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carry the "empire" yet farther west. The "Great American Desert" disappeared from the maps. During a series of years in which the rainfall was more adequate than usual, the agricultural areas leaped forward to the west from county to county. The first general advance was in 1883. Within five years, western Kansas and Nebraska and eastern Colorado were largely settled. To the east of the arid region is a strip of territory embracing portions of Kansas, Nebraska, the Dakotas and northwestern Texas, which has been designated as the "rain belt." Its name resulted from the theory that the humid region was gradually extending itself toward the west as a consequence of the breaking of the prairie sod, the laying of railroad and telegraph, and the advent of civilization. There was supposed to be a progressive movement of the "rain belt" as civilization advanced. While thorough cultivation undoubtedly makes a material modification in the effects of a given degree of aridity, it has been declared that the probability of a perceptible change in climate does not merit serious discussion. The theory received a serious setback from the periodical exodus which occurred when succeeding years brought a rainfall at or below the normal. There were years when the average rainfall (10 to 20 inches) decreased by almost half; there were months without a cloud; there were days in the southwest when the winds were so dry and hot that green corn was turned into dry and rattling stalks. When crops shriveled and died on millions of acres, men lost hope and means, and they were forced to abandon the homes that represented the earnings of a lifetime. Whole counties were nearly depopulated. These vicissitudes caused the tide of migration to ebb and flow, and continually wore out its resources. The desert had been removed from the maps. The supplications of the devout and the dynamite of the "rainmaker," a suggestion of the Indian medicine men who had held sway on the plains less than a century before, had vainly implored the heavens for the rain which alone was wanting for the production of profitable crops. Yet the blunt fact remained, and still remains, that many millions of acres were dead, vacant, and profitless simply because of their aridity. This land has little value now, for in many places a whole section does not yield enough to keep a fleetfooted sheep from starving.

South of Yellowstone park in the Wind river mountains of Wyoming rises Mount Union in majestic grandeur. Three streams take their course from this peak—the Missouri, the Columbia and the Colorado. Embraced in the branching arms of these streams is the industrial future of a region greater in extent than any European nation save Russia. Could this vast district be reclaimed for settlement, it would be a task second to none in the realm of social economics, for here millions of people could find homes. Within this region is contained practically all that remains of the public domain. The only element lacking to make the land valuable is moisture. New influences are at work to remedy this, the bitter failures of 20 years ago have been largely forgotten, and a second wave of settlement is sweeping over the plains. Rather slowly and unwillingly public attention became fixed upon irrigation. While the water supply is sufficient to irrigate only a small fraction of the arid domain, approximately three-fourths of a billion acres, several million acres are already under irrigation, and there is a good prospect that many more millions will be irrigated in the future. At present this area forms potentially the best part of our national heritage. Although most of the land would be typical for raising wheat, and the completion of the irrigation works which the government now has under way will add millions of bushels to the annual production of wheat, the better adaptability of other crops to intensive cultivation under irrigation will doubtless soon render it unprofitable to irrigate wheat extensively. The introduction of irrigation will make possible the growing of diversified crops in some sections where wheat alone can now be profitably raised. Where the supply of water is insufficient for irrigation, the only remedy is the development of drought resistant crops for dry farming. One of the greatest of these is durum wheat. If there is water enough to irrigate but one acre of ground on the dry farm, this will make a green oasis with shade and foliage for the farmer's home, a pleasant contrast to the monotony of the gray and dusty summer plains with their shimmering waves of heat.

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CHAPTER VIII.
FERTILIZERS

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.

NATURAL FERTILITY.

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 1 Yearbook U. S. Dept. Agr., 1899, p. 203.

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.''1 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

1 Yearbook IT. S. Dept. Agr.. 1899. p. 340.

2 U. S Dept. Agr., Bu. of Soils, Bui. 22 (1903), p. 63.

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