COMPOSITION OF SOILS. 35 



phosphoric acid and potash were contained in unlimited 

 quantities ; a balance of the plant-food constituents is 

 essential to full and complete growth and development. 



Exhaustion of Soils. Exhaustion of soils has refer- 

 ence mainly to the four constituents, nitrogen, phosphoric 

 acid, potash, and lime ; the amount of the others is usually 

 contained in excessive quantities in all soils. Exhaustion 

 is, however, a relative matter, since it is not possible 

 to completely exhaust a soil of its active constituents. 

 Exhaustion means properly the reducing of the constit- 

 uents to that point which makes the production of crops 

 unprofitable; hence the question of exhaustion is a vari- 

 able one, determined in a large measure by local circum- 

 stances. 



Exhaustion, too, may have reference to one constituent 

 only; for instance, there may be an abundance of nitro- 

 gen and phosphoric acid, and a deficiency of potash. 

 By growing a class of crops which take more of the 

 constituents that are present in relatively large amounts, 

 and less of those that exist in small amounts, the period 

 of exhaustion is deferred. 



Natural Strength of Soils. The power which soils 

 possess of gradually forming active ingredients is termed 

 natural strength. It is obvious that the character and 

 origin of the soil have an important bearing upon this 

 point. 



The natural strength of a light sandy soil may be 

 measured by a crop of wheat of five bushels per acre; 

 while the natural strength of rich valley or prairie soil 

 may be measured by an annual yield of twenty-five 

 bushels of wheat per acre; that is, in the one case, the 



