil samples from 88 fields analyzed, this dataset in no way is comprehensive and additional work is needed to better understand the trends observed from the 2020 soil survey.