Crusting is often a problem on medium-textured silty soils. The crust restricts water movement through the soil surface and causes water to stand on the field unless there is good surface drainage. These soils drain slowly. Soil structure also affects drainage. The spaces between the aggregates are interconnected with large pores where water and air can move freely.
Massive structure can occur in some loams or clayey soils.
Water and air movement is very slow where this structure occurs. Some clay minerals, such as montmorillonite, swell on wetting and shrink on drying, while others, such as illite, are non-swelling. Swelling clays restrict water movement when they become wet because swelling closes the pores, preventing water and air movement. Other soil properties may also be important in drainage. Organic matter promotes good structure, and, as the organic matter content increases, the internal water and air movement are improved.