LOSSES OF PLANT FOOD FROM SOILS 561 



exceed 7 pounds per acre, an amount sufficient only for a 30- 

 bushel crop of corn, or i| tons of clover hay. It may be kept 

 in mind that, so long as the surface soil contains more phosphorus 

 than the subsurface, erosion helps to deplete the soil of phosphorus ; 

 but when the phosphorus content of the surface becomes reduced 

 by cropping to a point below that of the subsurface, then erosion 

 tends to increase the phosphorus in the surface soil. 

 In regard to erosion, President Van Hise says: 



"It is plain that we must not permit soil erosion to take place more rapidly 

 than the soil is manufactured by the process of nature. To do this will be 

 ultimately to destroy our soils. If nature manufactures the soil at the rate of 

 one inch in a century, then the erosion must not exceed one inch in one century." 



Of course, this statement refers especially to residual upland 

 soils and to the making of soils from the slow disintegration of the 

 underlying rock. Most of the corn-belt subsoils include from 20 

 to 200 feet of loess and glacial drift above the bed rock. 



The loss of plant food by cropping is the most serious matter 

 on most of the valuable agricultural soils; but this loss is over- 

 looked by many, in part because of the continued increase in the 

 value of our farm produce. Thus the Report of the Secretary 

 of the United States Department of Agriculture places the total 

 value of farm produce in the United States for the year 1909 at 

 $8,760,000,000, compared with $4,417,000,000 for the year 1898, 

 which marks the beginning of his administration; but, before 

 deciding whether to rejoice or weep, we should consider that this 

 enormous increase in value is due not at all to improvement of 

 soil, but to increased acreage in crops and to increased prices for 

 food that must be paid by our own citizens. 



The corn acreage in the United States increased from 77,700,- 

 ooo in 1898 to 109,000,000 in 1909; and the price of corn in- 

 creased from 28 cents per bushel as an average of the years 1895 

 to 1900, to 47 1 cents per bushel as an average of the years 1901 

 to 1908. The average yield of corn has fluctuated from 16.7 to 



30.8 bushels per acre, but the average for the twenty years 1870 

 to 1889 was 25.6 bushels, while the average for 1890 to 1909 was 



24.9 bushels per acre, for the entire United States. 



