Soil Organic Matter: The Secret to a Healthy Garden

Soil organic matter is the unsung hero of a healthy garden.

What is soil organic matter (SOM)? It’s a component of soil, and as the name suggests it’s made of organic matter, as opposed to sand, silt, and clay. Organic matter means material that has come from any living organism, mainly waste and remains. It holds immense amounts of water, provides an ideal environment for a vast diversity of soil microbiota, and allows plants to access nutrients that would otherwise be locked away. Soil organic matter allows for a garden, and helpful organisms in it, to thrive. 

Let’s dive in and learn more about this building block of healthy soil and learn how to best make use of it in garden spaces.

SOM was not always soil; it’s made of plants and other organic materials that have been broken down by microbes and been made bioavailable to plants again. In other words, a plant can’t draw the nutrients out of a leaf just by contacting it with its roots - these nutrients must be made into “food” for the plant by microbes. This “food” primarily comes in the form of carbon. Additional “foods” like nitrogen can come from leguminous sources of SOM.

Soils with high organic matter content absorb and retain vast amounts of carbon, which would otherwise be in the atmosphere.

There are two types of soil organic matter: particulate organic matter (POM) and mineral-associated organic matter (MAOM). The stability of POM is directly determined by the total mass of living roots in an area. For example, a garden with long-rooted native plants will have a higher POM compared to a traditional ornamental garden where the plants have shorter roots.

In contrast, MAOMs regulating factor for stability is microbial activity. Microbial activity can strengthen or weaken depending on environmental conditions, whereas root biomass and growth remain consistent year to year. Nitrogen plays a key role in regulating soil carbon content, so if you want healthy soil, using plenty of organic compost is key. This will lead to better root growth, which will in turn make the soil more “nutritious” for your plants as POM increases. This feedback loop leads to better conditions for your plants as a whole, including better access to nutrients, higher water content, and larger diversity within the soil’s microbial community.

Interpreting your Soil Organic Matter Content

An easy way to visually assess organic matter content in soil is through its color. As a general rule, the darker the soil, the more organic matter it contains. An example of this are leaves that fall into lakes; they turn from full brown leaves to black particulate as they decay. This dark layer of soil contains incredible microbial communities, mainly fungi and bacteria.

A textural giveaway for high microbial activity is soil that sticks together regardless of moisture - microbes create microscopic bonds which keep soil particulates stuck together. Detritivores, as in waste-eating creatures like rollie-pollies, earthworms, and ants all help pare down larger pieces of organic matter for soil microbes. These microbes work symbiotically with plants to “build” the soil through organic matter and are set back by soil disturbance.

Landscape fabric illustrates this by blocking any organic matter from reaching the soil, ultimately leading to less healthy plants and “dead” soil. This type of soil contrasts high SOM soil, and is mostly clay or sand, lighter in color, more erosion-prone, and absent of much of the biodiversity of high SOM soils. In addition to benefitting the microbial community, SOM also helps with water retention.

Soil organic matter is directly related to water content and retention within soil

For each additional 1% of SOM per acre, that acre of land can hold approximately 30,000 more gallons of water, which is the same amount of water an average American uses per year. The ability for soil to retain water is helpful for both plant health and landscape health. It provides more water for plants and animals, reduces erosion, and reduces stress during droughts.

For a home gardener, especially with long-rooted native plants, it means less watering is needed and a higher diversity of life is supported. With climate change impacts and water overuse leading to an era of “water bankruptcy” per the UN, keeping one’s local ecosystem sufficiently watered is especially relevant. 

All of these factors combine to create better ecosystems for the various organisms supported by gardens. More carbon and water means larger plants with more flowers, which means more abundance for pollinators. Higher numbers of microbiota help detritivore populations thrive, which in turn helps animals higher up in the food chain by putting nutrients back into the ecosystem.

To put it simply, SOM is the basis for a temperate ecosystem’s circle of life. In order to create a strong foundation for all aspects of your native plant garden, we highly recommend improving your soil’s organic matter content.

-Written by Ben Krueger