Clay: is composed of fine, flat, waferlike particles that fit together tightly and take in water slowly. Chemically, clay is primarily silicon and aluminum in composition, with small amounts of sodium, magnesium, iron, calcium and potassium.
Once the clay particles absorb moisture, they hold it so tightly that it is almost impossible for plants to get any use out of it Beyond this, there is no space for air to penetrate. When clay dries, it is difficult to work, and plant roots have a hard time penetrating downward.
Rubbed between your fingers, wet clay soil feels smooth, soft and slippery.
Sand: has particles at least 25 times larger than the largest clay particles. Pure sand, while high in mineral content, contains almost no nutrients and has almost no capacity to store moisture. Air penetrates sand deeply, and water moves through too rapidly, dissolving away many nutrients.
Sand thus dries out quickly and sometimes reflects enough heat to damage vegetable crops. Most sandy soils contain enough clay and silt to retain some water and nutrients.
Rubbed between your fingers, sand feels grainy and gritty. Silt is a kind of intermediate stage between clay and sand. Silt particles pack down hard, almost like clay, but they are considerably larger. Their size is about halfway between the size of sand and clay particles. Silt topsoils are often found over dense layers of clay. They are often not fertile.
Rubbed between your fingers, silt feels a bit slippery, with a grainy texture.
Loam: is made up of clay, sand, silt and a good supply of decomposed organic material called humus. The grains have good structure built by the combined action of root growth, insects, worms and bacteria. This type of soil drains well, yet retains enough water for good plant growth. Air can circulate freely, and there is plenty of room for roots to grow.
Very few soils are ready to grow vegetables; most soils need to be worked before they will yield as they should. If soil has a high clay content, or too much sand or silt, if s got to be improved. Fortunately, it is possible to change the structure, drainage and circulation of most types of soil by the addition of organic material and inert materials such as gypsum. This is also dealt with under Organic Gardening in Chapter 3. Organic additives should be the first alternative considered by the gardener.
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