The dry-farmer must
consequently use all possible precautions to prevent evaporation
from the soil.
Conditions of evaporation from from soils
Evaporation does not alone occur from a surface of free water. All
wet or moist substances lose by evaporation most of the water that
they hold, providing the conditions of temperature and relative
humidity are favorable. Thus, from a wet soil, evaporation is
continually removing water. Yet, under ordinary conditions, it is
impossible to remove all the water, for a small quantity is
attracted so strongly by the soil particles that only a temperature
above the boiling point of water will drive it out. This part of the
soil is the hygroscopic moisture spoken of in the last chapter.
Moreover, it must be kept in mind that evaporation does not occur as
rapidly from wet soil as from a water surface, unless all the soil
pores are so completely filled with water that the soil surface is
practically a water surface. The reason for this reduced evaporation
from a wet soil is almost self-evident. There is a comparatively
strong attraction between soil and water, which enables the moisture
to cling as a thin capillary film around the soil particles, against
the force of gravity.
Pages:
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136