Organic matter, or OM, is the smallest component of soil but it is also the most important. Without it, life can’t exist. It has a profound effect on every other property of soil.
As we discussed briefly in the Soil Basics lesson, organic matter is made up of three distinct categories. Living organisms, actively decaying matter, and stable, older organic material. All serve different and vital roles in the health and makeup of soil.
Living organisms include bacteria, fungi, viruses, algae and protozoa. Earthworms, insects, plant roots, and even mammals like rabbits and moles also fall into this category. The tiny microorganisms like fungi and bacteria are instrumental in making nutrients available to plants. They serve as a food source for larger microorganisms like protozoa as well. Earthworms, insects and larger animals create spaces in the soil and move it around to stir it up and allow air and water to infiltrate the soil.
While all are important to soil health, perhaps none are quite as beneficial as the lowly earthworm. No other soil creature does as much to enhance soil structure, water and air pockets, nutrient conversion and many other functions. We could write an entire learning series (and will in the near future) on the benefits of earthworm activity to soil health.
Our next category is the recently dead and decaying organic matter. This is made up of plant and animal tissues that are in the process of decomposition. This is the primary food source for all those organisms in the soil. It is also the part of organic matter that must be constantly replenished; either by a healthy system that automatically replaces it with growth that is present in and above the soil or by amendments or additions applied to the soil. Manure, compost, and crop residues are just some of the ways that this component can be replaced.
The final component is the more stable, long-dead matter sometimes referred to as ‘humus’. This organic matter is very small and serves to improve many aspects of soil composition as well as hold onto and slowly release water and nutrients. Humus improves drainage and lightens the soil, preventing compaction. It neutralizes the effects of heavy concentrations of clay by improving aeration. It also improves water and nutrient retention in sandy soils and improves it’s structure by helping hold it together. This is called aggregation.
Organic matter is instrumental in driving the different cycles of the soil as well. It processes and produces nitrogen, water, oxygen and carbon. These are the main cycles that effect soil health, plant nutrition, and a healthy environment for plants to grow roots.
It plays an important role in other aspects as well. We’ve mentioned making nutrients available for plants. It also buffers the soil against rapid changes in pH, the level of acidity or alkalinity of the soil. Microorganisms can directly interact with plant roots to pull nitrogen from the air and deposit it for future plant use. Organic matter changes the color of soil and helps it stick together, or aggregate. It can also help protect against harmful chemicals, harmful organisms, or extreme changes in environment.
No other component of soil can effect soil health as much by its presence or absence. In sustainable production almost every problem can be solved in whole or in part by adding or protecting soil organic matter.
We will explore a lot of these benefits and interactions in more detail as time goes on; especially earthworms, microorganism interaction with plant roots, and the different soil cycles.
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