The object of fertilizing is to have available in the soil a sufficient quantity of all the elements the plant needs for making a maximum crop.

It is also necessary that the soil be in such perfect mechanical condition that plant food may be made available, and that the roots and rootlets of the plant may be able to easily and readily take up this available plant food.

Theoretically, in order to determine the amount of fertilizer a crop should require, it would only be necessary to have a chemical analysis and know the tonnage of the crop, and have an analysis to show the amount of fertility in the soil. Practically, these analyses constitute only one of a combination of factors that should be used in estimating what fertilization is necessary.

The crop indicates whether or not the fertilization is right for immediate returns, but the analyses should be a guide to the grower in estimating what should be done to maintain large crops continuously.

A great many formulas for fertilizing have been worked out by growers and experiment stations both in America and Europe. A number of these will be given - not that they should be used simply because they have been successful under other conditions, but that they may be used as a guide for experimenting.

There is removed in a 600-bushel crop of potatoes approximately:

160 pounds of nitrogen 60 " " phosphoric acid 160 " " potash.

An acre of soil eight inches deep weighs about 2,375,000 pounds. (This varies somewhat and this estimate is for soil on the Twin Falls North Side Irrigation Project, Jerome, Idaho.) The elements of fertility vary, but, as an example, the soil at Jerome, Idaho, contains:

.47 per cent, of potash.

.11 per cent. of phosphoric acid.

.06 per cent. of nitrogen.

This would be a total of Nitrogen..........1,425 pounds.

Phosphoric acid. . 2,612.5 pounds Potash...........17,692.5 pounds.

Even though it is not possible for plants to use all of this, if proper cultivation methods are used there is food enough to last for a great many years.

The three elements of fertility that are called the "essential elements" are nitrogen, phosphoric acid and potash. At least ten other elements enter into the plant and are important and necessary for the production of plants. All but these three are used in small quantities and it is generally considered by students of the soil that the seven magnesium, sodium, chlorin, sulphur, iron, silicon and calcium - are present in most soils in sufficient quantities to supply the needs of crops almost indefinitely. The one exception to this is lime, and its use is generally considered to be more to make conditions right for the making available of other elements than as an element of plant food itself.

The potato plant - leaf, vine, stem, root, and tuber - is composed of elements taken from the soil and air. The plant is started from stored-up nutriment in the tuber or part thereof that is planted. After the start, the rootlets take water (hydrogen and oxygen), nitrogen, the phosphates, potash and the other mineral elements from the soil. These are taken up by the movement of sap to the leaves. The leaves take carbon and oxygen from the air through the stomata or breathing pores on their under surface; the various elements are transformed by the sunshine, heat, protoplasm and chlorophyl, and water (hydrogen and oxygen) and carbonic acid gas are given off by the leaves. The food which is manufactured or transformed is deposited throughout the plant. A large part of it goes to the storehouse of the enlarged underground stem or tuber.

An average plant is made up somewhat as follows:

Phosphate, potash and other minerals (from the soil)

5 0 per cent.

Nitrogen (from the soil and air)

1.5 per cent.

Hydrogen (from water)......

6 5 per cent.

Oxygen (from water and air) ....

42.0 per cent.

Carbon (from air)...............

45.0 per cent.

From this it appears that Nature is lavish in its supplies, and that the soil, the condition of which is more or less under the control of man, contributes a comparatively small share in the plant's economy.

This part is very important, however, so much so that in many instances success or failure depends on how it is looked after.

In stating the part the elements have in the growth of the plant it must be considered that the statements are relative and not absolute.

Nitrogen

Nitrogen is largely instrumental in providing growth in plants, heavy, luxuriant stems and leaves being credited to a liberal supply of this element in available form. A deficiency is shown by weak, yellow growth. The elements of plant food in the soil are transformed or manufactured in the leaves in combination with sunlight and the gases of the air into the substances that form the edible tubers. In order that these processes be carried on with the best results there must be a strong, healthy manufacturing establishment - the plant.

Phosphoric Acid

Phosphoric acid hastens the maturity of crops and influences the production of seeds in fruits. In the potato it is supposed to influence the production of starch.

Potash

Potash is supposed to influence yield, and be a factor in the formation of starch.

Like many other things concerned with the production of crops and the working of the various elements in nature's laboratory, the exact offices of potash and phosphoric acid are not known, but can only be judged and estimated by experiments and results. In general, it is known that nitrogen is mostly concerned with growth of plants, and phosphoric acid and potash with the fruit or crop.

Nitrogen is obtained from the following sources:

(1) Organic - Natural products in which nitrogen is combined with other elements such as carbon hydrogen and oxygen, decaying or decayed vegetable matter, dried blood, dried meat, tankage, fish, garbage, tannery waste, cottonseed and linseed meal, guano, animal manures.

(2) Inorganic (chemical forms) - Ammonia in combination with other elements, nitrate of soda, and nitrate of lime or nitrolin (a new Swedish product).