There are many reasons you might need to buy topsoil. Natural topsoil is only about 2 to 3 inches deep on the top layer of earth. It erodes easily. Constant tilling, and wind exposure of tilled spaces, wear down topsoil quickly. Local topsoil varies widely from place to place. Color and texture are big clues to the type of natural topsoil you have. The three main types of topsoil are sand, silt and clay.
Light tan, grainy topsoil is sand. That type of topsoil is easy to dig, but presents problems for both building and growing. For gardeners, sandy topsoil leeches out too quickly. Nutrients and water dissipate through the soil and away from the plant roots because the soil drains fast. Sandy topsoil does not support structures well either. Any building, from small garden sheds, to big homes, requires deep placement of cement footings to hold structures in the loose soil.
Clay is heavy, reddish in color, and full of minerals. On the positive side, clay has a high nutrient level, and holds those nutrients. Unfortunately, there are more serious negatives. Clay is so heavy it packs down hard, and is difficult to dig. Clay soil holds water around a plant's root system, and can drown them. If you have clay soil it is important to aerate it regularly and provide drainage ditches around the rows of your plants and trees. Clay soil supports structures of all sizes very well.
Silt is a natural occurrence of a sand and clay blend. It appears dark, almost black, and falls apart easily in your hands. It offers the best of both worlds naturally. However, on its own silt does not provide enough of either of the good qualities of sand or clay for optimum plant growth. It is a little better at holding structures than sand, but still shifts too much for long-term strength. It drains better than clay, but can still allow flooding, and leeches out too many nutrients too fast.
The Perfect Blend--Loam
Optimal garden soil is a mixture, or blend of sand, silt, and clay. Loam is a perfectly even amount of all three types of soil. It drains well--but slowly--providing the best platform for nutrients for plants without stressing them with too much water, or causing root rot from standing water.
Commercial topsoil comes in two varieties: local for whatever location the soil business is located in, and blended. Blended topsoil in bags, or delivered by the yard in truckloads, is a mixture of topsoil meant to produce the perfect texture and holding power. Blended topsoil is always more expensive than local topsoil, even when local varieties are rich in nutrients.