THEORY OF SOIL FERTILITY 195 



tributed the greatest single discovery in agri- 

 culture in a century, that of the nitrogen- 

 fixing bacteria of leguminous plants, sampled 

 the fertile alluvium of the upper Rhine in 

 Holland, one of the most fertile soils in the 

 world, and he found it contained elements of 

 plant food as follows, in an acre 12 inches deep 

 and weighing 3,500,000 pounds : 



Soluble silicates 81,900 pounds 



Lime 143,220 



Potash 35,910 



Soda. 68,920 



Ammonia 2,100 



Phosphoric acid 16,310 



Sulphuric acid 31,360 



To produce a crop of barley of 33 bushels 

 to the acre, he calculated, required 55 pounds 

 of potash and 55 pounds of phosphoric acid, 

 with other minerals in proportion. Of nitro- 

 gen were required 54 pounds. Then by simple 

 division we find that this richest soil in the 

 world contained enough innate fertility, if 

 liberated by natural agencies, to produce less 

 than 38 full crops if ammonia only is con- 

 sidered, 296 full crops if phosphoric acid is 

 considered, and 653 crops, as to its potash con- 

 tent. This particular soil, it happens, has been 

 cropped for the better part of one thousand 



