84 AGRICULTURAL ECONOMICS 



Because of these variations with respect to fertility and 

 location, land is said to vary in economic productivity, or, in its 

 value-producing power. Variation in physical productivity of 

 land is due to variation in soil, variation in rainfall, and varia- 

 tion in temperature. The soil in the United States varies from 

 the sterile, arid sands of the great American desert to the well- 

 watered fertile loams of central Illinois, from land that will 

 yield nothing to land which will yield enormous crops of corn 

 year after year. 



The improved land of the United States is shown in the 

 accompanying map which is based upon the " improved land 

 in farms." In 1910 there were 478,451,750 acres of improved 

 land in the United States. The enumerators were instructed 

 to include under improved land, " all land regularly tilled or 

 mowed, land pastured and cropped in rotation, land lying 

 fallow, land in gardens, orchards, vineyards, nurseries, and land 

 occupied by buildings." The improved land is not evenly 

 distributed over the United States. For example 82.9 per 

 cent of the total area of Iowa, 78.2 per cent of Illinois, 33.7 

 per cent of Wisconsin, and 1.8 per cent of New Mexico is im- 

 proved farm land. Only 25.1 per cent of the total area of the 

 United States is given as improved farm land, whereas 73.2 

 per cent of England is improved farm land. The relatively 

 low percentage of improved land in the United States and the 

 uneven distribution of this land can be explained largely in 

 terms of topography, soil, and climate. A map showing the 

 topography of the United States helps one to understand the 

 low percentage of improved farm land in the large are as occupied 

 by mountains. (See Fig. 3.) A map showing the rainfall helps 

 one to understand why vast areas are eliminated from this class 

 of plowed or mowed land, there not being enough rainfall to 

 support tilled crops. (See Fig. 4.) The soil map shows the 

 great irregularity of the soils of the United States with regard 

 to their usefulness. A map showing the mean temperature 

 during the growing season also throws light on the distribution 

 of improved land. The resultant of all these forces is given in 

 a map showing the value of all farm crops by counties in the 



