The biggest problem with irrigation on soil health, is usually the problems with clay platelets... Here is the best little introduction to Soil Structure I've ever come across... Short, simple and yet very insightful... http://soils.missouri.edu/tutorial/page9.asp "... Air, water and plant roots can penetrate deeper in the soil; this can be important to plant survival during times of drought. The larger voids serve as short-term storage space for water, easily accessed by plants. ... Aggregation begins with flocculation of clay particles (platelets) into microscopic clumps called floccules; the cations that are caught between two platelets attract the negative charges on both platelets, binding them together. ... The polyvalent cations (including Ca2+, Fe3+ and Al3+) may also attract and bind with hydrophobic (water repelling) humus molecules allowing them to bind to clay surfaces. These clay-humus particles bind with each other and with grains of silt to form the smallest of the primary aggregates, perhaps as small as 0.01 mm. ... Notice what they say here about the heaving/shrinking, wetting/drying and freeze/thaw... "... As a soil dries out, the clay platelets move closer together and cause shrinking in soil volume. Cracks will form along tiny zones of weakness, and over the course of several wet/dry cycles this network of cracks becomes better defined. Plant roots, as they repeatedly remove water from the same vicinity, reinforce a drying pattern and contribute to physical aggregation of the soil. The process of freezing and thawing in the soil also contributes to the drying process as ice crystals form. And shrinking and swelling that results from wet-dry and freeze-thaw cycles creates tiny cracks or fissures (shrinking) and pressure (swelling) that break apart structureless masses of clay to eventually form soil peds or aggregates."