ash, and phosphoric acid.

The nitrogen is obtained from (1) nitrate of soda mined in Chile, (2)
ammonium sulphate, a by-product of the gas works, (3) dried blood and
other by-products of the slaughter-houses, and (4) cotton-seed meal.
Nitrate of soda is soluble in water and may therefore be washed away
before being used by plants. For this reason it should be applied in
small quantities and at intervals of a few weeks.

Potash is obtained in Germany, where it is found in several forms. It is
put on the market as muriate of potash, sulphate of potash, kainite,
which contains salt as an impurity, and in other impure forms. Potash is
found also in _unleached_ wood ashes.

Phosphoric acid is found in various rocks of Tennessee, Florida, and
South Carolina, and also to a large extent in bones. The rocks or bones
are usually treated with sulphuric acid. This treatment changes the
phosphoric acid into a form ready for plant use.

These three kinds of plant food are ordinarily all that we need to
supply. In some cases, however, lime has to be added. Besides being a
plant food itself, lime helps most soils by improving the structure of
the grains; by sweetening the soil, thereby aiding the little living
germs called _bacteria_; by hastening the decay of organic matter; and
by setting free the potash that is locked up in the soil.

CHAPTER II

THE SOIL AND THE PLANT

SECTION VIII. ROOTS

[Illustration: FIG. 16. ROOT-HAIRS ON A RADISH]

You have perhaps observed the regularity of arrangement in the twigs and
branches of trees. Now pull up the roots of a plant, as, for example,
sheep sorrel, Jimson weed, or some other plant. Note the branching of
the roots. In these there is no such regularity as is seen in the twig.
Trace the rootlets to their finest tips. How small, slender, and
delicate they are! Still we do not see the finest of them, for in taking
the plant from the ground we tore the most delicate away. In order to
see the real construction of a root we must grow one so that we may
examine it uninjured. To do this, sprout some oats in a germinator or in
any box in which one glass side has been arranged and allow the oats to
grow till they are two or more inches high. Now examine the roots and
you will see very fine hairs, similar to those shown in the accompanying
figure, forming a fuzz over the surface of the roots near the tips. This
fuzz is made of small hairs standing so close together that there are