Ch. V, 6] 



STRUCTURE OF SOILS 



243 



peculiarities of the resultant plant give a dew to the role of 

 that substance. 



In addition to the mineral matters the soil solution con- 

 tains small amounts of diverse organic substances, partly 

 beneficial to plants and 

 partly injurious. They 

 are mostly (set free by the 

 decay of humus, which 

 was originally living 

 tissue containing pro- 

 teins, carbohydrates, and 

 other classes of sub- 

 stances ; but some appear 

 to be formed as excretions 

 of hving roots. It was 

 an old belief, long aban- 

 doned but now revived 

 with new evidence, that 

 roots excrete substances 

 injurious to themselves, 

 though commonl}' harm- 

 less to other kinds ; that 

 the accumulation of such 

 substances tends to poison 

 a soil for the plants which 

 produce them ; and that 

 soils rendered barren by 

 long use of one crop are 

 not exhausted of neces- 

 sary mineral salts, as 

 commonly supposed, but 

 are poisoned bj- the ac- 

 cumulation of these excre- 

 tions. But these matters are still in debate, and their deci- 

 sion must await further evidence. 



Micro-organisms. Last in prominence, though not in 



Fig. 171. — Tj'pical illustration of the 

 methods and results of water culture ; 



'^ To- 



The plants are Buclvwheat. To dis- 

 tilled water in the middle jar were added 

 all of the mineral suits needed by the plant ; 

 to that on the left, all except potassium ; 

 to that on the right, all except iron. In 

 the latter case the upper, less shaded, 

 leaves are white, not green, in the plant. 

 (Originally from works of Pfeffer.) 



