PARSONS - “Soil health” is a term used commonly today. But what does that really mean? The USDA’s Natural Resources Conservation Service has defined soil health as “The continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and humans.” Okay, but what does that mean? To a farmer, soil health is the capacity of the soil to produce a crop – healthy soils produce crops of higher quality and higher yield.

So what makes soil healthy? Soil is composed of physical, chemical and biological components. The physical components are most commonly thought of when we think of soil – it’s the rocks and minerals that have been broken up into very small particles of sand, silt and clay. These compounds are regularly measured to determine the texture of the soil. A simple method of determining soil texture is to put a small amount, about a teaspoon full, of soil in the palm of your hand and add enough water to make it muddy. Mixing the soil and water in your hand, soil texture can be determined by how the soil feels. Sand is the coarsest material (62 um – 2 mm) and can be easily seen or felt in a soil sample as rough particles. Silt particles are smaller than sand (3.9 – 62 um; Figure 1), while clay particles are very small (less than 3.9 um). Soils high in clay have a soapy feel. The relative proportions of sand, silt and clay determine how the soil is classified. For example, a silt loam soil has 20 to 50 percent sand, 75 to 90 percent silt, and 0 to 30 percent clay. In contrast, a silty clay loam has 60 to 70 percent silt, 0 to 20 percent sand, and 25 to 40 percent clay. The textural composition of soil is determined by the soil formation processes, termed “pedogenesis”. These processes are regulated by the parent material (for example, the underlying rock; in our area, limestone is a common parent material), weathering, and plant cover. Some soils are developed from erosion – wind and water can carry soil and deposit it in new areas, creating loess (wind-blown) and alluvial (water-borne) soils. A good physical composition is the first ingredient for a healthy soil.

The second component of soil, and one that is also regularly measured, is the chemical component. This includes the nutrients nitrogen (N), phosphorus (P or DAP), and potassium (K or potash) that are commonly included in added fertilizers. Another important chemical characteristic of soils is the pH. The pH of soils can be adjusted with lime. Soil pH is critical as it changes the availability of the other nutrients. If a soil is too acidic (low pH) or alkaline (high pH), nutrients may be present in high quantities in the soil, but they bind too tightly to the physical components and become unavailable to plants. A similar phenomenon can occur with water in the soil. Clay mineral particles bind things very tightly. Even when clay soils have high water or nutrient content, the plant may not be able to take up the water and nutrients because the clay particles bind them too tightly. This is why it’s important to adjust the pH of the soil – to increase the amount of nutrients available to the plant.

The final component that is critical to the overall capacity of soil to provide a “vital living ecosystem” is the biological component. We are learning much more about the factors involved in the biology of soils and their role in soil health. The biological component includes the plants, animals, insects, earthworms, nematodes, arthropods, protozoa, fungi, and bacteria that live in the soil. The biological community is a very important component of soil health. While much of the soil biological community is visible, such as earthworms, the truly dynamic component is too small to be seen without magnification. This microscopic community, the microbial community or microbiome, is responsible for much of the activity that goes on in the soil. A teaspoon of soil can contain a billion bacterial cells, several to hundreds of yards of fungal hyphae (Figure 2), thousands of protozoa, and 10-20 nematodes. Some of these are beneficial, for example the Rhizobia bacteria that work with plants to fix nitrogen in certain plants such as soybeans. Arbuscular mycorrhizal fungi (AMF) are a group of beneficial fungi that form close bonds with plants, actually growing into the root cells of vascular plants and helping the plants take up nutrients. Other microorganisms are detrimental, such as the fungus that causes charcoal rot. The soil microbiome is truly a very dynamic, active, and diverse community. The microbial community performs much of the activities of breaking up and recycling plant residues, and capturing nutrients and water. The bacteria and fungi form close interactions with plants, creating symbiotic relationships that benefit both the plants and the microbes. The microbes mine nutrients and water from the soil, and transfer these to the plants. In turn, the plants release sugars (carbohydrates) that the microbes need for an energy source. This dense, symbiotic network is the key to soil health.

Much is known about how to manage the physical and chemical characteristics of soils to improve their productive capacity. We are learning the importance of the soil biological components and their contribution to agronomic productivity. Biological soil characteristics are important for their role in integrating physical and chemical characteristics of the soil for optimal productivity. This “vital living ecosystem” must be supported in order for it to support plants, animals and humans. Just like you and me, the most important thing the soil microbiome needs is food – and most of that comes from plant residues. Reducing tillage and increasing the amount of plant residues, for example by planting cover crops, helps nourish the soil microbiome and improves soil health.

If you would like more information or have questions, please contact me at (620)-820-6131 or by email at gsassenrath@ksu.edu.