Soil Organic Matter

Organic matter is at the very foundation of soil ecology and management. In an effort to provide some of the key components of organic matter, we offer the following seven C’s for your consideration.

Composition – What is Organic Matter?

http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html

We often use the term “organic matter” two different ways in farming. In one sense, we refer to plant material in the form of crops, green manures, animal manures, compost, or wood shaving / straw animal bedding all as examples of fresh organic matter that can be added to soil as mulch or food to increase or maintain soil biology. Sources of fresh organic matter may include weed seeds or available forms of nutrients that can be leached, and if incorporated into the soil at high rates may lead to rapid rates of breakdown and decay that can lead to short term reductions in nutrient availability.

The more common use of the term organic matter is for already decayed plant materials after they are no longer identifiable as the original plant and have been biologically degraded to a humus material or soil organic matter. Soil organic matter is the focus here and various forms of fresh organic matter are covered in other sections (Cover crops, Compost, Manure). We often define several types of soil organic matter including 1) the active component that is still breaking down quickly over weeks and months, 2) the slowly available component that is breaking down over a period of years, and 3) the stable or recalcitrant organic matter that is breaking down over decades.

Contributions – What does Organic Matter Do?

The formation of soil is a story of rocks and minerals being broken down to smaller and smaller pieces. Soil Organic Matter is the glue that helps provide structure to all the smaller particles, helps prevent compaction, and can lead to an increase in root growth. As soil organic matter and therefore soil structure increase, soil is capable of absorbing water faster, retaining more water, and resisting erosion by wind and rain. Soil organic matter also attracts and holds many soluble plant available nutrients so the nutrients are retained by the soil and are available to the plant. The presence of soil organic matter usually leads to an increase in the number and types of microorganisms which can lead to reductions in plant pathogens and diseases.

Catabolism - Why do we need Organic Matter to break down?

As plants grow, the energy of the sun is trapped and stored. Just as we get energy from eating plants, microorganisms in the soil get energy by continuing to break down plant derived organic matter. When a horse eats hay, the horse gets energy, a part of the hay becomes the horse, part is lost as feces, and part is lost as carbon dioxide each time the horse exhales. In ecological farming, the soil is alive like the horse and needs to be fed regularly. The soil microorganisms include bacteria, fungi, protozoa, nematodes, and many other life forms that make up the soil food web (Soil Biology).

Construction – How do we increase Soil Organic Matter?

There are many possible sources of fresh organic matter that can be added to soil for the creation of soil organic matter. Each cash crop we grow contributes some roots and plant parts that remain in the soil and some contribute more than others. We also grow cover crops or green manure to provide food for the soil food web and ultimately soil organic matter. Animal manure has been a primary component added to the soil for thousands of years and compost is a more recent addition to the picture. Some organic matter breaks down quickly and some takes longer to degrade. Compost is an example of organic matter that has already been degraded or stabilized. A biologically active soil needs to have mixture of fresh, partially degraded and previously degraded organic matter.

Conservation - How do we maintain Soil Organic Matter?

Environmental factors such as soil moisture, temperature and aeration that increase biological activity may lead to changes in organic matter degradation. Since soil texture can effect soil aeration, it also influences soil organic matter bread down (more breakdown in sandy soils than clay soils). While we need organic matter to breakdown slowly, some farming practices can lead to the undesired rapid loss of organic matter. Frequent cultivation can stimulate soil microorganisms by providing more oxygen. For this and other reasons that help reduce erosion, we want to minimize cultivation. Application of more than needed nitrogen fertilizer also stimulates soil bacteria to degrade more organic matter and should be avoided. Incorporation of excessive amounts of fresh organic matter may also over stimulate the soil food web and decrease organic matter. It is also likely that growing the same crop year after year will lead to reductions in organic matter.

Cycling - Keeping Carbon Moving

The organic matter or carbon cycle is based on continually supplying carbon as a food source for microorganisms, the loss of some carbon as carbon dioxide, and the building up of long term carbon in the soil that contributes to soil aggregation and formation. It is a dynamic process necessary for nutrient availability and cycling.

Conclusion

Soil organic matter contributes greatly to soil quality and plant health. Managing soil organic matter entails consideration of a range of factors that influence carbon cycling and ultimately the long term health of the soil. As ecological, organic and sustainable farmers of the future, growing our understanding of organic matter is indeed at the foundation of soil ecology and management.