The structural management of a clay soil is not such a simple problem as that of sandy one. In clays and similar soils of temperate regions the potential plasticity and cohesion are always high because of the presence of large amounts of colloidal clay. When such a soil is tilled when wet, its pore space becomes much reduced, it becomes practically impervious to air and water, and it is said to be puddled. When a soil in this condition dries, it usually becomes hard and dense. The tillage of clay soils must be carefully timed. If plowed too wet, the structural aggregates are broken down, and an unfavorable structure results. On the other hand, if plowed too dry, great clods are turned up which are difficult to work into a good seedbed.