188 A TEXTBOOK OF GENERAL BOTANY 
percolation of any water. As the clay soil in this wet condition 
dries it shrinks and cracks, thus forming hard, tenacious clods. 
Weight of soil. A given volume of sand is heavier than the 
same volume of clay, as the weight of the individual grains of 
sand is sufficient to overcome considerable friction and to cause 
the particles to be compactly arranged, so that the pore space 
is decreased to a greater extent than is the case with lighter 
particles such as clay. 
Water has a tendency to bind soil particles together, as is 
shown in the case of sand, which can be molded to some ex- 
tent when wet but falls apart when dry. Owing to the greater 
amount of surface afforded by the small particles of clay, water 
binds particles of clay together with much more force than it 
does grains of sand. It is partly for this reason that clay is 
tenacious and harder to plow than sand, and it is owing to 
these properties that a farmer regards clay as a heavy soil and 
sand as a light soil. 
Water absorption and transpiration. When soil. contains a 
considerable quantity of water, it gives up water to plants 
more readily than when it is dry. The amount of water in 
the soil may therefore be a very important factor in determin- 
ing whether or not a given rate of transpiration is excessive. 
In order that a plant may live it must, over an extensive period, 
absorb as much water as it loses through transpiration. If the 
rate of transpiration continues to be greater than the rate of 
absorption, the plant will ultimately wither and die. When 
there is a sufficient supply of water in the soil, a plant may be 
able to replace the amount of water lost, even when the rate 
of transpiration is comparatively high; while if there is less 
water in the soil, the same rate of earns may cause the 
plant to succumb to drought. 
Available soil moisture. Plants not only cannot absorb water 
as rapidly from a dry soil as from a moist soil, but they will 
wilt long before all the water is removed from the soil. When 
the water is reduced below a certain amount, a plant cannot 
absorb it fast enough to replace loss through transpiration, and 
