largely on the known relationship of the water requirements of
plants to the natural precipitation of rain and snow. It is a most
elementary fact of plant physiology that no plant can live and grow
unless it has at its disposal a sufficient amount of water.

The water used by plants is almost entirely taken from the soil by
the minute root-hairs radiating from the roots. The water thus taken
into the plants is passed upward through the stem to the leaves,
where it is finally evaporated. There is, therefore, a more or less
constant stream of water passing through the plant from the roots to
the leaves.

By various methods it is possible to measure the water thus taken
from the soil. While this process of taking water from the soil is
going on within the plant, a certain amount of soil-moisture is also
lost by direct evaporation from the soil surface. In dry-farm
sections, soil-moisture is lost only by these two methods; for
wherever the rainfall is sufficient to cause drainage from deep
soils, humid conditions prevail.

Water for one pound dry matter

Many experiments have been conducted to determine the amount of
water used in the production of one pound of dry plant substance.
Generally, the method of the experiments has been to grow plants in
large pots containing weighed quantities of soil. As needed, weighed
amounts of water were added to the pots. To determine the loss of
water, the pots were weighed at regular intervals of three days to
one week. At harvest time, the weight of dry matter was carefully
determined for each pot. Since the water lost by the pots was also