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 

 clover because of 

 some poisonous sub- 

 stance thought to 

 be produced by the 

 decay of the clover 

 roots. If planted to 

 other crops for a few 



49. ^ Poisonous substances in the soil, SCaSOnS it will recover 



its former productive- 

 ness. The injurious 

 results of even a 

 single crop of sorghum 



Poisonous substances in the soil, 

 formed by decaying vegetable matter, some- 

 times keeps 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. 



(94) 



