When organic matter decays and weathers, it forms a thin layer of soil. Our gardens rely on this soil to be nutrient-rich, drain well and hold water well enough to keep our plants hydrated. Soils are defined by their texture--the soil's particle size--as well as its proportion of clay, silt or sand. There are four primary types of soil, with the fourth being a mixture of the other soils. Each type offers it own advantages and disadvantages.
Silt is made of quartz and other minerals. Silt has a smooth texture, but looks like a dark sand. It is very fertile and nutrient-rich. Silt can hold moisture, but because of its texture is easy to work with even when it is wet.
Sand is made up of small particles of minerals and rocks. Sand is primarily formed from the weathering of shale, quartz, limestone and granite. This soil is gritty, but drains well and warms up faster in the spring compared to other soils. As a result of its ability to drain well, plants planted in sand may not get enough water and can become dehydrated.
While clay is nutrient-rich and retains water well, it is the most challenging soil to work with. Clay is very fine grain and has little air space. It compacts easily and holds in moisture so plants often either are not able to grow their roots, or if they do establish themselves, they become waterlogged quickly.
Loam is not really a soil type of its own, but rather a mixture of the three other soil types. This soil contains some mixture of silt, sand and clay. Loam is gritty, retains water and also has good drainage. Most any plant will thrive in loam soil.