Soil Organic Matter: The Living, the Dead, and the Very Dead
Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

Soil organic matter makes up only a few percent of most soils, but it has a great deal of influence on soil properties, and in turn, agricultural productivity.

What does it do?  The list of soil properties affected by soil organic matter is long. It includes: aggregate stability (how well tiny clumps of soil hold together, which affects soil structure); cation exchange capacity (the ability of soil to hold onto positively charged nutrients for plant growth), nutrient release rate by mineralization (how much nitrogen, phosphorus and other elements are given off by microbial activity), and water-holding capacity.

How much is there?  The amount of soil organic matter in a particular location is primarily due to natural factors like temperature (cool locations accumulate more organic matter), soil texture (clayey and silty soils tend to have more organic matter than sandy soils), and the drainage (poor drainage promotes soil organic matter build up). However, management also affects soil organic matter level, and it is not unusual for that level to decline over time in cultivated fields. Frequent tillage, periods of bare ground, and removal of crop residues all contribute to reductions in soil organic matter.

What is it?  Soil organic matter is made up of plant and animal residues in different stages of decomposition, cells of soil microorganisms, and substances that are so well-decomposed it’s impossible to tell what they were to begin with.

Living organisms are also considered to be part of soil organic matter, and they play a big role in contributing organic residues to the soil and in formation of more stable types of organic matter. Plant roots and various soil animals (rodents, earthworms, mites, etc.) all provide organic materials to the soil that eventually become part of the soil organic matter cycle. There are four main processes in that cycle, and all of them rely on soil microbes: decomposition of organic residues, release of nutrients (mineralization), release of carbon dioxide (respiration), and transfer of carbon from one soil organic matter ‘pool’ to another.

Not just one kind of organic matter. In addition to organic matter that is alive, there are three types, or pools, of “dead” soil organic matter: active, slow, and passive. These are determined by the time it takes for them to completely decompose.

Active soil organic matter is primarily made up of fresh plant and animal residues that break down in a very short time, from a few weeks to a few years. This kind of organic matter is associated with a lot of biological activity. Passive soil organic matter, also known as humus, is not biologically active, meaning it provides very little food for soil organisms. It may take hundreds or even thousands of years to fully decompose! Slow soil organic matter is somewhere in between active and passive soil organic matter. It consists primarily of detritus, partially broken down cells and tissues that are only gradually decomposing. Slow soil organic matter is somewhat resistant to decay and may take a few years to a few decades to completely break down.

What is Humus?  Hummus (with two m’s) is a middle-eastern food made from chick peas. Humus (one m) is the passive fraction of soil organic matter. It is a dark, complex mixture of organic substances that have been significantly modified from their original form over time, and it also contains other substances that have been synthesized by soil organisms. Usually, humus represents the majority of total soil organic matter, and it is relatively stable over time.

Humus has a lot to do with the ability of a soil to retain nutrients and water. Humus also supplies organic chemicals to the soil solution that can serve as chelates, which can hang onto trace elements and increase their availability to plants.

What is active soil organic matter?  Active soil organic matter is ‘fresh meat’ to microbes. It is the readily digestible and easily decomposed portion of fresh organic (meaning carbon-containing) residues. Active soil organic matter plays a very different role than passive organic matter does. As it is decomposed by soil organisms it helps stabilize soil aggregates, it releases nutrients by mineralization, and it provides food for microbial activity, which can lead to suppression of plant diseases and enhanced plant growth.

The amount of active organic matter in the soil can change quickly, in just a year or two, and it’s highly influenced by soil management practices. To maintain the same level of active soil organic matter requires a constant supply of fresh organic materials, usually from growing plants. Crop roots, crop residues and cover crops all contribute to active organic matter. Soil must also be managed to minimize the loss of organic matter through oxidation (from aggressive tillage) and erosion (from ground left bare).

The amount of active soil organic matter and the proportion it makes up of total soil organic matter are good indicators of soil health. Unfortunately, most soil tests don’t differentiate between the different forms of soil organic matter; they just report the percent of soil that is total organic matter. As a result, farmers aren’t able to determine what is going on with their soil organic matter pools, and they can’t monitor how their management practices are affecting active soil organic matter levels.

The good news is that soil scientists are working to develop affordable tests active soil organic matter, and these should be commercially available in the near future.

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