308 NATURE AND PROPERTIES OF SOILS 



The first outstanding feature of the above table is the con- 

 trol on drainage losses exerted by cropping. The loss of 

 nitrate nitrogen is reduced to an exceptionally low figure, 

 while the saving of potash, sulfur, and lime is quite apprecia- 

 ble. No phosphoric acid is lost even from the bare soil. The 

 losses due to cropping and leaching combined from a planted 

 soil are generally but little greater than the drainage losses 

 alone from a soil kept continuously bare except in the cases 

 of the phosphoric acid and the potash. 



The next point of interest is the difference in the nutrients 

 removed by a rotation of crops, such as maize, oats, wheat, 

 and hay as compared with permanent meadow. The latter, 

 although absorbing less nutrients than the rotation crops, 

 exert as marked a conserving effect on the nutrients appear- 

 ing in the drainage as do the crops in rotation. The compara- 

 tive removal of nutrients from the soil by cropping and leach- 

 ing are well shown by the following diagram, in which the 

 weight of the symbols indicates where the loss of any par- 

 ticular nutrient is the greater. 



RELATIVE LOSSES OF NUTRIENTS FROM A PLANTED SOIL THROUGH 

 CROPPING AND DRAINAGE 



164. Possible exhaustion of the soil. — It is interesting at 

 this point to compare the amounts of nutrients removed an- 

 nually from a soil cropped in rotation with the amounts which 

 are present in an average soil to the depth of four feet. As- 

 suming reasonable figures for the pounds of sulfur trioxide, 

 lime, phosphoric acid, nitrogen, and potash and considering 

 that these nutrients are wholly available, the following sig- 



