Soil is composed of many particles of varying sizes. Soil scientists have classified soil particles into three major groups: sand, silt and clay. Sand particles are the largest and tend to hold little water but allow good aeration. Clay particles are very small in size and tend to pack down so that water does not drain well and little or no air can penetrate. Silt particles are medium sized and have properties in between those of sand and clay.
A loamy soil, then, is one that combines all three of these types of particles in relatively equal amounts. Loamy soil is ideal for most garden plants because it holds plenty of moisture but also drains well so that sufficient air can reach the roots.