Organic Matter: Key to Soil Management

Adapted from the third in a series of five UMass Extension fact sheets. 
The series is available online at http://www.umassvegetable.org.
John Howell, University of Massachusetts Extension

Soil organic matter (SOM) improves the moisture holding capacity of sandy soils and the aeration of clay soils and helps the overall tilth of any soil.  In New England soils, SOM is the chief contributor to cation exchange capacity, a measure of a soil's ability to retain nutrients.  The breakdown or decomposition of SOM releases nutrients which can be used by plants.

What is organic matter?
By definition, organic matter contains carbon.  Carbon is a source of energy for microorganisms (microbes) in the soil, including bacteria, fungi, and actinomycetes.  Some of these are pathogens which cause plant disease, but in a healthy soil the vast majority are beneficial.  Organic matter provides food for a diverse population of microbes in the soil and this helps prevent any one type of organism, such as a plant pathogen, from dominating.

SOM is continuously being produced and broken down by living microorganisms.  Dr. Fred Magdoff of the University of Vermont coined an appropriate statement: "There are three kinds of SOM: the living, the dead and the very dead."  The living fraction of the SOM is made up of living plants and animals, including microbes that are found in the soil.  When they die, stalks, leaves and other plant parts retain recognizable characteristics for a while.  This is the dead fraction of the SOM.  Sooner or later the dead organic matter decays and cannot be recognized for what it was, eventually becoming humus.  This is very dead organic matter.  In addition, animals eat plants or other animals and pass some of their food through their bodies as manure which is rich in nutrients and organic matter.

SOM is broken down by microbes as they consume it for food.  Any factor that affects soil microbial activity also affects SOM break down.  In the microbe, respiration combines most of the carbon from SOM with oxygen to form carbon dioxide gas. For this process to continue, there must be an exchange of oxygen and carbon dioxide between the atmosphere and the soil pore spaces.  Gas exchange can be restricted if the soil is compacted or saturated with excess water.  This slows the rate of SOM decomposition.  While excess water inhibits decomposition, a certain amount is necessary to support microbes.  Therefore, conditions of moisture stress can be expected to slow the decomposition of SOM.

Soil microbes are also influenced by soil pH. This is especially true of bacteria.  Under acid conditions, bacterial activity in breaking down organic matter is greatly reduced.  Soil fungi responsible for breakdown of SOM are generally less affected by low pH.  In most cases, however, bacteria are responsible for most of the decomposition of SOM, and as a rule this process is significantly slowed if pH levels drop below 6.0.

Soil temperature has a marked influence on microbial activity.  The optimum soil temperatures for bacterial activity are in the 70 to 100º F range, but activity occurs as low as 40º F, although at greatly reduced rates.  In the South, where soils are warm for most of the year, there is more annual decomposition of SOM

than in the North and generally soils are lower in organic matter.

Organic matter consists of numerous compounds which vary greatly in their ease of decomposition.  Sugars, starches and proteins are rapidly decomposed by microbes, while lignin, fats and waxes are resistant to this process.  Fresh organic residues consist mostly of easily decomposed compounds which break down rapidly under favorable conditions.  The result is a rapid reduction in the volume of SOM.

The resistant materials remain and form the dark-colored material called humus.  Humus continues to decompose, but at a very slow rate.  Carbon dating has shown some humus to be thousands of years old.  Humus forms the colloids which contribute to increased cation exchange capacity and good soil structure.

In summary, a moist, warm, well-aerated soil with a pH between six and seven provides ideal conditions for decomposition of SOM.  These are the conditions that promote optimum growth of most vegetable crops.  So it seems clear that productive farming practices are usually quite destructive to SOM!  But this isn't as bad as it sounds.  Imagine what would happen if SOM didn't break down.  We would be buried under tons of un-decomposed dead plant material.  The decomposition of SOM is a beneficial process.  It provides energy for a diverse group of soil microbes, releases nutrients for plant growth and leaves us with humus.  The challenge is to continuously replace what is lost and, if practical, increase SOM.

Adding To Soil Organic Matter

Compost quickly comes to mind when thinking of ways to add organic matter to the soil.  Everyone from homeowners to farmers can make compost.  Most farmers don't have enough raw materials to satisfy their needs.  Some are bringing in additional materials such as municipal yard wastes to compost on site.  Others are purchasing compost from the increasing number of commercial composters.  Regardless of the source, compost should be finished before use.  Finished compost has no recognizable bits of matter and will not heat up after turning.  Compost should also be tested for nutrient content.  Finished compost should have a low ammonium content, high nitrate level and a pH near neutral.  Repeated use of a compost high in a particular element could cause a nutrient imbalance.

Animal manure is an excellent source of nutrients and organic matter.  About half of the nitrogen in fresh dairy manure and 75% of the nitrogen in poultry manure is in the form of ammonia.  Ammonia is subject to loss through volatilization if not incorporated immediately after spreading.  In the soil, ammonia is converted to nitrate and is available for plant use.     However, nitrate is subject to leaching and large applications should generally be avoided.  There are times when readily available nitrogen is needed, but fresh manure should be applied with caution.  Many people prefer to compost manure before field application.  This stabilizes the nitrogen.  Manure can be mixed with other materials for composting.

Cover Crops are gaining favor as a way of increasing organic matter.  Winter cover crops have been used for years, primarily to protect soil from erosion.  Winter cover crops can also take up much of the nitrogen left over at the end of the growing season.  Winter rye has been an old standby.  It can germinate and make quite a bit of growth, even if planted as late as October.  Winter rye is efficient at taking up leftover nitrogen.  It remains