hem and
shake them; continue adding water and shaking them till every pebble
is covered with a film of water; let any surplus water drain off. Then
weigh again; the difference in the two weights will be approximately
the weight of the film water that the pebbles can carry. Repeat this
with sand and compare the two amounts of water. A striking
illustration can be made by taking two slender bottles and placing in
them amounts of colored water equal to the amounts of film water held
by the pebbles and sand respectively. In the accompanying illustration
(Fig. 29), _A_ represents the amount of water that was found necessary
to cover the pebbles in tumbler _B_ with a film of moisture. _C_ is
the amount that was necessary to cover with a film the particles of
sand in _D_. The finer soil has the greater area for film moisture. It
has been estimated that the particles of a cubic foot of clay loam
have a possible aggregate film surface of three-fourths of an acre.

CHAPTER VI

LOSS OF SOIL WATER

LOSS OF SOIL WATER AND MEANS OF CHECKING THE LOSS

We noticed in previous paragraphs that soil might at times have too
much water in it for proper ventilation and so check the growth of the
roots of the plant. Now is it possible that soil water may be lost or
wasted and if so can we check the loss?

In the experiment to find out how well the soils would take in the
rainfall (page 40) we noticed that the clay soil took in the water
very slowly and that on a field of clay soil part of the rain water
would be likely to run off over the surface and be lost. Free water
may be lost then, by surface wash.

We noticed methods of checking this loss, namely, pulverizing the soil
with the tillage tools and putting organic matter into it to make it
absorb the rain more readily.

We noticed that water poured on the sand ran through it very quickly
and was apt to be lost by leaching or percolation. This we found could
be checked by rolling the soil and by putting organic matter into it
to close the pores.

We learned that roots take water from the soil for the use of the
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 los