The combination of individual sand, silt, and clay particles into larger clumps called aggregates creates soil structure. Good garden soil is crumbly. Nutrients and water are held within the aggregates. The spaces between aggregates allow water and air to move easily though the soil and provide enough space for plant roots to grow.
Plants grow best when good soil structure is maintained. Do not walk on, work in, or drive machinery over wet soil. It is also important to refrain from excessive tilling. Any of these actions can damage soil structure, compacting the pores, which decreases the movement of air and water through the soil.