Healthy soil is the engine that begets fertility — but high-octane, fertile soil is not necessarily healthy
I have been involved in agriculture for more than five decades and have seen many different systems of crop production applied in many different places. I have listened to arguments for and against “organic,” “conventional,” “no-till,” “sustainable,” and “regenerative.” But, at the end of the day, I have to ask if a particular system is good for the soil or not, if it produces nutrient-dense food or not, and if it is profitable for the farmer or not.
I’d like to focus in this article on whether or not a given cropping system makes and keeps the soil healthy. Healthy soil not only supports the ability of plants to become healthy food; it cycles and purifies the water we drink and supports all of the life that surrounds us.
Soil health can be defined as the continued capacity of soil to function as a vital, living ecosystem that sustains plants, animals, and humans. As we have examined soil health, we have identified some useful indicators to describe how healthy a soil might be. This is akin to assessing our own health with a set of indicators — blood pressure, oxygen level, blood sugar, heart rate, etc. By looking at a number of these indicators, a physician can create a picture of our health and develop a course of action to make any needed improvements.
There are many indicators of soil health that have been proposed by NRCS (Natural Resources Conservation Service) and others to help us get our arms around how well a soil is doing and its capacity to function at a given point in time. While assessing soil health is not an exact science, it is helpful to determine how a soil is trending under a given management system, to at least weigh if the system is improving or degrading the soil.
Soil Health Indicators
Soil health indicators can be categorized into physical, chemical, and biological properties.
Some physical indicators of soil health include aggregate stability (resistance of soil aggregates to breaking down when wet), infiltration rate (how fast water enters the soil), bulk density (a measure of compaction and pore space), available water holding capacity (soil’s capacity to store water for plants), and surface cover (usually the percent of the soil surface covered with plant residues).
Chemical indicators have long been used in an attempt to assess plant nutrients or a chemical balance in the soil. Some chemical indicators include pH (acidity/alkalinity), electrical conductivity (measures soluble salts), plant nutrient availability (typically nitrogen, phosphorous, potassium, and others), and cation exchange capacity (the soil’s ability to hold positively charged ions such as potassium, calcium and magnesium).
Because the soil is a living system, biological indicators are perhaps most important in assessing soil health. Biological indicators might include soil organic matter (the total amount of living and dead organic material in the soil), soil respiration (a measure of carbon dioxide released by the respiration of the organisms of the soil food web), phospholipid fatty acid (PLFA) or DNA sequencing (the chemical signature of what particular organisms are present in the soil), earthworm counts, and enzyme activity (a measure of the amount of biologically produced enzymes active in the soil).
Some other visual assessments of soil health might include soil color, plant root development, and soil aroma. Visual indicators are a bit more subjective, but they can add to an overall portrait of soil health.
Soil Health Guidelines
As I have studied soil health over time, I have come up with my own favorite soil health indicators that incorporate multiple factors together to tell me the most about the present condition of a soil. My favorite indicators are soil aggregate stability, soil microbial diversity and biomass, and earthworm populations. Soil aggregate stability and earthworm populations are fairly easy to assess yourself (though testing labs will do aggregate stability for you). Soil microbial diversity and biomass testing is now widely available from labs that utilize PLFA analysis or DNA sequencing. If I want to compare the soil health of two agricultural systems, soil aggregate stability, soil microbial diversity and biomass, and earthworm populations are my go-to gauges.
Here are some general guidelines that I use when judging these indicators to determine if a soil is healthy or not. Healthy soil should have:
- Soil aggregate stability: greater than 50 percent water-stable soil aggregates
- Microbial biomass:
- Bacteria: 2,000-5,000 lbs/acre
- Fungi: 2,000-5,000 lbs/acre
- Protozoa: 200-300 lbs/acre:
- Nematodes: 100 lbs/ac
- Microbial diversity: fungal:bacterial ratio of 1:1
- Earthworm population: more than 5 earthworms per cubic foot of soil
Water-stable soil aggregates are where biology meets physics. Microbes and fungi produce “biological glues” like glomalin and polysaccharides that stick soil particles together into crumbs and clumps of soil we call “aggregates.” A rudimentary soil aggregate stability test can be done by simply collecting a few soil aggregates and allowing them to dry for a few days, then dropping them in water to see how long they hold together. While this assessment is a bit subjective, it can give you a good idea of how a soil aggregate stability test that assesses the actual percentage of aggregates that are water-stable might go.
Many of the glues that hold soil aggregates together also serve as food for members of the soil microbiome. If the glue-making organisms can flourish to the extent that they can make the glues faster than other organisms can eat them, soil aggregates should remain stable.
It is also important to know both the quality and quantity of microorganisms in the soil to determine its health. Lab tests like the PLFA can identify exactly who is living in your soil, such as bacteria, fungi, protozoa, and nematodes, as well as the extent of each that are present. A diverse and balanced community can generate nutrient cycling amenable to plants with minimal external inputs. In particular, the ratio of fungi to bacteria can indicate how well a soil can cycle nutrients and maintain soil aggregates. A ratio of 1:1 fungi to bacteria is recognized as a good balance that can feed plants and maintain the structure of the soil at the same time.
Lastly, earthworms are a great indicator of soil health because they are easy to observe and do well when the rest of the soil microbiome does well. Earthworms eat most of the members of the soil microbiome, so when there is a diverse and abundant community of microbes in the soil, earthworms will be well fed. Earthworms reproduce based on weight rather than age, so the faster they grow, the sooner they are able to reproduce.
Soil Fertility vs. Soil Health
The term “soil fertility” has been around a long time and is often used to indicate the ability of a soil to supply the nutrients plants need to grow in order to produce a harvestable crop. Soil health, as mentioned previously, is described as the capacity of the soil to function.
We can think of soil fertility as the fuel, while soil health is the engine. A healthy soil with a diverse soil microbiome will generate fuel biologically and then “run” at a rate necessary to feed the plants growing in it. A fertile soil can generate fuel in a number of different ways — from the addition of organic matter (manure, green manure, or compost), significant tillage that aerates the soil and super-charges soil bacteria to break down soil organic matter, or the addition of plant nutrients in either synthetic or natural forms.
A fertile soil isn’t always healthy, but a truly healthy soil is almost always fertile because the biology is constantly recycling minerals and making them available to the plant instead of nutrients being available in only short bursts when organic inputs are added to the soil. Plants exude a great deal of sugar and other exudates into the soil to feed soil biology in a way that matches the needs of the plants. If plants get their nutrients in intermittent bursts, they may become less reliant on the steady supply from the soil microbiome for their nutrition, or some of the excess nutrients may be lost to leaching or volatilization. This can result in both soil and plants suffering in the long term.
No matter what your crop production system may be, I think it is important to determine if your soil is truly healthy or if it is just rapidly cycling nutrients from inputs. This is why I suggest that folks assess their soil health using at least the three soil health assessments mentioned above. Stable soil aggregates, a robust soil microbiome, and the lowly earthworm can give you a good idea how well your soil engine is running to support your farm in the long term. Performing additional tests of soil health indicators can give you an even better understanding of the status of your soil, especially if there is a particular area of concern, such as excess salts or poor water infiltration.
We shouldn’t assume that following a given crop production system will make or keep our soil healthy — the proof is always in the pudding, and monitoring soil health can help us stay on the right track when managing the soil.
