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Fertilizing consists in the physical application to the soil of elements which are immediately or mediately available for plant food, or which aid in changing from unavailable to available forms of plant food any elements already existing in the soil. It is meant, of course, to exclude water, the contribution of which is irrigation, but any elements held in suspension or solution by irrigation waters, and falling under the conditions of the definition, are fertilizers.
Historical—The Homeric Greeks were familiar with the use of manure as a fertilizer. Cato mentions irrigation, frequent tillage and manuring as means of fertilizing the soil. To these Virgil adds ashes. The ancient Peruvians were skillful in the application of manure, a practice that has existed in parts of Russia from time immemorial. The earliest records on agriculture show that the value of fertilizing had already been taught by experience. The degree to which intensive cultivation had developed, the natural fertility of the soil, and the incidental occurrence of materials that could be used as fertilizers have always been, in general, the factors determining the extent of the practice.
Soil Composition and its Relation to Plant Life.—From a physical point of view the soil of the field may be analyzed as follows: (1) The soil proper, consisting of various sizes and arrangements of grains made up of insoluble or imperfectly soluble minerals; (2) humus, more or less decomposed organic matter derived from the decay of former animal and plant life: (3) the soil moisture, covering the soil grains, and containing in solution a varying amount of the soluble soil constituents: (4) the soil atmosphere, differing from air in composition to some extent, and usually saturated with water vapor; and (5) soil ferments, or bacteria, which so permeate the soil that it should be considered as a living mass and not as dead, inert matter. Indeed, the inanimate parts of the soil have their highest significance as the environment of the bacteria which they contain, and in part nourish. To understand the effect and value of fertilizers, a knowledge of the chemical and physical composition of soils, and of the relation of their composition to plant growth is essential. These things must be clearly understood, because fertilizers act upon the plant indirectly through their influence upon the composition of the soil. At the beginning of the nineteenth century Sir Humphrey Davy said that the substances which constitute the soil “are certain compounds of the earths, silica, lime, alumina, magnesia and of the oxides of iron and magnesium; animal and vegetable matters in a decomposing state, and saline, acid or alkaline combinations.’’’ He also fully understood that the soil furnished nourishment for the plants, and that different plants flourish best in different soils. While he described the soil elements, often with surprising accuracy, and was the most expert chemist of his time, he did not adequately appreciate the plant foods contained by the soil, and his conception of the functions of the elements which he described was often extremely vague. For example, he held that the silica which plants contain imparts to them their rigidity. He recognized in a general way, however, that phosphoric acid, potash and lime enter into the composition of plants, and he successfully combated many unscientific notions. The derivation of soils from rocks was also known in his time. Mineral or artificial manures were first studied systematically by Liebig, whose views found their way into the United States before the middle of the century. The publication of his work in 1840 marked a new era in agricultural chemistry. Before his time it was very generally held that organic substances were the chief food of plants. This has been called the humus theory. It was rejected by Liebig, who went to the opposite extreme and held that organic matter has no part in plant life, Practical knowledge of the use of manures, wood ashes, slaughterhouse refuse, gypsum, lime and plaster as fertilizers was widely diffused and acted upon before the time of Liebig, but it required his work to bring about a full appreciation of plant requirements and of the important office of the soil. Through the vehement discussions of his work, Boussingault, Lawes, Gilbert and others were led to a critical study of these problems. The exact needs of plants for mineral nutrients were carefully investigated by means of experiments of water-culture and sandculture. This work was carried on by the foreign experiment stations between 1865 and 1873, and its results contributed very materially to, the subsequent development of the enormous industry of manufacturing and selling commercial fertilizers. With prophetic vision Liebig said: ‘‘Manufactories of manure will be established in which the farmer can obtain the most efficacious manure for all varieties of soils and plants.’’’ Systematic work in the chemical analysis of soils in the United States began in 1850, when D. D. Owens made an extensive chemical examination of the soils of Kentucky in connection with its geological survey. The most recent developments seem to show that the amount and proportion of the elements contained by the soil are of less importance than was formerly supposed. It is of far greater importance that such elements as are present should be in a form available for plant food. Just what form an element must assume to be most available seems to be in a large measure an unsolved problem yet, but evidently the texture and the structure of the soil are fully as important as the chemical condition of its elements. By texture is meant the relative sizes of soil grains, and by structure the arrangement of these grains under field conditions. After exhaustive investigations on many types of soil, the conclusion has been reached ‘‘that on the average farm the great controlling factor in the yield of crops is not the amount of plant food in the soil, but is a physical factor, the exact nature of which is yet to be determined.” Most of the fertilizing which has been done has been according to the theory that the soil is a lifeless mass composed of so many elements, and that some elements were absent, or not present in sufficiently large proportions, it being the object to contribute in the form of fertilizer the elements which were needed. While the benefits of fertilizers have been unquestioned for over a century, it is, nevertheless, doubtful whether quite the right path has been followed by investigations which endeavored to determine just how those benefits arose. Air and soil are the media through which the growing plant receives its nourishment, but this is more than a mere mechanical process. In some cases at least there must be some sort of digestion or decomposition of foods before there can be assimilation. Silica, highly insoluble and apparently the least suited of all the mineral constituents of the earth to enter the vital organism of the plant, however finds its way into the plant tissues. Phosphorus, one of the most important mineral foods of plants, exists in the soil, or is applied in fertilizers, almost exclusively in the form of mineral phosphates, but appears in the plant largely in organic combination, while the mineral phosphates which do appear are not those which pre-existed in the soil, such as those of lime, iron and alumina, but chiefly those of potash. It is also found that soils of different composition, texture and structure supply different quantities of water to the plant, irrespective of the percentage of water actually present in the soil. As water conveys the nutritive solutions to the plant, when the supply of water is inadequate, there may also be a deficiency of nutrient materials. It is probable, then, that fertilizers, by temporarily increasing the concentration of the solution, increase the food supply. Such fertilizers seldom permanently affect the nature of the solution, and the concentration with respect to the mineral plant food constituents per unit of solution is considered approximately constant. In the same and in different soils, however, the water content varies widely, and usually the greater the water content, the more diluted is the solution.
1 Yearbook U. S. Dept. A gr. 1899. p. 340. * U. S. Dept. A gr., Bu. of Soils, Bul. 22 (1903), p. 63.
In 1902 such exceedingly delicate and sensitive methods for analyzing soils in the field were devised that “the amounts of nitrates, phosphates, sulphates and the like, which may be present, as indicated by water solutions, can be determined to within four or five pounds per acre one foot deep.” Fertilizers applied in the spring can be traced from the place of application down through the different depths of the soil which they invade. Much progress has been made toward determining by analysis the fertilizers needed by a particular soil.