Page:Popular Science Monthly Volume 82.djvu/241

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GERMAN geographer has estimated that the world contains 1,700 million people, and that they are increasing at the rate of twelve million a year. During each succeeding decade, therefore, provision must be made for feeding a new population greater than the present population of the United States. This demands an enormous, steadily growing increase in the world's output of agricultural products. How to provide for this increase is one of the largest material problems that confronts our generation and the generations to come. Many factors must contribute to its solution. New land must be brought under cultivation by a wider distribution of population, by increased facilities of transportation, by better utilization of the available water supply through storage and irrigation. A larger yield per acre must be secured by improvement of the varieties of food-yielding plants through biological selection and breeding, through the adoption of more economical methods of farming, and especially through increasing and maintaining the fertility of the land by the scientific use of fertilizers in adequate amount.

This last aspect of the problem is the one with which this article is concerned. It is a vital part of the food problem, one which can not be eliminated by advances in any of the other directions just referred to; for plants can not live on water and air alone. They consist, to be sure, in largest proportion of compounds of carbon, hydrogen and oxygen; and they have the marvelous power of producing these compounds under the influence of sunlight from the carbon dioxide of the air and the water of the soil. But they contain also as essential constituents certain other elements, especially nitrogen, phosphorus and potassium, which they can not obtain from the air, which they must therefore extract from the soil. These elements are, however, present only in small quantity even in virgin soil; and they soon become exhausted through the harvesting of successive crops. It is therefore necessary, in the long run, to return to the soil the quantities of nitrogen, phosphorus and potassium that are contained in the vegetable products taken from it.

The sources from which we can obtain these three plant-foods cheaply and abundantly is so large a question that only one of them, nitrogen, will be here considered. Of the three this is by far the most expensive—by far the most difficult to obtain in sufficient quantity at low cost.