Soil is more than the dirt under foot. It is a mixture of several components, including rock, three types of mineral particles--clay, silt and sand--air, living and decayed organisms and water, in varying percentages. It is these percentages that determine the three different soil types: clay, sandy or loam, which is the ideal soil for gardening.
Clay soil is dominated by clay particles and holds more water than either sand or loam. Clay is very slow to dry out in the spring, which causes a delay in planting, according to the "Encyclopedia of Gardening" by the American Horticultural Society. Clay also does not absorb heat from the sun as readily as other types of soil, delaying the onset of plant growth, and then inhibiting root growth once the seeds germinate.
Sandy soil has the opposite problem of clay, according to "Gardening for Dummies" by Michael MacCaskey and Bill Marken. Although sandy soil warms up readily in the spring, water drains through it quickly. When plants are watered, the nutrients pass through the soil like a sieve, so frequent, light fertilization is a must, as is frequent watering to keep the roots of the plant from drying out. Sandy soil can be improved through the addition of organic material to improve fertility.
Loam soils are ideal for gardening, containing the perfect mix of sand, clay and silt particles, according to "America's Garden Book" by Louise and James Bush-Brown. Loam has a porous texture to provide good drainage and optimal aeration as well as enough organic matter, called humus, to sustain normal plant life. Additionally, loam is spongy enough to retain moisture without puddling after a heavy rain.