Why Organic Matter is Key to Sustainable Soil Management
You’ve probably heard a lot about the importance of organic matter in your soil and getting the soil organic matter (SOM) to the proper levels according to your soil’s tilth and structure. To use banking as a metaphor, you should deposit more than you take out of your soil bank, but why does it matter?
The proper level of organic matter allows plants in your soil to form healthy root systems, locate the nutrients they need, and hold or drain water while feeding the microorganisms present. It provides and sustains life, prevents erosion, and enhances the health of your soil, promoting long-term success and increased crop productivity.
Understanding Soil Organic Matter
Soil organic matter is the fraction of soil comprised of decomposing matter. Decomposing matter includes plant debris, spent cover crops, rotting crops, food scraps, and grass clippings. It’s often called soil organic carbon because carbon is the bulk of organic matter. Different areas of the United States naturally have higher levels of SOM due to native vegetation, climate, and glacial activity, according to the University of Minnesota Extension.
The levels of decomposition distinguish between three types of SOM:
- Living: Organisms like insects and fungi, roots, and plant residue
- Active (dead): Actively breaking down and partially decomposed organic debris
- Passive (very dead, humus): Stable and in the final stage of decomposition, held on tightly to silt and clay particles.
Earthworms, mycelium, and insects like beetles burrow in the soil, eat their way through, and break down plant debris, leaving behind waste and recycled nutrients. As they travel, soil particles mix and bind, building soil aggregates and increasing organic matter levels. Soil in an agricultural setting should strive for 3% to 6% organic matter for best results. NC State Extension Publications
Benefits of Organic Matter in Soil
Let’s discuss what good things come about when organic matter is present in the soil.
Enhanced Soil Fertility
Healthy soil is constantly recycling macronutrients like nitrogen, phosphorus, potassium, and calcium, micronutrients like copper, iron, boron, and zinc, and non-mineral elements like carbon, hydrogen, and oxygen needed to sustain life. Mineralization is when vital nutrients are made available for plant growth after microorganisms feed.
If you want to learn more about macronutrients and micronutrients and how they affect plant growth, this printable reference on Nutrient Management from Cornell University is an excellent primer. For a deeper understanding of how plants take up the 15 essential nutrients - check out the Nutrient Management pages from the University of Hawaii.
A symbiotic relationship exists between life in the soil, such as roots, earthworms, and fungi, and life above the soil surface, like trees, animals, sunlight, and rainfall. The form of recycling and the rate at which components are passed through from the soil to the air and back down again depending on what’s growing, the season, and weather patterns.
Living creatures release carbon dioxide into the atmosphere; photosynthesis is the primary way the environment reabsorbs and sequesters carbon. This recycling of nutrients provides what’s needed for crops in the garden at any given time, leading to healthier plants and increased yields.
Related: How to Test Soil pH Before Planting — and Why You Should
Improving Soil Structure
Soil structure is the basis of a lot of soil health education because it affects how effectively the soil retains water, avoids erosion, drains, and sequesters carbon. Various soil particles (sand, silt, and clay) are more attracted to some than others and bind together. Imagine reaching into your soil and pulling up a handful. When you open your hand, the chunks of soil you see are soil aggregates. Sandy soil falls apart in small pieces, clay soil may stay all together in one large lump, and silty soil will resemble more of a crumbly cake.
The texture of silty soil improves soil water infiltration, retains moisture, provides channels for earthworms to tunnel through and aerate, and helps roots anchor to them. Accomplish and maintain SOM between 3 and 6% of your overall soil’s makeup for fluffier soil that’s more enjoyable to work with and where less surface crusting will occur.
Adding organic matter does not change a soil’s texture—the percentage of sand, silt, and clay in the soil—but adding organic matter will alter soil structure by increasing the pore space and improving drainage. Gardeners can be successful with any soil texture, as long as they know the attributes and limitations of that soil.” North Carolina Extension Gardener Handbook
How Soil Cation Exchange Capacity (CEC) Boosts Nutrient Retention and Yields
Test your soil for cation exchange capacity (CEC) to determine its nutrient-attracting and holding capacity for positively charged minerals (cations) like calcium, magnesium, and potassium. The results of this test will vary depending on soil structure and organic matter levels. The score will indicate plants’ ability to access the nutrients they need, leading to better yields and healthier crops. Soils high in organic matter and clay have more negatively charged particles and, therefore, a higher CEC, meaning they hold more nutrients. (1)
Related: How to Determine the Texture of Your Soil
Boost in Water Retention
Soil organic matter works seemingly like magic in different soil compositions. In clay-like soils, having the proper levels of organic matter helps drain water, whereas in sandy soils, it helps retain it. Imagine holding sand in your hand, then adding compost and mixing it. The structure becomes more stable, and fewer particles will likely slip through your fingers. On the contrary, if you mix a hunk of clay with a handful of compost, the clay will become less thick, more absorbent, and easy to work with. Increasing SOM will make your farm more resilient to changing weather patterns and events like flooding, harsh winds, and drought-like conditions.
The more water the soil maintains, the less growers have to irrigate their fields, making farming a more sustainable venture. Your crops will also build resilience to drought-like conditions by seeking the required moisture deep within the soil.
Reduced Soil Erosion
Soil erosion will naturally decrease when the soil gains the ability to retain water. Stable soil can drain water more efficiently, anchoring the particles and any roots of live plants in place. Topsoil high in SOM stays in place during harsh winds and heavy rainfall.
Consider what earthworm castings (poop) do for the soil. Organic matter, including sticky microorganism waste, is the glue of soil, helping to bind the particles together and anchor them to each other. As they tunnel through the soil, feeding on decomposing matter, they aerate it. As they leave behind waste, SOM increases, and soil erosion decreases.
Supported Biodiversity
You may have heard the term “feed the soil” and wondered what it means. Feeding the soil refers to nourishing the microorganisms, healthy bacteria, and fungi that hide beneath the soil surface. Soil is safest with something growing in it. As plant matter grows and breaks down, it provides food and sustains all the good guys invisible to the naked eye, which is vital to a successful garden. Make the perfect habitat for beneficial soil organisms to invite them to come and stay.
Related: Top 5 Benefits of Using Cover Crops in Home Gardens
Enhanced Soil Resilience
Supporting the “good soil guys” like healthy bacteria, beneficial nematodes, and the plethora of microorganisms in our soil that help our gardens thrive allows them to be the primary residents of our soil, reducing the number of parasitic nematodes and creating a more stable environment. Stable, healthy soil is more resistant to soil-borne diseases, less likely to erode during heavy rainfall and wind, and more tolerant to drought-like conditions.
Contact your local Natural Resources Conservation Service (NRCS) agent to inquire about soil survey results from your growing region. The results of these surveys may help you decide how best to treat your soil and what best practices to use. Healthy soil is a significant building block for growing healthy crops in the long term.
Practical Strategies for Increasing Soil Organic Matter
Everything mentioned here is one big intertwined cycle. Here are some practical ways to increase your SOM levels that are easy to implement now.
Composting and Mulching
The subject of compost and mulching is vast and can get complicated quickly, but it doesn’t have to be difficult. Adding items from your yard, around your farm, and materials leftover from harvest day are great ingredients for making compost and creating mulch.
Some farmers use leaf mulch exclusively to maintain soil health and keep their SOM healthy. If the ingredients are coming from elsewhere, for example, woodchips from your local municipality, be 100% sure you know the source before bringing it onto your property. Certified Organic or Certified Naturally Grown growers should follow the appropriate guidelines within their certification.
Crop Rotations and Cover Crops
Cover crops are an effective and easy way to add organic matter and cover your soil when unused. Think of them as your “phone-a-friend” line of defense. There’s a family and type of cover crop for every garden size, need, and zone. You can even combine them to get more bang for your buck!
- Need to suppress annual weeds in a new, infertile garden plot? Sow early-season field radishes to break up compaction, block weeds from germinating, reduce spring rain and snow melt erosion, and capture nutrients for future crops.
- Buckwheat is an excellent summer cover crop. It’s quick to germinate and mature, attracts beneficial insects and pollinators, and instantly boosts your SOM levels as it breaks down.
- Select crimson clover to add nitrogen to the soil and help maintain moisture.
Properly terminate the cover crop by crimping, mowing, smothering, flail-mowing, rolling, or tarping. Leave the debris in place and allow it to break down. The decomposing matter is food for the soil; it provides necessary nutrients for the next round of crops, attracts earthworms and beetles, and becomes a part of your soil, increasing SOM. The nutrients that cover crops add to your soil will decrease your need to amend with synthetic fertilizers.
Practice crop rotation so the same nutrients aren’t always in demand. Leave debris in the beds to boost organic matter. Rotate crops in different families to break the pest, disease, and weed cycle. Increase biodiversity in plants to attract various pollinators and beneficial insects.
Related: Cover Crops for Successful Land Management
Reduced Tillage Practices
Motorized tillage in soil is like someone picking up your house, shaking it a few times, and then setting it back down. Getting your things back in the proper order will take a long time. The same disarray occurs for the organisms living in your soil each time the soil experiences tillage.
The Sustainable Agriculture Research and Education states that living organisms in the top 6 inches of an acre of soil with 3% organic matter will weigh about 1.5 tons, the equivalent weight of two Holstein milk cows. That is a lot of shaken houses.
Tillage disrupts the soil structure, hinders microbial activity, and accelerates plant debris breakdown by adding too much air to the soil. This loss of plant debris and microbial activity leaves the soil structure even more vulnerable to environmental stressors like heavy rainfall and wind.
To maintain healthy organic matter levels:
- Reduce tillage and avoid driving heavy machinery over gardens
- Use a 6-mil light deprivation silage tarp to kill back grass and prevent annual weeds from germinating
- Add compost, manure, and crop residue as needed
- Utilize cover crops to prevent soil loss, suppress weeds, and build fertility
Related: Using Silage Tarp for Organic Weed Suppression Control and Bed Prep
Challenges in Managing Soil Organic Matter
Maintaining organic matter levels on your farm can be challenging as it’s breaking down at different rates throughout the growing season. Develop a balance of cover crops, off-farm organic matter like aged manure or compost, and green waste like grass clippings and leaves from your farm to promote soil microbes, healthy bacteria, and beneficial nematodes. Simply put, you want to keep the good guys happy so they can fight off any bad guys the soil may encounter.
Unpredictable and changing weather patterns also cause concerns for maintaining adequate SOM. Knowing the limitations of your soil based on its structure, SOM, and region will help you get in tune with your crops. Annual soil tests should explain your soil’s organic matter levels and recommendations for future management based on the remainder of the test results.
Related: Beginners Guide to Soil, Media, Amendments, and Fertility
FAQs
What are the main benefits of organic matter in soil?
Organic matter promotes microbial activity, holds soil in place, enhances resilience, boosts plant health, and increases yields.
How can I increase the organic matter content in my garden soil?
Rotate cover crops, reduce tillage practices, and add organic materials like leaf mulch, organic straw, grass clippings, and woodchips.
Why is water retention important for soil health?
When plants have access to the water they need, healthy roots form, becoming more resilient to environmental stressors. The soil becomes more robust and resilient to drought, flood, and disease. The soil structure may also improve.
What role do microorganisms play in soil management?
The millions of beneficial microorganisms cycle through nutrients, break down debris, aerate the soil, and help stimulate plant growth. They also keep harmful bacteria in check so your gardens can thrive.
Can organic matter help prevent soil erosion on sloped lands?
Yes, absolutely. Start by planting cover crops to build up the soil organic matter, add fertility, and hold that soil in place.
Written by Jenna Rich of Partners’ Gardens, LLC.
Resources
- Fundamentals of Soil Cation Exchange. Purdue University Extension. https://www.extension.purdue.edu/extmedia/ay/ay-238.html.