This is part 4 of a 4-part series. If you missed the earlier series you can read them here:
- Part 1: What Is Soil Health?
- Part 2: The Role of Beneficial Nematodes
- Part 3: The Role of Beneficial Protozoa
Organic matter is all the microbial activity, fresh plant residues, molecules bound to soil particles, and much more. Organic matter is the most important piece of the puzzle and is what makes soil fertile.
Three Parts of Soil Organic Matter
For ease of discussion about organic matter soil scientists have divided The soil food web into three parts. Dr. Fred Magdoff from the University of Vermont termed the three parts of soil organic matter as "the living, the dead, and the very dead." Even though most agricultural and garden soil only contains between 1-6% organic matter by weight and is only about 6 inches deep, it is the basis for healthy plants, animals, and people. So, what’s the living part of organic matter?
The Living Part of Soil Organic Matter
In parts 2 and 3 we went over some of the living organisms in soil, protozoa and nematodes. And looked at bacteria and fungi. According to USDA SARE, "The living portion represents about 15% of the total soil organic matter. The range of organisms is so great it is estimated that they represent about 25% of the world’s total biodiversity." The living soil organisms that make up that 15% of organic matter have a major role in soil fertility. Live organisms in soil need to decompose other parts of organic matter to live. They need nutrients to survive and, in the process, convert many organic soil molecules into inorganic molecules for other microorganisms and plant roots nutrients.
Microbes, with the proper amount of moisture and heat can reduce a compost pile by 70-80%. An open compost pile may also have a few vertebrates as residents, especially in winter, because it’s putting off heat. Their burrowing and nesting activities in the compost, or field residues, break the organic material into smaller pieces so the microbes can be more efficient.
The organisms that create soil fertility aren’t all microscopic. They include earthworms, whose burrowing creates channels for root growth and water infiltration. They leave a sticky substance along the burrow that supports soil aggregation and stabilization. Microorganisms are food for other microorganisms, insects and other arthropods. These organisms live on or in the soil for all or some part of their lives, and are food for other larger soil dwellers such as moles, voles, salamanders, and other arthropods.
The robins that eat the earthworms also contribute to the fertility of the soil by pulling up the worms (leaving a channel for water and roots) and depositing feces. Other birds scratch the surface, such as thrushes who feast on ants. Colonies of ants create intensive tunneling systems. That behavior mixes nutrients through the soil and allows water to reach plant roots.
And of course, we can’t leave out plant growth, both above and below ground. When a plant is alive, by photosynthesis it is creating nutrients out of thin air. Through mutualistic relationships with the microbial community in the rhizosphere plants gain access to macro and micronutrients. Without nutrients and microbial partnerships plants could not survive.
And Then There’s the Dead Organic Matter
Most microorganisms have a very short life span. If they don’t get eaten first, they die a natural death in about 12 hours. Then they become part of the “dead” organic matter. That seems obvious, but we can’t see them so we forget about them.
When we look at a field or garden, we see crop residues and compost in various stages of decomposition, easily identified as dead. They are and they aren’t. They’re alive with “living” microorganisms decomposing them into plant nutrients.
The dead part of soil organic matter experiences rapid changes. Soil microbes are continually decomposing the dead part of organic matter, crop residues, dead microorganisms, and other bits of organic detritus. They are creating food for themselves, other microbes, and plants. Live insects chew on dead residue, increasing the surface area for microbial activity.
The dead organic matter is highly unstable in the soil. Farming and gardening practices that include tilling, crop rotations, and cover cropping can negatively or positively affect the microorganisms in the area. Tilling crop residue under the soil increases the speed it decomposes.
Is rapid decomposition a good thing? Not usually. More CO2 is emitted into the atmosphere as microorganisms decompose the materials. On the up side, more plant nutrients are immediately available for plant uptake, but that’s a short-term nutritional burst.
Crop rotations and mixed cover cropping systems both positively impact the quantity of microbes in the soil as well as their diversity (richness). Greater diversity of living organisms leads to a greater diversity of dead organisms. As dead organic matter is decomposed nutrients are released into the soil in plant available forms.
The Organic Matter That’s Very Dead
The dead organic matter is highly mobile in soil. The nutrients released when organic matter is decomposed by microbiology are taken up by soil microbes and plant roots or they will be leached out of the root zone. These nutrients include the nitrates and sulfates naturally found in soil with high organic matter.
The very dead, on the other hand, is the stable part of soil organic matter that is hard for microorganisms to decompose. This part of the soil is usually called humus and it can remain in soil for hundreds or even thousands of years. Although it sounds like it’s inert, it has properties that make it important for soil health. Humus is also food for fungi and its presence is indicative of fungal activity.
The dead organic matter can be equated to that immediate sugar rush you get after eating too many cookies. The very dead is more like the effect of eating a variety of vegetables for your entire life and being healthy. The chemical and physical properties of very dead organic matter allow it to hold nutrients and release them over a long period to the soil and vegetation.
Humus, or the very dead organic matter, improves soil structure, tilth, and cation exchange capacity (CEC). Humus contains both fulvic and humic acids - which transport nutrients from the soil to the plant rhizosphere, increasing nutrient availability. Depending on farming and gardening practices humus can continue to benefit your property for generations. Organic matter, especially humus, creates the dark color of your soil. That dark color is very beneficial in the spring because a dark soil warms up much faster than a grayish soil. Sooner to work in spring and full of nutrients.
How To Incorporate More Organic Matter for Healthy Soil
Is it hard to incorporate organic matter into your soil? That depends on your farming and gardening practices but even more, on your mindset. Leaving plant residue on the soil surface as organic matter will look untidy. That’s simply because we’re so used to seeing a field or garden as a flat blank slate to plant in.
But think of your property differently. Think of it as a prairie, or maybe pasture. That mixed vegetation is living organic matter feeding microbes that are going to feed your crops.
Potential for Your Crop
Have you thought extra vegetation would take moisture away from your crops? That’s not an unusual opinion. But not only does the pasture cover retain moisture for your crop, it also prevents erosion of your precious topsoil. After you roll or mow it down the roots and crop residue become dead organic matter, keeping your microbial community happy. Using a system of crop rotation with those cover crops will increase the biological activity in your soil, increasing soil fertility. There are both direct and indirect ways organic matter improve soil health and crop productivity.
Direct Ways Organic Matter Increases Crop Productivity
- Through organic matter decomposition nutrients are made into plant-available forms
- The soil is able to retain more calcium, potassium, magnesium, and ammonium
- Micronutrients such as zinc and boron are available in plant-available forms
- Plants and microbes form mutualistic relationships for nitrogen fixation
- Certain microbes make mineral forms of phosphorus plant-available
Indirect Ways Organic Matter Increases Productivity
- Microorganisms produce substances that promote better root growth
- Plants take up more nutrients and water with stronger root systems
- Soil structure is improved
- Clay soils have better water infiltration and CEC (Cation exchange capacity)
- Sandy soils have increased water retention and increased available nutrients
- Fewer problems with pests or disease
Impacts of Organic Matter: Living, Dead, Very Dead
All three forms of organic matter are needed in a balanced soil. Not only to maintain high productivity in your fields or gardens but on a global level. The amount of organic matter on your property has worldwide impacts on the carbon cycle, the nitrogen cycle, and the hydrologic cycle.
The Carbon Cycle
According to the USDA, "Soil organic matter contains an estimated four times as much carbon as living plants, and in fact carbon stored in all the world’s soils is two to three times the amount in the atmosphere. As soil organic matter is depleted, it becomes a source of carbon dioxide for the atmosphere."
There’s more carbon in soil than in all plants, animals, and the atmosphere combined. But those carbon compounds will remain in the soil only through farming and gardening practices that cause the least soil disturbance, remain covered with vegetation all year, and use few synthetics. The "living, dead, and very dead" all play important functions in retaining organic carbon.
The Nitrogen Cycle
Organic matter levels in your field can determine how much nitrogen plants can take up in mineralized form. There is an incredible amount of nitrogen tied up in organic matter. Every percentage point of organic matter in the top 6 inches of soil (topsoil) contains about 1,000 pounds of nitrogen. It’s just sitting there until the microbial community converts it into ammonium. A strong microbial community of decomposers can convert that nitrogen into plant-available forms needed for optimum plant growth.
The manufacturing of synthetic nitrogen fertilizers is a highly energy intensive process. It uses nitrogen from the air mixed with hydrogen from natural gas at high temperatures, taken through a series of products to get the plant available form of nitrogen, ammonium nitrate. That process emitted 1,250 million tons of CO2 in 2018, approximately 2.4% of global greenhouse emissions.
Planting cover crops that include legumes fixes nitrogen from the air. Hmmmm…. not as energy intensive and doesn’t generate millions of tons of CO2.
The Hydrologic Cycle
How fast, or slow, water infiltrates into your soil affects your fields – it also affects your aquifer, nearby streams and rivers, and ultimately the ocean. Soils with high organic matter, relatively undisturbed, increase the rate water filters through clay and decrease the rate water flows though sand.
Water that percolates into the subsoil isn’t available for plant use. It may be charging up the aquifer but it depends on what chemicals that water is taking with it whether that will have a good effect on your area water supply.
Soil erosion is a major concern for many farmers and landowners. Rainfall causes water erosion in many shapes and forms; splash, sheet, rill, gulley, and even valley erosions. All of these can be stopped or mitigated through the use of organic matter. Not just planting plants, but the entire complement of "living, dead, very dead" are needed. Plants with weak root systems can’t handle a downpour, strong plants may bend but won’t break.
Organic matter is a complex of living plants and organisms, crop residues and compost, and humus interacting to maintain healthy soil. Your farm or garden is part of a global ecosystem. How you envision and maintain your soil matters for the health not just of your crops but of the world.