Though some gardeners may be blessed with perfect soil, most of us garden in soil that is less than perfect. If your soil has too much clay in it, is too sandy, too stony or too acidic, don’t despair. Turning a poor soil into a plant-friendly soil is not difficult to do, once you understand the components of a healthy soil.
Soil is composed of weathered rock and organic matter, water and air. But the hidden “magic” in a healthy soil is the organisms—small animals, worms, insects and microbes—that flourish when the other soil elements are in balance.
Minerals. Roughly half of the soil in your garden consists of small bits of weathered rock that has gradually been broken down by the forces of wind, rain, freezing and thawing and other chemical and biological processes.
Soil type is generally classified by the size of these inorganic soil particles: sand (large particles), silt (medium-sized particles) or clay (very small particles). The proportion of sand, silt and clay particles determines the texture of your soil and affects drainage and nutrient availability, which in turn influence how well your plants will grow.
Organic Matter. Organic matter is the partially decomposed remains of soil organisms and plant life including lichens and mosses, grasses and leaves, trees, and all other kinds of vegetative matter.
Although it only makes up a small fraction of the soil (normally 5 to 10 percent), organic matter is absolutely essential. It binds together soil particles into porous crumbs or granules which allow air and water to move through the soil. Organic matter also retains moisture (humus holds up to 90 percent of its weight in water), and is able to absorb and store nutrients. Most importantly, organic matter is food for microorganisms and other forms of soil life.
You can increase the amount of organic matter in your soil by adding compost, aged animal manures, green manures (cover crops), mulches or peat moss. Because most soil life and plant roots are located in the top 6 inches of soil, concentrate on this upper layer. To learn more about making your own compost, read All About Composting.
Be cautious about incorporating large amounts of high-carbon material (straw, leaves, wood chips and sawdust). Soil microorganisms will consume a lot of nitrogen in their efforts to digest these materials and they may deprive your plants of nitrogen in the short run.
Soil life. Soil organisms include the bacteria and fungi, protozoa and nematodes, mites, springtails, earthworms and other tiny creatures found in healthy soil. These organisms are essential for plant growth. They help convert organic matter and soil minerals into the vitamins, hormones, disease-suppressing compounds and nutrients that plants need to grow.
Their excretions also help to bind soil particles into the small aggregates that make a soil loose and crumbly. As a gardener, your job is to create the ideal conditions for these soil organisms to do their work. This means providing them with an abundant source of food (the carbohydrates in organic matter), oxygen (present in a well-aerated soil), and water (an adequate but not excessive amount).
Air. A healthy soil is about 25 percent air. Insects microbes, earthworms and soil life require this much air to live. The air in soil is also an important source of the atmospheric nitrogen that is utilized by plants.
Well-aerated soil has plenty of pore space between the soil particles or crumbs. Fine soil particles (clay or silt) have tiny spaces between them – in some cases too small for air to penetrate. Soil composed of large particles, like sand, has large pore spaces and contains plenty of air. But, too much air can cause organic matter to decompose too quickly.
To ensure that there is a balanced supply of air in your soil, add plenty of organic matter, avoid stepping in the growing beds or compacting the soil with heavy equipment and never work the soil when it is very wet.
Water. A healthy soil will also contain about 25 percent water. Water, like air, is held in the pore spaces between soil particles. Large pore spaces allow rain and irrigation water to move down to the root zone and into the subsoil. In sandy soils, the spaces between the soil particles are so large that gravity causes water to drain down and out very quickly. That’s why sandy soils dry out so fast.
Small pore spaces permit water to migrate back upwards through the process of capillary action. In waterlogged soils, water has completely filled the pore spaces, forcing out all the air. This suffocates soil organisms as well as plant roots.
Ideally, your soil should have a combination of large and small pore spaces. Again, organic matter is the key, because it encourages the formation of aggregate, or crumbs, or soil. Organic matter also absorbs water and retains it until it is needed by plant roots.