A firm, mellow soil.

A moist soil.

A soil supplied with available plant food.

A warm soil.

A ventilated soil.

These are the most important facts about plant growth so far as the
plant grower is concerned. In other words, these conditions which are
necessary for root growth and development are the most important
truths of agriculture, or they are the foundation truths or principles
upon which all agriculture is based. Having found these conditions,
the next most important step is to find out how to bring them about in
the soil, or, if they already exist, how to keep them or to improve
them. This brings us, then, to a study of soils.

[Illustration: FIG. 12.
To show where growth in length of the root takes place. Forty hours
before the photograph was taken the tip of the root was ¼ inch from
the lowest thread. The glass cover was taken from this in order to get
a good picture of the root.]

[Illustration: FIG. 13.
Radish seeds sprouted on dark cloth. To show root hairs.]

[Illustration: FIG. 14.