II. Reducing the loss of soil moisture
A. REDUCING SOIL EVAPORATION. Water in the soil exists as a continuous film surrounding each grain. As water near the surface evaporates, water is drawn up from below to replace it, thinning the film. When it becomes too thin for plant roots to absorb, wilting occurs.
1. Shelter belts of trees or shrubs reduce wind speeds and cast shadows which can reduce evaporation 10 to 30 percent by itself and also reduce wind erosion.
2. Mulching reduces the surface speeds of wind and reduces soil temperatures.
3. Shallow tilling can create a dirt mulch 2 to 3 inches deep which dries out easily but is discontinuous from the subsurface water, preventing further loss. Tillage must be repeated after each rain to restore the discontinuity. This is most workable where rainfall occurs in a few major rainfalls with relatively long intervals in between.
B. REDUCING TRANSPIRATION. All growing plants extract water form the soil and evaporate it from their leaves and stems in a process known as transpiration.
1. Weeds compete not only for soil nutrients, but water as well and so their control is critical.
2. Selection of crop is significant as well. Dwarf varieties have less surface and so lose less water. Some plants close their stomae when it is hot, reducing their water loss. Others, like corn, curl their leaves during hot afternoon and open them at night, effectively changing their surface area in response to conditions.
3. In dry farming, the number and spacing of plants is reduced so that fewer plants compete for soil moisture. The exception to this occurs when allowances for insect, bird, and rodent loss must be made at planting.
4. Where rainfall is frequently marginal to insufficient, drought "insurance" can be obtained by clear fallowing a sufficient area. An area clear of growing vegetation with a properly maintained stubble and soil mulch can retain 20 to 70 percent of the precipitation received until the next year. Where 5 to 6 acres each year per family have been so set aside in India, the specter of famine due to drought has been eliminated.
5. Post harvest tillage will create stubble and dirt mulches and destroy weeds before the onset of the dry season.
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