When you hear the word ‘compost,’ what is the first thing that comes to mind?
Maybe you’re thinking about a pile of food scraps, or raking up leaves in the fall. Perhaps the first thing that follows it is ‘fertilizer’. One of the most important things that compost adds to your soil is organic matter.
We talk a lot about microbes being the true power of any living soil, but we would be remiss if we didn’t give credit to all the work done by organic matter in your growing system. Organic matter is the part of soil made up of plant and animal matter, both dead and alive. It is the basis for fertile and productive soil, even though it makes up a small portion of your soil’s composition.
We’re about to herald the achievements of organic matter and all it can provide, but typical agricultural soils only have about 3-6% organic matter by weight! The reason it’s so important when it exists in such small quantities is because it influences and modifies all essential soil properties. Nutrient availability, aeration, and supporting soil microbes, are just a few functions performed by organic matter.
What Counts as Organic Matter?
There are three basic classifications for organic matter in your soil: the Living, the Dead, and the Very Dead (if you’re wondering, yes that is the official terminology).
The Living element of organic matter includes the vast array of microbes present in soil consisting of bacteria, fungi, viruses, protozoa, and algae. The living element of organic matter includes things like worms, voles, insects, rabbits, and plant roots! Living material makes up 15% of the total soil organic matter. Because of the vast diversity of life below the soil, it is estimated that 25% of the world’s biodiversity is contained in the soil.
Fresh residues (detritus), also known as Dead organic matter, are derived from expired organisms, plant materials, and manures. This component of ‘dead’ matter is considered active, because it is easily broken down in the soil, and used as a food source for the ‘living’ organic matter. As these residues are broken down, different chemical compounds are released that can be used as food sources for other organisms. Proteins, amino acids, sugars, and starches are among the nutrients provided by dead organic matter.
There’s no ‘only mostly dead’ in soil (sorry Miracle Max), but there is a Very Dead classification, which is where things get interesting. If 'dead' organic matter is still considered active, then 'very dead' organic matter is considered inactive. This material consists of organic substances in soil that are difficult for organisms to break down and decompose. While some people use humus as a term to describe all organic matter, humus is actually just one of the 'very dead' types of organic matter in soil. Humus resists decomposition, meaning that it can exist in soil for hundreds and even thousands of years. Over this time, it will slowly release nutrients into the soil as well as improve the overall soil structure by promoting aeration and drainage. Some humus molecules even have the ability to attach to heavy metals and pesticides and trap them, limiting movement into plants and animal bodies, and waterways.
Plant Benefits
Other than just creating better soil for plants to grow in, the presence of organic matter directly impacts your plant health:
The majority of the nutrients that plants consume come directly from the soil and are sourced from decomposing organic matter. Nutrients in soil often exist as components of large molecules, which are a form that plants cannot break down or consume. Microbes and other decomposers are responsible for breaking these down into a simpler form that plants can easily use (the living organic matter in your soil at work!).
Soil texture, or ‘tilth’ identifies how porous the soil is. This texture is important for two reasons: First, the ‘loose’ quality of the soil will determine how easily roots can penetrate the soil and become established. Second, good tilth also means that water easily and readily penetrates the soil, able to reach even the deepest root growth. When water is stored in the soil, it is available for plants to rely on during times of heat, drought, or stress.
Humus and humic substances in the soil can help plants realize their full genetic potential. This results in the growth and development of larger and more colorful produce, brighter and more beautiful blooms, and even the stimulation of healthier root development!
Environmental Benefits
Increased organic matter in soil will not only impact plant health but can provide benefits to the environment!
Healthy soil is characterized by diverse microbes, which can support drought resilience, salt resistance, disease management, and even sequester carbon dioxide from the atmosphere into the soil. The more species present in your soil, the more functions they can perform for a healthier environment. Microbes rely on organic matter as a food source, and as a place to grow their populations. Without organic matter, microbes aren’t able to thrive in their environments, meaning that soil lacking in organic matter will naturally have lower populations of microbes.
When soil is porous, it can readily and more easily absorb water. This limits erosion by allowing the water to go into the soil, rather than washing away and taking the soil layer with it. Runoff of chemical fertilizers is also greatly reduced when water is readily absorbed, limiting the impact of fertilizer on rivers, lakes, and streams.
Figure 2.6. Changes in soil surface and water-flow pattern when seals and crusts develop.Ch 2. What Is Organic Matter and Why Is It So Important - SARE
Organic matter is the most important soil component when it comes to reducing pesticide leaching. Microorganisms in the soil can change the chemical structure of many pesticides, industrial oils, petroleum products, and other potentially harmful compounds, rendering them harmless. Organic matter in the soil will trap these substances within the soil layer, rather than allowing them to seep into groundwater, and give them time to be detoxified by microbes.
Although it only makes up a small percentage of typical soil, organic matter fills an essential niche. We owe a lot to organic matter! When soils are tired and need a refresh, compost is an easy-to-apply solution that supplies organic matter and microbes. There are other ways to preserve soil organic matter including limiting tillage, reducing heavy machinery on your soil, selecting microbially friendly pest control and fertilization products, and using cover crops to keep your soil active.