plant and send it up to the leaves, which in turn send it out into the
air, or transpire it, as this process is called. We learned also that
the amount transpired is very great. Now water that is pumped up and
transpired by the crops we are growing we consider properly used. But
when weeds grow with the crop and pump and transpire water we consider
this water as lost or wasted.

Water may be lost then by being pumped up and transpired by weeds. And
this is the way weeds do their greatest injury to crops during dry
weather. The remedy is easily pointed out. Kill the weeds or do not
let them get a start.

There is another way, which we are not apt to notice, by which water
may be lost from the soil. When the soil in the pans in a previous
experiment (page 26) had been wet and set aside a few days it became
very dry. How did the water get out of this soil? That at the surface
of the soil evaporated or was changed into vapor and passed into the
air. Then water from below the surface was pumped up by capillary
force to take its place just as the water was pumped up in the tubes
of soil. This in turn was evaporated and the process repeated till all
of the water in the soil had passed into the air. Now this process is
going on in the field whenever it is not raining or the ground is not
frozen very hard.

Water then may be lost by evaporation.

How can we check this loss?

Suppose we try the experiment of covering the soil with some material
that cannot pump water readily.