Essentially the function of the soil in relation to the plants that grow in it is fourfold: It must supply water; it must supply nutrients; it must supply gases (carbon dioxide and oxygen); and it must be firm
enough to support the plant securely. The ideal soil is a middle-of-the-road mixture, holding moisture and nutrients while letting excess water drain away to make room for air.
Don’t make the mistake of assuming that your garden contains only one type of soil; several different soils can exist in one backyard. Each natural soil is composed of fine rock particles, organic matter, and microorganisms. A good soil is 50 percent solids and 50 percent porous space, which provides room for water, air, and plant roots. The solids are 80 to 90 percent inorganic matter and 10 to 20 percent organic materials. Water and air should each occupy about half of the porous space.
Types of soil
There are four basic types of soil, and the texture of each is determined by the different proportions of various-sized soil particles. These four types of soil are clay, sand, silt, and loam.
Clay soil. A clay soil is composed of particles that are less than 1/31750 of an inch (1/200 mm) in diameter. These minute particles pack together more closely than larger particles and have a greater total surface area. Clay soil can hold more water than other soils. It often drains poorly, but drainage can be improved by the addition of organic matter to break up the clay particles. If you try to work with a clay soil when it’s wet, you’ll compress the particles even more closely; then, when the soil dries, you’ll be left with a surface something like baked brick or
The small particles of clay soil hold water and often drain poorly.
The larger particles of sandy soil often let water drain through too quickly.
concrete. Properly managed, however, clay soils can be the most productive of all.
Sandy soil. A sandy soil is made up mostly of particles that are over 1/3175 of an inch (1/20 mm) in diameter. They are much larger than clay particles and irregular in shape, so they don’t pack as closely together as clay particles. Because they have less total surface area, these larger particles hold less water than smaller particles and are much more porous. Sandy soil drains like a sieve, but can be improved by the addition of organic matter, which helps retain moisture and nutrients.
Silt soil. In a silt soil the size of the particles is intermediate — between clay and sand. Depending on the size of its particles, a silt soil can act either like a clay soil or like a fine sandy soil. Silt consists of small, gritty particles that can pack down very hard, and it’s not very fertile. Silt soil is often found on top of heavy clay, which slows or stops drainage.
Loam. Loam is a mixture of clay, silt, and sand particles. A good garden loam is something to cherish, particularly if it also contains a heavy supply of organic matter. All soil improvement is aimed at achieving a good loam — when you add organic matter or make other improvements to your clay or sandy soil, you’re trying to provide the type of loam that lucky gardeners have without all that extra work.
A do-it-yourself test of soil mixture
The best way to determine the approximate texture of the soil in your garden is by feeling it with your
hands. Try this test: Take a small handful of moist garden soil, and hold some of the sample between your thumb and the first knuckle of your forefinger. Gradually squeeze the soil out with your thumb to form a ribbon. If you can easily form a ribbon that holds together for more than one inch, you have a very heavy clay soil. If a ribbon forms, but it holds together for only three-quarters of an inch to one inch, your soil is a silty clay loam. If the ribbon forms but breaks into pieces shorter than three-quarters of an inch, you have a silty soil. If a ribbon won’t form at all, you have a sandy soil.
