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 order to insure that the crop is supplied with nitrogen, phosphates, and potash.

==Nitrogenous manures== specially stimulate the growth of the foliage, stems, and roots of plants, and are therefore of the greatest benefit to Carrots, Parsnips, Turnips, Beet, Celery, Asparagus, Rhubarb, all the Cabbage tribe, and leafy crops generally.