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Heavy clay soil presents problems to homeowners because its structure is so tight and dense. When clay soil has very little organic matter content, the pore spaces within the soil are very small. Rainfall and/or sprinkler water penetrates the soil very slowly and percolates downward very slowly. Thus, the water infiltration rate is usually much slower than any sprinkler application rate.

Gypsum may be used to help flocculate clay in muddy ponds, but it isn't terribly effective at improving water infiltration rates in clay soil.

Here is an excerpt of a website explaining the effects of gypsum on caliche (clay soil high in calcium), found at : http://www.wtamu.edu/~crobinson/DrDirt/gypsum.html

"Gypsum is a salt - calcium sulfate - and when added to calcareous clay soils (the typical high calcium soil in Colorado), does no more than increase the already high calcium content. Thus, gypsum + calcareous clay = gypsum + calcareous clay. In other words, adding gypsum to a soil that does not need calcium is a waste of money. Also avoid adding gypsum to a saline soil (soil high in salts). Gypsum increases salt levels."

The best way to improve soil structure is to add high quality organic matter, like compost. This is more practical in garden beds than in the lawn. Add up to 4 inches of compost and incorporate it to a depth of 8 inches. Avoid using un-composted manures since they will raise salt levels.

In the lawn, aerate twice annually. After core aeration, topdress with high quality organic matter, such as peat moss or compost. During the growing season, mow with a mulching mower to return clippings to the soil; this will help increase organic matter.

To help improve irrigation water infiltration and avoid runoff, cycle the irrigation . Apply a small amount of water (about one-quarter inch or less) during the first cycle, then repeat after one to four hours, then repeat again after one to four more hours. By cycling the irrigation this way, you will allow water to percolate deeper into the soil. Vary the frequency of irrigation days according to the season and weather conditions. During spring, irrigate only every 10 to 14 days. During summer, increase irrigation frequency to about every 5 days. Clay soil holds water much longer than sandy or sandy loam, so you can irrigate less frequently.