Gypsum, John, Gypsum.

http://www.ars.usda.gov/main/site_main.htm?modecode=36021500 found 35 lbs per inch of rainfall per acre will increase soil porosity or drainage 300-500 per cent more depending on soil type.

Ben Franklin plastered the hills of Philadelphia for his crops, spelled out gypsum and it could be seen for miles with the greener crops.

http://www.pbs.org/benfranklin/l3_world_agriculture.html

Ed