Organic matter and soil: the “why’s” and “how’s”

Why does organic matter improve soil?

If you farm or garden, do you check the organic matter levels of your soil? It is a good indicator of soil health since you can’t have good health without it. Organic matter feeds the microbes that build the soil.

One of the goals of any good farmer or gardener should be to increase the organic matter of their soil.

There are three ways to achieve this:

  1. Reduce tillage because that just stimulates residue breakdown and loss of organic matter.
  2. Grow crops that produce large amounts of organic matter like corn or wheat or plant cover crops that that are not harvested and grown for their biomass (green manures).
  3. Add animal manures or compost back to the soil.

You can grow healthy and productive crops and plan a high residue crop rotation.   A good rotation can include sod and grass crops that leave lots of roots in the soil, crops that leave lots of surface residue, and/or cover crops that supply both.

Organic amendments can also be used to build soil organic matter. There is no end to the number of possible organic amendments and by-products available for your soil.

Organic matter helps build soil structure, provides a source of nutrients to crops, aids in reducing compaction and surface crusting, increases water infiltration and builds soil structure. It is the raw material that microbes in the soil convert to organic matter which eventually decomposes into humus. Humus is a recalcitrant organic matter that is resistant to further decomposition. Raw organic material is unstable since it readily decomposes with 90% of it disappearing as microbes consume it and respire carbon dioxide. The remaining 10% forms stable humic compounds that fortify the soil.

Humic compounds build soil structure and tilth. Porosity, aeration, water-holding capacity and permeability are all favorably affected by humus. The addition of easily decomposable organic residues leads to the synthesis of organic compounds that bind soil particles into structural units called aggregates. These aggregates help to maintain a loose, open, granular condition. Humus increases the ability of the soil to resist erosion and enables soil to hold more water.

When you begin to add organic matter to the soil, the first step is to increase the active organic matter that microbes feed upon. Gradually, the species and diversity of organisms in the soil begins to shift, and the amount of stabilized organic matter will increase. It may take a decade or more for the total organic matter levels to increase. However, don’t get discouraged, the beneficial effects of adding organic matter appear long before the total organic matter levels are measurable.

If you haven’t already, it’s time to begin measuring and tracking organic matter since it is a good indicator of soil health. Measures below 2% are low, which means your soil needs help. Shoot for 3% or more but remember building organic takes time and a sustainable strategy. It takes time to build organic matter but even less time to drive it back down.

Dr. Dan Davidson posts articles on soil health and management related subjects. If you have suggestions for topics or questions, feel free to contact him at or call 402-649-5919.