152 THE CHEMICAL ANAL YSIS OF SOILS [chap. 



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 01 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. 



* Partly 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. 

 For example, a wheat crop on poor soil would often be 

 doubled by the use of 2 cwt. of nitrate of soda per acre, 

 i.e., by the addition of 35 lbs. of nitrogen in nitrate of 

 soda to a soil that already contained in the top 9 inches 

 more than 2000 lbs. per acre. Again, 4 cwt. per acre 

 of superphosphate, containing about 60 lbs. of phosphoric 

 acid, will be necessary in the usual rotation to secure 

 a good swede crop, though there may be already 2000 

 to 3000 lbs. of phosphoric acid in the soil. We are 

 then driven to conclude that the nitrogen, potash, and 

 phosphoric acid are present in the soil in some other 

 mode of combination than the form in which they exist 

 in manures ; so that although they may be in the soil 



