Incidentally, this
points to planting that shall be proportional to the moisture
contained by the soil. See Chapter XI.
As the temperature rises in spring, with a decrease in the relative
humidity, and an increase in direct sunshine, evaporation from the
soil surface increases greatly. However, as the topsoil becomes
drier, that is, as the water fihn becomes thinner, there is an
attempt at readjustment, and water moves upward to take the place of
that lost by evaporation. As this continues throughout the season,
the moisture stored eight or ten feet or more below the surface is
gradually brought to the top and evaporated, and thus lost to plant
use.
The effect of rapid top drying of soils
As the water held by soils diminishes, and the water film around the
soil grains becomes thinner, the capillary movement of the
soil-water is retarded. This is easily understood by recalling that
the soil particles have an attraction for water, which is of
definite value, and may be measured by the thickest film that may be
held against gravity. When the film is thinned, it does not diminish
the attraction of the soil for water; it simply results in a
stronger pull upon the water and a firmer holding of the film
against the surfaces of the soil grains.
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