Sand is probably not what you think of when you think of growing food—it's supposed to be at the beach or in the desert, not in your garden. But as our population and the demand for both food and water grows, farmers will likely have to find ways to grow crops efficiently in less-than-ideal soil, while conserving as much water as possible. Now, physicists have developed some recommendations for dealing with one challenge of growing in sand—water retention—including pre-wetting and mixing absorbent particles in with the soil.
The problem with sandy soil is well known to gardeners and farmers alike: Unlike soils made up of smaller particles and more varied, irregular shapes, sand doesn't hold water and nutrients well. Instead, water falling on sand tends to form a shallow, uniform top layer that collects into narrow vertical channels as water travels deeper—much like water droplets forming and then falling from the edge of a wet roof. As a result, most of the sand never gets wet, and the parts that do drain quickly, making it difficult for plants to take in water.
Unlike soils made up of smaller particles and more varied, irregular shapes, sand doesn't hold water and nutrients well.
Searching for a solution, Yuli Wei and colleagues at the University of Pennsylvania and the Complex Assemblies of Soft Matter lab first performed a series of experiments designed to probe the effects of soil particle size and water flow rate on soil irrigation. Controlling those factors in real soil, however, is difficult to say the least, so the team used boxes of tiny glass beads, ranging in size from 180 micrometers to a millimeter in diameter as a stand in for sandy soil, and they devised a sprinkler system that would allow them to control both how much water fell and how fast the droplets were moving when they hit soil. While channels formed regardless of particle size, irrigation flow rate, and droplet speed, the team found that using larger beads resulted in narrower water channels that formed closer to the surface, while soils comprising smaller particles led to much wider channels and a deeper layer of water near the surface—a consequence of increased capillary forces in finer soils. The team found similar results when increasing the irrigation rate, though droplet speed had no effect.
Next, the experimenters turned to controlling the formation of water channels and improving overall irrigation. One technique that worked was thoroughly mixing a small amount of water into the soil before turning on the sprinklers (or before the rain came). Even in small amounts, pre-wetting was enough to encourage water sprinkled on the surface to diffuse through the soil rather than form narrow channels. A potential alternative to pre-wetting is to add a layer of super-absorbent hydrogel particles—a mix of potassium acrylate and acrylamide—underneath the surface. As the hydrogel wets, its particles swell and form a kind of dam, so that the soil above slowly fills as water falls. Either way, there's more water in the right places for crops to grow.