=Another Peculiarity of Agriculture—Limitations on Division of Labor.=—In one other fundamental feature there is a difference between agriculture and industry. In industry, as will be seen presently, the individual worker confines his attention to one operation in the process of production. He does not make a shoe, but only part of a shoe. But in agriculture, this division of labor cannot be carried so far. [Sidenote: Why division of labor does not apply to agriculture.] The workers engaged in agriculture cannot be ploughmen, sowers, reapers, or threshers only; they must take a hand at all these things when the time comes. This is because the tasks connected with agriculture change with the seasons. Agricultural labor must, therefore, be much more versatile than labor employed in large-scale industry. Most industries, moreover, are able to run along at an even pace throughout the year, affording steady employment to a fixed number of workers. But in most forms of agriculture, the amount of labor required is much greater at some seasons of the year than at others, thus giving the farmer a labor problem of great difficulty to contend with. [Sidenote: The chemical elements in soil.] =The Exhaustion of the Soil.=—The agricultural production of the country depends upon the fertility of the soil. Agricultural soil contains various chemical properties which are exhausted by long-continued cropping, particularly if only one type of produce is grown. These chemical elements are, more particularly, nitrogen, phosphorus, and potash. Some crops draw chiefly upon one of these chemical elements and some upon the others. Wheat and corn take large amounts of nitrogen from the soil, while potatoes draw a larger proportion of potash. The exhaustion of the soil is prevented in two ways, first by rotation of crops and, second, by the use of fertilizers. Rotation of crops involves the growing of different products in successive years, such as wheat, potatoes, and hay. It is not always practicable. [Sidenote: Fertilization.] Fertilization involves the putting of chemical elements back into the soil. It may be effected by the use of natural manure or artificial fertilizers or by ploughing under the soil a green cover crop. Land retains its fertility to the degree that chemical elements are conserved in it.