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What Makes Up a Healthy Soil?

What Makes Up a Healthy Soil?

Soil consists of particles derived from rocks (geologic minerals), pore spaces containing water and air, both living and dead organic materials, and chemicals dissolved in water, all bound to soil particles and in a solid form. Healthy soil has good soil structure, can cycle nutrients, can filter and retain water, is easily penetrated by plant roots, has a diversity of organisms living in it, has adequate plant residues protecting surface, and consistently grows crops and or native vegetation.

Soil Texture

The soil at any location is formed through geologic time from rocks deposited at the location. The soil texture is formed due to soil-forming factors at the location and cannot be easily changed by management. The soil particle sizes are sand (0.0021 to 0.078 inches), silt (0.000079 to 0.0021 inches) and clay (less than 0.000079 inches) and measured at different quantities in the soil determine the soil texture. The soil textural triangle in Figure 1 shows how differing amounts of sand, silt and clay determine texture. It is commonly said that an ideal soil is 50% pore space (water + air), 5 % organic matter, and 45% minerals. The 45% of the soil that is minerals is the sand, silt, and clay component. Organic matter and soil minerals form organized units, which are referred to as soil aggregates. A healthy soil will contain a variety of soil aggregate sizes ranging from small (0.0021 to 0.01 inches) to large (greater than 0.079 inches), which remain together when the soil is wetted and when the soil is dry. Having a variety of pore sizes present in the soil allows rain and irrigation waters to infiltrate, but also lets it store water and chemicals in the small pores within soil structural units.

Organic Matter

The organic matter components of soil can be easily changed by management. Practices that land managers choose can either return or remove organic matter (carbon) to the soil. The amount and type of soil carbon will affect what types of organisms and materials make up the soil organic matter. Organic matter in the soil consists of dead aboveground plant material that has been moved into the soil, living and dead plant roots, soil organisms, sugars, carbohydrates, fats, lignin, cellulose, and humic and fulvic acids. A healthy soil should have a balance of these different types of organic materials. Humic and fulvic acids are types of soil organic matter that are most commonly associated with soil clays and remain in the soil for a long period, whereas particulate organic matter from biology growing in the soil can be rapidly changed. Some carbon compounds, such as sugars, carbohydrates, fats, lignin, and cellulose, are formed through primary production and biologic activity.

Common Organisms

Common organisms in the soil include bacteria, fungi, actinomycetes, protozoa, nematodes, worms, insects, amphibians, reptiles, and mammals. A gram of surface soil (0.035 ounces) can contain approximately 100,000,000 bacteria, 10,000,000 actinomycetes, 10000 protozoa, and 10 to 100 nematodes (Brady and Weil 1999). A cubic yard of soil can contain up to 1,000 earthworms and 1,000 other fauna, consisting of insects, amphibians, reptiles, and mammals (Brady and Weil 1999). These organisms all play a role in soil functioning. Bacteria, fungi, and protozoa help to decompose organic materials and make the nutrients contained in dead soil organic matter available to plants. Certain fungi also create associations with plant roots to help them take up nutrients. Nematodes and protozoa eat other soil organisms and release broken down forms of these organic compounds as plant available nutrients. Worms create pores in the soil; eat organic matter, such as plant residues; and help to decompose these materials. Larger organisms (such as insects, amphibians, reptiles, and mammals) use the soil as a home, create large pores, and mix soil materials.


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Photo Credit: gettyimages-dmytro-diedov

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