246 SOILS AND FERTILIZERS 



which plants grow, it is thus possible, by a well-chosen 

 rotation, to save plant-food material that would otherwise 

 be lost. 



308. Production of toxic substances from plants. — Tl^at 

 soil sometimes contains organic substances that exert an 

 injurious effect on the growth of certain plants is indicated by- 

 recent experiments and was surmised by some early writers 

 on the subject. De Candolle was probably the first to ad- 

 vance the idea in 1832. He suggested that at least some 

 plants excrete from their roots substances that are injurious 

 to the growth of the plants themselves and others of their 

 species, although the excreta may be harmless or even bene- 

 ficial to other plants. This he considered one of the reasons 

 for the failure of many crops to succeed when grown contin- 

 uously, while the same soil may be productive under a rota- 

 tion of crops. 



Of recent years this subject has been investigated exten- 

 sively in the United States and to some extent in Europe. 

 There appears to be no doubt that toxic substances of an 

 organic nature sometimes occur in soils, and there is evi- 

 dence that some of them are connected with the growth of 

 certain crops to which they are injurious. In most soils 

 containing toxic substances the injurious effect is exerted on a 

 large number of plants rather than only on those that have 

 been previously grown. It is still a question to what extent 

 excretion from roots or partial decomposition of plant residues 

 are responsible for the poor growth that results from the 

 continuous growth of crops on the same soil. 



309. Management of a crop rotation. — The advantages 

 of a crop rotation are so apparent and are connected so closely 

 with the profits to be derived from farming that there can be 

 no doubt regarding the advisability of practicing a rotation, 

 even when some one crop may be much more profitable than 

 any others that can be grown. Thus even in regions and on 



