What makes one soil well-draining and another soil a seasonal lake? For the most part here in Central Florida, it all comes down to fines. Fines are really small particles, considerably smaller than beach sand. Clay and silt are both examples of fines. The amount of fines in a given quantity of soil has a large determination on the characteristics of that soil, but for our case, we’ll just focus on how it relates to water.
Because fines are so small, a cubic inch of pure fines has a larger collective surface area than a cubic inch of beach sand. This extra surface area means there are more places for water molecules to latch onto. Furthermore, that cube of fines will also have fewer gaps that water can escape through. These qualities of fines make them very good at grabbing and then storing water. Too many fines and we get soil that becomes saturated and stays saturated. We call these high-fine soils “heavy.” Heavier soils not only provide for worse drainage but also are more capable than lighter soils of wicking moisture up from deeper levels, which can raise your water table. They also swell and contract more than low-fines soils when wet or dry respectively. You can see this for yourself by observing a dry lake bed. The lake was able to hold water because a high concentration of fines in the lake bed prevented the water from draining quicker than it could be replenished. However, once it did dry out, the soil contracted and cracked.
Growers who are faced with heavy soils can benefit from a combination of raised rows and mulching. The raised rows aid in keeping the plant roots from being permanently saturated. The mulch helps to regulate moisture and prevent the soil from forming a crust. Just like in a dry lake bed, contracted soils on a farm can form a tough crust at the ground surface that is difficult for fine roots to penetrate. Because the crust is so tightly bound, it actually becomes slightly hydrophobic, meaning it repels water. If you irrigate using a drip irrigation system on a raised bed, the water will have a tendency to run off the sides of the bed and into the rows until the crust swells enough to allow water to penetrate. This tends to form channels over time that divert water away from your crops. A layer of mulch evenly distributes water over the soil, reducing channeling and increasing the amount of water available to your plants.