, and these soils are called
organic soils or humus soils. The black soils of the woods, swamps and
prairies, contain large amounts of humus.

_Work of Animals._--Earth worms and the larvae of insects which burrow
in the soil eat soil particles which pass through their bodies and are
partially dissolved. These particles are generally cast out on the
surface of the soil. Thus these little animals help to move soil, to
dissolve soil, and to open up passages for the entrance of air and
rain.

SOIL TEXTURE

We have seen that the soil particles vary in size and that for the
best development of the plant the particles of the soil must be so
arranged that the delicate rootlets can readily push their way about
in search of food, or, in other words, that the soil must have a
certain texture. By the texture of the soil we mean the size of its
particles and their relation to each other. The following terms are
used in describing soil textures: Coarse, fine, open, close, loose,
hard, stiff, compact, soft, mellow, porous, leachy, retentive, cloddy,
lumpy, light, heavy. Which of these terms will apply to the texture of
sand, which to clay, which to humus, which to the garden soil, which
to a soil that plant roots can easily penetrate? We find then that
texture of the soil depends largely on the relative amounts of sand,
silt, clay and humus that it contains.

CHAPTER IV

RELATION OF SOILS TO WATER

IMPORTANCE OF WATER TO PLANTS

We learned in a previous paragraph that plant roots take moisture from
the soil. What becomes of this moisture? We will answer this question
with an experiment.

=Experiment.=--Take a pot or tumbler in which a young plant is
growing, also a piece of pasteboard large enough to cover the top of
the pot or tumbler; cut a slit from the edge to the centre of the
board, then place it on top of the pot, letting the stem of the plant
enter the slit. Now close the slit with wax or tallow, making it
perfectly tight about the stem. If the plant is not too large invert a
tumbler over it (Fig. 21), letting the edge of the tumbler rest on the
pasteboard; if a tumbler is not large enough use a glass jar. Place in
a sunny window. Moisture will be seen collecting on the inner surface
of the glass. Where does this come from? It is absorbed from the soil
by the roots of the plant and is sent with its load of dissolved plant
food up through the stem to the leaves. There most of the moisture is
passed from t