208 THE FARMER OF TO-MORROW 



The same simple computation can be applied 

 to any particular soil specimen. By this means 

 it has been variously computed that the aver- 

 age agricultural soil in the United States will 

 be reduced to low fertility in from seventy- 

 five to one hundred and fifty years a small 

 period of time when compared with the thou- 

 sand years which the soils of western Eu- 

 rope have been farmed, and more than four 

 thousand years in China, Japan and Corea. 



It is not pertinent to the discussion at this 

 point to give the rates of fertilizing which 

 Doctor Hopkins advises by means of chemi- 

 cals. On normal soils he believes that, of the 

 mineral elements, only phosphorus is deficient. 

 On abnormal soils, such as drained swamp 

 lands and some sandy soils, the potash element 

 too is deficient. This is conceded by all inves- 

 tigators in the science of soil fertility. But 

 Doctor Hopkins believes that even on normal 

 soils, when hay, straw, potatoes and root crops 

 and market vegetables are sold off the farm, 

 potassium must eventually be purchased and 

 returned to the soil. Lime is deficient in some 

 soils, and all investigators agree that in special 

 instances it must be returned to the soil in the 

 form of ground limestone. They disagree, 



