The chemical elements composing the natural food of ordinary crops are
ten in number, viz.--carbon, hydrogen, oxygen, nitrogen, sulphur,
phosphorus, potassium, calcium, magnesium, and iron. These are obtained
from the soil and air, and unless all of them are available plants will
not grow. The absence of even one of them is as disastrous as the want
of all, and a deficiency of one cannot be made up by an excess of
another; for example, if the soil is deficient in potassium the crop
suffers and cannot be improved by adding iron or magnesium. All the
food-elements are found in adequate quantities in practically all soils
and the surrounding air, except three--nitrogen, potassium, and
phosphorus. These are often present in reduced amount, or in a state
unsuited to plants; in such cases the deficiency must be made up before
remunerative healthy crops can be grown, and it is with this express
object that manures are added to the soil.

One of the best known substances employed in this way is farmyard
manure, which is indirectly derived from plants and contains all the
elements needed for the growth of crops. It is, however, of very
variable composition and rarely, or never, contains these elements in
the most suitable proportions, and its value can always be greatly
improved by supplementing its action with one or other of the so-called
artificial manures or fertilisers. Although it is strongly advisable to
add farmyard manure or vegetable composts to the soil of all gardens now
and again, in order to keep the texture of the soil in a satisfactory
condition, excellent crops can be grown by the use of artificial
fertilisers alone. To obtain the best results from these some experience
is of course necessary, but the following details regarding the nature
and application of the commoner and more useful kinds should prove a
serviceable guide in the majority of cases.

Artificial manures may be divided into three classes:--

1. The Nitrogenous class, of which nitrate of soda and sulphate of
ammonia are examples.

2. The Phosphatic class, such as superphosphate, basic slag, and steamed
bone flour.

3. The Potash class, including kainit and sulphate of potash. The
several examples of each class contain only one of the three important
plant food-elements, and as a single element can only be of use when the
others are present in the soil, it is generally advisable to apply one
from each class, either separately or mixed, in orde