What is Soil Organic Matter?

What is Soil Organic Matter?

Soil organic matter (SOM) is commonly described as the “backbone” for organic life in the soil.  The preservation of soil carbon is vital for living soils, and plays a crucial role in our food and farming systems and mitigation for rising global temperatures.  

It has been estimated that approximately two-thirds of atmospheric carbon is the result of burning of fossil fuels, with the remainder due to agricultural land disturbances.   In an effort to conserve SOM, climate scientists such as James Hanson advocate for improved agricultural and forestry practices that take steps to improve soil fertility through increasing its carbon content, such as alley-cropping and coppicing.

Primarily, SOM is derived through the process of photosynthesis where atmospheric carbon becomes "fixed" as simple carbohydrates and sugars in the form of root exudates.  Generally, SOM refers to the fraction of the soil composed of dead plant and animal matter throughout different stages of decomposition.  SOM can be divided into three major types:  

  1. Plant residues and the living microbial community. 
  2. Active soil organic matter referred to as detritus
  3. Stable soil organic matter known as humus.

Most productive agricultural soils have somewhere between 3 and 6% soil organic matter.   There are various examples of regenerative farms that have upwards of 12% to 15% soil organic matter, although climate, temperature, and soil type are crucial factors to consider. When we talk about “humus”, we are referring to that uppermost fraction of the soil at its final stages of decomposition, characterized by that rich, black/coffee hue.

There are numerous chemical, physical, and biological benefits to agricultural soils with higher levels of soil organic matter.  Most importantly is soil organic matters’ role in shaping soil health and structure.  It accomplishes this through enhancing porosity leading to better aeration and water infiltration and holding capacity.  As the backbone to all life, soils with greater amounts of soil organic matter provide fuel for all the organisms along various levels of the soil-food chain.  From the microscopic bacteria, fungi, protozoa, and nematodes invisible to naked eye (quick note: the number of bacteria in a handful of healthy soil exceeds the human population of earth by more than  a factor of seven) to larger arthropods and earthworms, all life requires a steady flow of organic matter to survive.   As these creatures and critters go about their lives hunting for food and avoiding predation, their movement further structures the soil as channels are carved and conditions improved for water and air to deeply penetrate and support life.  When consumed and passed as waste, nutrients like nitrogen, phosphorus, and potassium previously immobilized in the cell walls of these organisms are released and taken-up by plant roots.

Because undisturbed soils typically contain higher levels of soil organic matter, and soil microbes need a continuous supply of soil organic matter to survive, implementing regenerative practices such as no-till, cover cropping, and composting can help improve soils and plant health. 

 That is to say:  healthy soils grow strong and healthy plants.  Improve the health of your soils by implementing practices to increase soil organic matter.




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