It was not until 1804 that any light was thrown upon the subject. In that year was published 'Recherches Chimique sur la Vegeta tion' by Theodore de Saussure. This brilliant French investigator was the first to appreciate the significance of the ash ingredients of plants; to point out that without them plant life was impossible, and to show that only the ash of the plant was derived from the soil.
Justus von Liebig is commonly regarded as having laid the foundation upon which the commercial fertilizer industry has been built. Certainly his reports to the British As sociation in 1840 and again in 1842 made the British agriculturists regard as they never had done before the importance of a sufficient sup ply of certain readily soluble mineral sub stances in the soil. He supported the conten tion of De Saussure regarding the importance of mineral matter in the plant, and its extrac tion from the soil. He refuted the theory, at that time popular, that plants absorbed their carbon from humus, but made the mistake of attaching little importance to the presence of humus in the soil. He showed the importance of potash and phosphates in manures, and in his earlier writings spoke of the value of nitrogen for fertilizing crops, but afterward made the mistake of denying the usefulness of nitrogenous manures for plants, holding that the ammonia washed down by rain affords a sufficient supply.
By the middle of the 19th century it was well understood that potash, phosphoric acid and nitrogen were valuable constituents of fertilizers, and that the other mineral elements of plant-food, with the occasional exception of calcium, were always present in sufficient quantity in arable soils. It has since been shown that the family of plants known as the Legioninosce have the ability to secure a large part, at least, of their nitrogen from the air, and that as these plants are very rich in nitro gen they leave a supply of nitrogen in organic matter in the soil when plowed under.
Complete and •Incomplete Fertilifers. Fertilizers, containing nitrogen, phosphoric acid and potash, mixed together so that an appli cation of the preparation will result in bringing each of these substances in contact with the soil wherever applied, are called complete fertilizers. The proportions in which the con stituents are mixed vary with the different brands and factories. Sometimes a fertilizer of this kind will be advertised for use on a certain crop, and will contain the nitrogen, phosphoric acid and potash in the proportion in which the manufacturer believes they will produce the best yields of that crop. He has,
however, no means of knowing the require ments of the soils on which the fertilizer is to be used.
Incomplete fertilizers contain only one or two of the customary fertilizer ingredients.
Available and Unavailable Fertilizer Ma fertilizing constituents in a fer tilizer may be present in a readily soluble or difficultly soluble condition, depending upon the chemical combinations and, to some extent, upon the physical condition in which they are found. Thus phosphoric acid when in the form of phosphate rock as it is taken from the deposits is a very difficultly soluble substance, and plants can avail themselves of it only in small quantities. After the same rock has been treated with sulphuric acid the phosphoric acid is in a form in which it can be readily used by plants. To a more limited extent the same is true of the phosphoric acid in basic slag, which, when the slag is in an unground condi tion, is not of much use to plants, but when the slag has been finely ground furnishes a valuable form of plant-food.
When the fertilizing material is in a condi tion in which it can be readily used by the plant it is said to be "available," and when it can be used only with great difficulty it is said to be °unavailable.° It is important that the purchaser should know in what form the con stituents are present in a fertilizer.
Nitrogenous Nitrogen is ab sorbed by most plants only in the form of a soluble salt; hence for the cereals and many other crops it is desirable either to have it in this form or to incorporate it in the soil in a condition in which it will be readily converted into a soluble condition. Plants like the clovers, alfalfa, peas, beans, etc., have the power of using the nitrogen of the air, and hence do not require nitrogenous fertilizers in such large amounts. These plants may be used to increase the supply of nitrogen in the soil. Nitrogen is the most expensive constituent of fertilizers, and is extremely important, as it is used in large amounts by plants and is likely to be deficient in soils. The form in which nitrogen is present in a fertilizer may make a great difference in its value and in the way in which it can be best applied. Nitrogenous fer tilizers differ in having their nitrogen either in the form of a soluble salt or combined as organic material.