In the semi-arid portions of the West, maximum moisture accumulation in fallow fields is the goal of farmers rotating wheat with fallow. Stewart Wuest, ARS soil scientist at the Soil and Water Conservation Research Unit in Adams, Oregon, recently gave a lecture at the Unit on his research and observations on this subject over the last 20 years. His research shows each one-quarter inch of stored rainfall in the fallow year can be worth two bushels of wheat in the crop year; hence decisions of when to till, or not till at all, become very important.
Years ago, tillage was timed to kill weeds before they went to seed, or grew vigorously enough to use up significant amounts of moisture. With the advent of Roundup and other chemicals to control weed and volunteer growth, growers could pay more attention to maximizing moisture retention. So what factors are important in that regard?
A typical gentle spring rain does not flow into the soil like fluid through a straw. Water clings to the surface of cracks and channels, moving horizontally or vertically (up or down) yielding to the forces of surface tension between water molecules and attraction by soil particles. Moisture will tend to move from areas of high moisture to areas of low moisture, as happens when water moves out to the sides of an irrigation furrow.
To demonstrate these forces, take a small piece of a newspaper and let a drop of water fall on it. You will notice some of the water will spread into the surrounding paper, demonstrating the attraction of soil particles. A part of the drop will remain where you dropped it, demonstrating surface tension of the water molecules. Now turn the paper vertical. The drop will start to move, then either stop at the edge of the wetted area or continue a wetting front downward until the forces of attraction, surface tension, and gravity equalize.
During a rain event, the film covering the crack or crevice will thicken until gravity overcomes the forces of attraction and surface tension, causing the water to move deeper into the soil. Water then moves downward until all forces equalize. When moisture evaporates from the upper layers of soil, surface tension and particle attraction operate in the opposite direction, moving moisture from lower layers upward. Eventually gravity, particle attraction and surface tension forces equal evaporative forces, and moisture loss ceases.
Untilled ground provides lots of surface cracks, worm holes, decayed root channels, etc., that allow rapid water entry and prevent runoff from heavy rains. When the weather turns hot, however, the continuous, uninterrupted soil particle-to-particle contact (consolidated soil) provides an avenue for moisture to move up and out through evaporation from the surface.
Tillage effectively stops moisture movement in both directions until soil consolidation can be re-established. Tilling too early in the spring can mean late spring rains cannot move deep enough to avoid complete loss through evaporation. Tillage creates voids through which moisture will not travel until the soil above is completely saturated. Only then will droplets form that will cross a void and enter the lower soil layers. Without a continuous path of consolidated soil to move water lower in the profile, small rains will be held close to the surface and will be totally lost to evaporation.
With a heavy rain, water can flow into soil like water through a hose. One problem, though, is such movement can carry small soil particles that plug the cracks and crevices, slowing or even stopping the downward movement of water. This is especially true of recently tilled soil not protected with copious amounts of residue. Significant erosion can then result on slopes, and persistent puddles or mini-lakes may form in flat areas.
Temperatures severe enough to cause significant evaporative losses from untilled soil usually do not occur until June or even July. Dr. Wuest’s research indicates even small amounts of rain can add to moisture stores in untilled soil until early June, and since even one-quarter inch of stored moisture can add two bushels to next year’s crop, one should not be in a hurry to break the soil conduits with tillage.
Research has also shown that differences in total moisture storage between tilled and untilled soil are not significant, though untilled soil will generally not have enough moisture in the seed zone until fall rains begin. If tillage is your preferred practice, it should be shallow and leave surface residue in place. Surface residue will beneficially reduce summer soil temperatures and is very important in reestablishing the moisture conduits when fall rains begin. Residue protects the soil from the tendency of wet soil particles to adhere tightly to one another, sealing the soil against water penetration and forming a crust when the rain stops. Partially buried residue will serve as a conduit to reestablish soil consolidation and allow rapid water movement to lower profiles.
Jack DeWitt is a farmer-agronomist with farming experience that spans the decades since the end of horse farming to the age of GPS and precision farming. He recounts all and predicts how we can have a future world with abundant food in his book “World Food Unlimited.” This article was republished from Agri-Times Northwest with permission.
