FERTILIZERS. Any substance applied to the soil to aid the growth of plant§ may prop erly be called a fertilizer. The name has, however, become associated with a class of materials manufactured and sold for this pur pose, the handling of which now constitutes an important industry.
The constituents of the plant that it derives from the soil are nitrogen phosphorus, sulphur, potassium, sodium, calcium, magnesium, iron, aluminum, manganese, silicon, chlorine. Of these nitrogen, phosphorus or phosphoric acid, and potassium or potash, are the substances most likely to be deficient in the soil and therefore the ones that are contained in fertilizers. A fertilizer may contain any one or more of these substances.
The chief distinction between the functions of farm manure and commercial fertilizers may be stated in a general way to be that farm manure increases crop production by improving the condition of the soil, as well as by furnish ing plant nutrients, while commercial fertilizers may act directly as a plant-food material with out materially affecting soil structure.
It is evident, therefore, that commercial fertilizers are not an adequate substitute for farm manure or green manures for producing permanent improvement. Their function con sists in supplementing the available supply of plant-food in a soil which may be deficient in any one or more of the substances usually con tained in fertilizers. On virgin soils commercial fertilizers are usually superfluous; but as loss of plant-food goes on under cropping, restitution must be made and, as this is usually not ade quately done with farm manure, fertilizers are finally called upon.
Where there is large loss of plant-food from soil constituents due to leaching, there must be a large excess of potential fertility, in order that the growing crop shall at all times be sup plied with available plant-food. A deficiency in any constituent will check growth by com pelling the plant to depend upon a less readily available supply. Commercial fertilizers are useful by presenting readily available food to the plant when it is beginning growth and when a deficiency in the supply is likely to produce a permanent injury.
As the price of land increases, as the cost of labor becomes greater and as the value of the crop augments it becomes more important that maximum crops shall be raised, and for these reasons commercial fertilizers usually have greatest sale where agriculture is most intensive. There are also many special crops requiring more of one plant-food element than of another, and the needs of these can best be met by the use of commercial fertilizers.
Historical.— The value of animal excre ments applied to soils on which crops were grown has been appreciated by the husbandman as far back as records go. Why this manure is beneficial, and what relation its constituents bear to those of plants, are matters which have only been worked out during the last century, and knowledge of which has led to the use of fertilizers composed of mineral salts, commonly known as commercial fertilizers.
The earliest record of the use of artificial manures for increasing the yield of crops is contained in a book entitled 'A Discourse Con cerning the Vegetation of The title page also contains the following announce ments: °Spoken by Sir Kenelm Digby, at Gresham College, on the 23d of January 1660.° The author advocates the use of saltpetre to increase the yield of crops, and says: By the help of plain saltpetre, dilated in water and mingled with some other fit earthly substance, that may familiarize it a little with the corn into which I endeavored to introduce it, I have made the barrenest ground far outgo the richest, in giving a prodigiously plentiful har His dissertation does not, however, show any true conception of the reason for the in crease in the crop through the use of this fertilizer. The almost total absence of any knowledge of th_ composition of plants, and the crude state of chemistry at that time, made this quite impossible.