Dec 16, 2011
Soil … it’s more than dirt

Soil is a living, breathing organism that, when properly managed, is a farm’s best defense against an environmental disaster and a farm’s best offense for financial stability. By understanding the makeup of soil, farmers can shield themselves against potential dangers and fortify their enterprises’ financial success.

Soil is comprised of abiotic (non-living) and biotic (living) components. These components work in tandem to ensure there is a strong soil structure. They also work together to form a place where plants, animals and microbes can live and grow.

Abiotic soil elements are what most people think of when they think of soil. It is air, water, mineral matter and organic matter. The majority of this segment of soil is in mineral matter. Mineral matter is the sand, silt and clay that make up soil. The proportion of these in relation to the others is termed soil structure. Soil structure is important in determining management strategies, but there are minimal management practices that effect soil structure besides soil erosion.

Water and air comprise 50 percent of the abiotic portion. Their amount is dependent on the soil structure. Soils that are higher in clay particles can hold more water or nutrients, but they also will hold it tighter and it may be harder for plants to take it up. The amount of water and air in the soil profile also has a direct effect on the soil biota.

Soil organic matter is the last of the abiotic elements. This is probably the most misunderstood part of the soil. Soil organic matter is less than 5 percent of the total mass of soil, but it is one of the most important aspects. Soil organic material changes a soil organism’s habitat and provides a food source for soil biota. When microorganisms feed, they change soil organic material so that inorganic nutrients, nitrogen, phosphorus and sulfur are released. Another term for this feeding is decomposition. Since microorganisms are continually changing the soil organic material, replenishment of soil organic material is necessary to maintain good soil health.

Biotic soil elements are the living organisms found in the soil. Most of these are microscopic, but they are vital for healthy soil. The most abundant biotic organism is plant roots. Plant roots and shoots are an energy and carbon source for soil organisms. Soil microorganisms can be found at high concentration near plant roots.

Bacteria and fungi are the most important organisms that are found in soil. They are responsible for organic matter decomposition. They bind the soil to form soil aggregates. Bacteria are important aspects in the nitrogen cycle, and some fungi help plants uptake water and nutrients. Protozoa also help soil decompose by feasting on plant roots, bacteria and fungi.

Nematodes (microscopic worms) are the most numerous animal in the soil. They increase soil decomposition by eating bacteria, fungi and plants. Arthropods and earthworms are two other classes of animals that are important for good soil health. Arthropods are small insects that graze on smaller organisms, bacteria, fungi and plants. Earthworms are important for soil aeration – their burrows are important macropores in the soil that increase soil water infiltration. They also increase soil aggregation and nutrient cycling when soil passes through their systems.

So, what do the components of soil have to do with soil quality? Soil that is managed to encourage good soil biota has proven to have good soil organic matter, higher water-holding capacity, a higher water infiltration rate, a good carbon to nitrogen ratio and a higher nutrient availability and release.

These qualities equate to cleaner water that infiltrates through the soil profile and less water run off because water can infiltrate through the soil profile.

Economically, farms can benefit because they will not have to apply as much nutrients or irrigation. There is also the potential of higher yields.

By Christina Curell, Michigan State University

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