How Organic Matter Helps Your Soil
Compost, well-rotted manure, and other forms of organic matter can improve just about any type of problem soil. Here's how and why.
The first step to being ready for gardening season is to prepare your garden soil for planting. That doesn’t just mean tilling; it also means incorporating organic matter into your soil before you sow the first seed or transplant the first seedling. Without the right organic nutrients in the ground, your plants will never reach their potential. Compost, well-rotted manure, and other forms of organic matter can improve just about any type of problem soil. Here's how and why.
About 90 percent of soil (the solid part) is composed of minuscule bits of rocks and minerals—the actual building blocks from which the soil was formed. Most soils are made of a mix of sand, silt, clay—particles that are determined by their size. Sandy soil is made up of relatively large soil particles. Clay soil is made up of relatively small particles. And silt falls somewhere in between. There are generally 10 or more textural classes of soil that include sandy clay, silty clay, and loam, which is a balance of fine clay, medium-size silt, and coarse sand.
The remaining 10 percent of soil is made of organic matter, which has everything to do with how well the soil nurtures plant growth (a process known as soil fertility). It’s the continual decomposition of organic matter that creates humus and releases plant nutrients. To boost your soil’s fertility, you must put out a welcome mat—in the form of the right conditions—for soil organisms to thrive.
About 50 percent of the volume of good soil is made up of pores, which are the spaces between particles that allow air and water to penetrate.
What Is Organic Matter?
What is the definition of organic matter? It’s anything that was once alive. In other words, dead leaves and faded flowers are organic matter; foam packing peanuts and plastic straws are NOT organic matter. When living things die and biodegrade, microorganisms break down the once-living matter into simpler compounds—first into humus (a chemical substance that stores plant nutrients, holds moisture, and improves soil structure), then humic acid (molecules that help plants take in water and nutrients), and eventually into basic elements. This process is called mineralization.
BTW: In case you were wondering, the main source of organic matter is plant tissue. In forests, that tissue comes in the form of fallen leaves. On farms, it’s the portion of the crops left in the ground after harvesting. In your garden, the primary source of organic matter will likely be grass clippings or shredded leaves.
Organic Matter Examples and Benefits
So where do you get the organic matter to use in building up your soil? Chances are it won’t be super expensive. Look for composted materials (purchased or DIY), green cover crops (aka green manure), peat moss, sawdust, shredded tree leaves, grass clippings, well-rotted animal manure, vegetable waste, and the dead bodies of insects and microorganisms. Here’s what not to include: diseased plants; grass clippings treated with noxious chemicals; bones and scraps of meat; and pet feces. Increase your soil’s organic content (ideally up to 5 to 6 percent) by tilling organic matter into the soil’s top several inches or mulching it with multiple types of organic matter. Make the addition of organic matter an annual activity to keep your garden soil healthy and humus-rich.
Healthy soil holds water without getting soggy and lets air penetrate to plant roots and soil organisms. The soil’s organic matter provides surfaces where nutrients can be held in reserve, which helps supply nutrients for plants over the long term. And healthy soil filled with organic matter increases the presence and activity of beneficial microorganisms and macroorganisms, such as earthworms. Many microorganisms work to break down organic matter, releasing nutrients into the soil. Some beneficial microorganisms in the soil also attack plant diseases, helping your garden stay healthier.
Microorganisms and Macroorganisms
Microorganisms (aka microbes) are the microscopic plants and animals that add life to the soil by feeding on once-living matter. They include bacteria, fungi, actinomycetes, algae, protozoa, yeast, germs, and nematodes. How prevalent are they? There are about 50 billion microbes in 1 single tablespoon of soil. Yikes! (But show some respect, please. Without microbes, dead plant and animal life would never decompose—and who wants a dead dinosaur lying around in their backyard?)
Macroorganisms, which can be seen with the naked eye, also serve a specific purpose related to building up soil. They range in size from the tiniest of mites to rodents—the biggest group of mammals in the world. The macroorganism you should care about most, though, is the earthworm, which tills and aerates the soil and transforms compost and other decaying organic matter into helpful humus and worm castings. (Dark brown, porous humus keeps soil healthy and helps it hold water.) Don't simply add earthworms to your garden thinking they’ll improve the soil. Instead, build up the soil with organic matter to attract earthworms.
How Organic Matter Helps Sandy Soil
As noted earlier, sandy soil is made up of relatively large soil particles that fit together loosely, and without sticking. Water runs through such soil quickly—which makes it dry out faster. Those large particles also make it harder for the soil to hold nutrients to nurture plants.
Organic matter helps sandy soils by acting like a sponge, allowing the soil to hold moisture during times of drought, and making nutrients available for a longer period before they leach out of the soil. Organic matter can help make some nutrients more available to your plants. As it breaks down, organic matter reduces erosion by helping the soil hold together better.
How Organic Matter Helps Clay Soil
Clay soil, which is loaded with nutrients, is made of very small particles that hold together tightly. As a result, there's much less air space in the ground for plant roots than what you find in sandy soil. This dense soil structure doesn't usually drain very well.
Organic matter comes to the rescue by helping push apart those tiny clay particles and creating more air space. Water drains more freely and plant roots grow more readily. Because organic matter keeps little clay bits from sticking together, the soil resists compaction, staying lighter and looser.