. A semipermeable membrane in the Apexa hydration system from DuPont, Wilmington, Del. (plastics.dupont.com), lets clean water vapor pass through to the soil, while retaining most undesirable salts. A double-walled structure containing ribs forms internal channels to hold water. Strips of the membrane are laid in the soil at appropriate depths for specific crops.

The membrane is a proprietary DuPont polymer that's highly permeable to moisture vapor but impermeable to water. When the ground is dry, water releases to the soil when water vapor diffuses through the membrane walls. Water condenses outside the membrane, presenting water to plant roots through the soil. Also, because the system uses a monolithic membrane, there are no holes to clog. The hydration process is self-regulating, helping conserve water. When the surrounding ground is saturated, no moisture transfers.

"The Apexa hydration system can change how the world's food is grown," says Chris Mucha, DuPont program manager. "It can lower overall watering costs and address salinity concerns, while helping conserve precious fresh-water supplies," he said. "More than traditional irrigation systems, this hydration technology is able to deliver water when and where it's needed, automatically, without costly gages or monitoring," he adds.