90 



BACTERIA AND SOIL FERTILITY 



That is, every time we harvest a fifty-bushel crop of wheat 

 from the soil we remove ninety-six pounds of nitrogen, sixteen 

 pounds of phosphorus, and fifty-eight pounds of potassium. 



It is evident now that the sane method of procedure would be 

 to examine our bank account of soil fertility and see which is 

 going to give out first, if drawn on in these proportions. One 

 method of examination would be to examine the crust of the 

 earth as a whole and find out its durability for crop production 

 under this condition. The manner of doing this is shown in 

 the following table : 



Relative Supply and Demand of the Six Elements 

 Essential to Plant Production 



Essential 



Plant-food 



Element 



Pounds in 2^- 



Pounds in 50 



Number 



Pounds in 



Number 



000^000 of 



Bushels of 



Years 



20 Tons 



Years 



Average Crust 



Wheat {grain 



Supply 



0} 



Supply 



of Earth 



only) 



Indicated 



Sugar-beets 



Indicated 



Phosphorus 



2j200 



12 



183 



18 



123 



Potassium 



49,2O0 



13 



3,785 



157 



^'^Z 



Magnesium 



48,000 



4 



12,000 



16 



3,000 



Calcium 



68,000 



I 



68,000 



12 



Sfi66 



Iron 



88,600 



0-3 



295,000 







Sulfur 



2,200 



0.1 



22,000 



4 



5>S^o 



It is clear that in the average crust of the earth, phosphorus 

 is the_ limiting element of crop production. There is, however, 

 a variation from place to place. The maxima and minima of 

 the three elements — ^nitrogen, phosphorus, and potassium — for 

 soil in the United States as given by Hopkins are as follows: 



1. In nitrogen — content from 1,000 to 35,000 pounds 



2. In phosphorus — content from 160 to 15,000 pounds 



3. In potassium — content from 3,000 to 60,000 pounds 



Now let us examine two western soils, those from the Logan 

 (Greenville) and Nephi, Utah, Experimental Farms. Their 

 composition is as follows : 



