Soil health is often defined as “the continued capacity of the soil to function as a vital, living ecosystem that sustains plants, animals and humans.”
We often think of soil health management as a new strategy, but it’s not. Healthy soils, with effective nutrient and hydrologic cycles, were functioning well before man decided to manage them. Agriculture in the early 1900s tended to focus more on plowing up the prairie soils with industrial technology and machinery rather than focusing on the soil’s ecology. At the time, soils were largely viewed as a medium for growing crops.
The Dust Bowl of the 1930s changed perceptions of soil and its importance to our work and lives. For instance in 1949, Aldo Leopold in A Sand County Almanac said, “Land, then, is not merely soil; it is a fountain of energy flowing through a circuit of soils, plants and animals.” Leopold went on to state, “When we see land as a community to which we belong, we may begin to use it with love and respect.”
Much has changed in how we view the soil today. We realize now that soil is living, and we’re working to learn more about its biological components. What has not changed is our responsibility as land stewards, and that process starts with how we see the soil.
You can use the following five indicators of soil health on any farm with just a shovel, your eyes and your nose.
Soil color provides a tremendous amount of information. Soil color can tell us the amount and state of organic matter and iron oxide, age, and other physical processes. In general, the darker the soil, the higher the organic matter content. Soil is typically darkest in the uppermost layers of the soil profile, and it lightens as depth increases. Soil organic matter and soil organic carbon are primary drivers in biologically active soil systems. In some cases, the dark color can be due to the presence of reduced iron and manganese in our deep prairie soils.
Today, we use soil color to not only gain a general sense of organic matter but to classify soils across the globe with a standard soil color system. Albert H. Munsell first standardized the soil color system as we know it today based on a system with three components: hue, value and chroma. It was primarily standardized for use in industry as a way for companies to order standard, consistent colors for materials. The U.S. Department of Agriculture later adopted the Munsell system as its official classification of soil colors. Following much success in its use by soil scientists, the USDA later helped develop the industry-standard Munsell Soil Color Book.
Healthy soils are biologically active soils. The presence of biological activity can give you insight into the soil’s state of health. Essentially, we are referring to the presence of earthworms, earthworm castings, dung beetles, etc., or evidence of their activity.
Earthworms are not only major decomposers of organic material, they are underground engineers. Earthworms create burrows through the soil profile, which increases porosity, enables water to move down and creates channels for roots. Earthworm excrement, known as castings, help increase nutrient cycling because pound-for-pound they contain significant amounts of nitrogen, phosphorus and potassium.
Dung beetles are another indicator of biological activity. Dung beetles are found on every continent except Antarctica, and they provide tremendous ecological services. These beetles take dung from the soil surface, roll it into a ball, lay their eggs in it and bury it deep in the soil. This creates a food source for their young and brings nutrient-rich organic material into the soil profile, which increases nutrient cycling and availability.
Soil structure is the arrangement of soil particles in different sizes and shapes. Structure often determines the amount of pore space between particles. Pore space is the space between soil aggregates, which the USDA Natural Resources Conservation Service define as “groups of soil particles that bind to each other more strongly than to adjacent particles.” More pore space allows for greater water infiltration.
The ability of a soil to hold its particles together and form soil structure is referred to as “aggregate stability.” Soil aggregation can occur by physical processes, such as when positively charged cations bind with clay particles. Soil aggregation can also occur biologically by organic adhesives. The amount of organic matter in a soil is a primary driver of aggregate stability. Commonly, aggregate stability increases as the percentage of soil organic matter increases.
Soil texture, or the amount of sand, silt or clay content, also plays a large role. Generally, soils with higher clay content inherently have greater amounts of organic matter. It is more difficult for sandy soils to build soil structure largely due to lower organic matter concentrations.
When looking at a soil profile or even a shovel slice, we can often see evidence of layers of resistance. These resistance layers can be seen in the soil structure with the presence of platy structure or horizontal layers. A couple of common sources of this effect are the continuous compaction of a soil from the soil surface and previous tillage creating what is known as a plow pan. Both of these restrictive layers limit root penetration and water infiltration.
One common indicator of a resistance layer can be found in the plant roots themselves, specifically in taproot species. On these sites, taproot plants will show signs of “J” rooting, which means a plant root grows down to the resistance layer and turns 90 degrees because it cannot penetrate the resistance layer. In extreme cases, water infiltration is also halted at this layer, which limits the soil’s water holding capacity and ultimately exacerbates the effects of drought.
The fifth indicator isn’t visual, but it depends on another one of our senses: smell. The earthy smell of a biologically healthy and active soil is the presence of an organic compound called geosmin.
In 1965, American scientists isolated the primary odor of soil to a single compound, which they called geosmin from the Greek, geo (earth) and osme (odor).
Geosmin is an organic product produced by active soil bacteria. Essentially, if your soils are cycling organic matter, they will have that fragrant earthy smell. Soils can have other smells, but they are not associated with soil health. Soils absent of oxygen can have a rotten egg or sulfur smell. This is often a sign of poor drainage.