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 and leaching is the most 

 serious matter on most of the valuable agricultural soils. 



Lyon and Bizzell (Jour. Ind. and Eng. Chem., Oct., 1911) report 

 a loss of ii pounds of potassium, 76 of magnesium, and 407 of cal- 

 cium, from uncropped soil; and 8 pounds of potassium, 31 of mag- 

 nesium, and 1 66 of calcium, from cropped soil (average for corn and 

 oats), in drainage water per acre from a four-foot stratum of clay 

 loam soil, from May 23, 1910, to May i, 1911 ; and Bartow (Illinois 

 State Water Survey Bulletin) reports 90 analyses of Illinois well 

 waters drawn chiefly from glacial sands, gravels, and till, showing, 

 as an average, n pounds of potassium, 130 of magnesium, and 330 

 of calcium, in 3 million pounds of water (see Table 74). These 

 data confirm the results of the Rothamsted investigations (pages 

 174, 175, 413), showing an excessive availability of magnesium and 

 especially of calcium ; and they clearly indicate that in many cases 

 those elements may be of much greater importance for soil improve- 

 ment than potassium, even from the standpoint of plant food 

 maintenance, and in addition to their value for correcting soil 

 acidity. (See also pages 105 and 633.) 



