What is Organic Matter? Benefits

January 16, 2015 Print Friendly and PDF

Green manures are grown to add nutrients and organic matter to the soil. Green manures are incorporated into the soil while green or shortly after flowering. Source: dirtworks.net


Corn husks, cobs and stalks provide ideal material for composting. Photo credit: Emilian Robert Vicol Flickr CC BY 2.0

Organic matter is derived from the breakdown of plant and animal residues. It is composed of forty-five to fifty percent carbon, along with oxygen, hydrogen, nitrogen, phosphorus, and sulfur.

Organic materials, when applied as mulches or incorporated into the soil, will slowly undergo breakdown by soil microbes into soil humus.

Examples of organic matter include plant residues such as lawn clippings, leaves, corn stalks and straw, manures, sludges, wood and food processing wastes, and green manures (plants grown for the purpose of incorporation into the soil).

Organic matter and soil humus (highly decomposed organic matter) provide the following benefits to soil health:

  • Provide 90% to 95% of nitrogen in unfertilized soils; nitrogen is the key nutrient and controlling factor for plant growth.
  • Supply plant-available phosphorus and plant-available sulfur when soil humus is present (25% or more).
  • Improve soil structure by increasing aggregation of soil particles which in turn promotes aeration, infiltration, and percolation.
  • Furnish 30% to 70% of a soil’s cation exchange capacity. The higher a soil’s cation exchange capacity, the greater its ability to hold onto nutrients until needed by the plant and microbes.
  • Increase plant available water, or water available to plants in-between field capacity (water remaining in the soil after saturated soil has drained) and permanent wilting point (when plants wilt but cannot recover despite the addition of water).
  • Act as chelates which help mobilize metal micronutrients, increasing plant-availability of these nutrients.
  • Reduce soil erosion.
  • Organic matter is a carbon (food) supply for beneficial soil microbes.
  • Nutrients in organic matter are not readily leached from the soil.

Composting plant residues (either fast composting or slow composting) may be desirable. Composting will facilitate microbial decomposition of the organic matter into readily-incorporated material.


Additional Resources:


Utah: How to Add Organic Matter to Soil
Oregon: Improving Garden Soils with Organic Matter


Michigan: Managing for Soil Organic Matter
Illinois: Making and Using Compost for Organic Farming
Minnesota: Why Manage Soil Organic Matter?


South Carolina: Interpreting Soil Organic Matter Tests


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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.