When gardeners consider soil types, it mostly comes down to texture. Soil texture refers to the size of the soil's mineral grains, which affects such qualities as drainage, pH--how acidic or basic soil is--and even fertility. As typed by the grain size most affecting a given soil's properties, sand, silt and clay are the three basic soils. Loam is a type of soil that has favorable proportions of the three sizes. Finally, potting soil is a special case used for plants grown in containers.
Sandy soils are mostly sand with clay and silt supplementing. The finest sand particles range from about 0.02 to .02 mm; coarse sand has bigger grains, measuring up to 2 mm.
Sandy soil has the largest pores--spaces between soil particles that hold air and let water drain. Unfortunately, sand's good drainage can cause problems for plants that need moisture. Additionally, nutrients can wash right through. This leaching also makes the soil tend toward acidity (below pH 7).
It's easiest to use plants in sandy soils that enjoy the conditions, including blueberry, thyme and quince. To make sandy soil heavier, add organic matter such as coir (coconut shell fibers) or compost. Lime can increase pH.
The finest type of particle, clay, comprises about a third of clayey soils. Clay grains are only about 0.002 mm and can hold together closely. Pores are also small, decreasing drainage and making clay soil the best at retaining water.
Because water doesn't leach away nutrients, clay soils tend to be rich. Clay soil also tends to have a higher pH than sandy soils, so the soil might be basic (above pH 7).
Heavy clay soils can have aeration problems, smothering roots. Too, some plants need a drier soil than clay can provide. Amend clay soil with organic matter. Plants that grow in clay soils include lilac, honeysuckle and dogwood.
Silty soil includes many particles between the size of sand and clay. This soil can be a problem because the silt sticks to the other minerals to form a crust that makes it difficult for roots to move and plants to sprout. It's not a good soil without amending it. Besides regularly adding organic matter, gardeners must avoid compacting silty soil.
Loam is the best soil for most plants, blending sand, silt and clay along with organic matter in proportions that allow one ingredient to offset flaws in other ingredients. Sand is the highest proportion of the inorganic material, with clay the least represented.
Potting soil is recommended for growing plants in containers, especially indoor plants, because the soil is free from weed seeds and pests. It comes in many formulas, some geared toward specific plants such as African violets.
In general, potting soil contains fertilizer and both organic and organic materials, with ingredients varying greatly among brands. Read packages before purchasing.