It is important for gardeners to know about soil components, soil texture, and soil structure. Since soil is what supports all life on our planet, it is important to understand a little about what makes up our soil and how it is structured. Soils do such a wonderful job of producing out food, forests, and ornamental plants that gardeners may sometimes take them for granted. Soils are as valuable and essential to life as water and air. By digging into and assessing your particular soil can help you prevent common plant problems and pave the way for years of gardening pleasure. You will find that you can make your own compost with a little knowledge of soil components.
What is typical soil made of?
Soil has three major components. An average handful of garden soil contains the following:
Clay, silt, and sand in various proportions make up 45-50 percent of soil by volume. These minerals come from rocks that have broken down over millenia through the action of wind, rain, ice, and vegetation. The rocks, or parent material, that many mineral soils are derived from are sometimes inadvertently struck or unearthed by homeowners.
Sand is the largest of the particles; you can see individual particles without a magnifying glass. Silt is next smallest in size. Individual clay particles are so small that you’d need a microscope to view them. Pore space makes up another 45-50 percent of the soil. The large (macro-pore) and small (micro-pore) spaces between individual soil particles are filled with a combination of air, water, roots, microorganisms, mites, earthworms, and beetles. During ideal growing conditions, 50 percent of these pore spaces are filled by air and 50 percent by water.
Soil organic matter makes up 1-5 percent by weight. This is the biologically active portion that drives the soil ecosystem and releases nutrients necessary for plant growth.
Soil organic matter makes up a small percentage of the total volume of soils but has a big impact on soil structure and fertility. It is made up of dead and decaying plants and soil creatures-from bacteria to earthworms. More than 90 percent of soil organic matter is found in the top 8 inches of soil, where the majority of plant roots are located. It gives top soils their dark brown color and earthy aroma and supports diverse populations of soil microorganisms and invertebrates (centipedes, beetles, mites, earthworms). This vast world of creatures produce the “glue” that improves soil structure by binding tiny clay particles and large sand particles into stable aggregates.
There may be up to 1 billion microorganisms in a gram of topsoil. These bacteria and fungi digest organic matter, releasing nutrients available for uptake by plant roots. The end of the decomposition process is humus, a stable, prized material that holds water like a sponge. As with clay particles, humus is negatively charged an attracts and holds positively charged nutrient ions, reducing their being leached by rainfall. Gardeners depend heavily on this storehouse of recycled, organically held nutrients to produce healthy plants
Soil Texture and Soil Structure
Soil texture is the term used to describe how a soil feels. It’s simply a measure of the relative proportion of sand, silt, and clay in a soil. For example, a soil with a mineral portion that is 60 percent sand, 15 percent clay, and 25 percent silt is classified as sandy loam. Sandy soil feels gritting; silty soils have a powdery texture when dry; and soils high in clay feel sticky when wet. Texture also determines how quickly the soil warms up in spring and how easy it is to till.
Sandy soil feels gritty, warms up relatively quickly, and is easy to till. Soil texture is not easily changed. Although soil structure can be improved with amendments, soil texture is more a function of the parent soil material and more difficult to alter. For example, adding sand to a heavy clay soil can produce a hard, cloddy, and inhospitable soil.
There are dozens or perhaps hundreds of soil types classified according to texture, slope, drainage, and parent material. It is not uncommon to encounter several soil types in a single home landscape. Contact the Natural Resource and Soil Conservation Service ( an agency of the United States Department of Agriculture). They may have a copy of your country’s soil map. You’ll be able to find out what specific type of soil covers your property.
Soil texture has a huge effect on the establishment and growth of plants. For example, coarse-textured sandy soils contain many large pore spaces, which allow for rapid water drainage. That’s good during a wet spring but a potential problem during hot, dry weather. Rapid drainage also leads to the quick leaching of some nutrients out of the root zone.
With sandy soils, you’ll find yourself fertilizing plants more frequently during wet weather and watering more often during, hot, dry weather. Fine textured soils with a high percentage of clay have many tiny pore spaces that hold water and drain slowly. Roots need air and water to grow. During wet weather clay soils may become oxygen-deficient, causing root death and plant wilting.
Soil Structure is a measure of how individual soil particles are arranged. In soils with good structure, soil particles are connected into larger crumbs or aggregates, creating many large and small pores that encourage air and water flow and root growth. The structure of poor mineral soils can be improved through the regular addition of organic matter-compost, aged farm manure, shredded leaves, grass clippings, and bark mulch.