172 THE CHEMICAL ANALYSIS OF SOILS [chap. 



of others whose fertility has been proved by experience 

 or by actual manurial experiments, there are yet many 

 considerations which prevent much weight being attached 

 to the results thus obtained. 



A comparison of the total amount of any of the 

 elements of plant food in the soil with the amount that 

 is withdrawn by an ordinary crop shows at once that 

 even in the poorest soils there is sufficient material for 

 something like a hundred average crops. 



The density of the surface soil has already been dis- 

 cussed ; it will be sufficiently accurate for our purpose 

 if we consider that the top 9 inches of one acre of 

 an ordinary arable field weighs 2,500,000 lbs. On this 

 basis, and without taking into account the -fact that 

 the roots of most cultivated plants range far deeper than 

 9 inches, there is yet present about 2500 lbs. per acre 

 of nitrogen, potash, and phosphoric acid in a soil con- 

 taining only o-i per cent, of these constituents, which 

 is about the lower limit usually found. The following 

 table shows the amounts of these food materials 

 nitrogen, phosphoric acid, and potash which are taken 

 from the soil by an average crop grown in rotation : 



* Bartly derived from the atmosphere. 



It is clear from a comparison of this table with the 

 quantities previously specified, that even the poorest soil 

 contains the nutrient material required by any ordinary 

 crop many times over, yet we know that crops respond 

 vigorously to dressings of manure which only add a 

 fraction to the plant food already stored in the soil. 



