CHAPTER XIV 



PRODUCTIVENESS OF SOILS 



134. Fertility and Productiveness Compared. A soil 

 may be fertile, that is, rich in food elements, but not pro- 

 ductive because of the presence of some harmful sub- 

 stance in the soil. A familiar example is the "clover 

 sickness" of northern soils. A soil naturally suited to 

 clover will grow 

 several splendid crops, 

 and then become 

 "sick of clover," as 

 they say, because 

 clover will not thrive 

 any longer. The soil 

 is still rich in all ele- 

 ments of fertility, but 

 not productive for clo- 

 ver because of some 

 poisonous substance 

 thought to be ex- 

 creted or produced by 

 the decay of the clo- 

 ver roots. If planted 

 to other crops for a 

 few seasons it will re- 

 cover its former pro- 

 ductiveness. The in- 

 jurious results of 

 even a single crop of 



Fig. 49. Poisonous substances in the soil, 

 formed by decaying vegetable matter, some- 

 times keep a fertile soil from being pro- 

 ductive. (Wheat seedlings grown in: (1) 

 Pure distilled water ; (2) soil extract ; (3) 

 same soil extract from which the poisonous 

 substances have been removed by absorp- 

 tion with carbon black.) Bureau of Soils. 

 United States Department of Agriculture. 



(95) 



