CH. V, 6] 



STRUCTURE OF SOILS 



243 



peculiarities of the resultant plant give a clew 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 living roots. It was 

 an old belief, long aban- 

 doned but now revived 

 with new evidence, that 

 roots excrete substances 

 injurious to themselves, 

 though commonly harm- 

 less to other kinds ; that 

 the accumulation of such 

 substances tends to poison 

 a soil for the plants which 

 produce them ; and that ^% 171 ' T Typ !? al n f lustration of the 



methods and results of 



soils rendered barren by x T V 



lono- imp of onp oron arp 



rop are 



water culture ; 



The P lants are Buckwheat. To dis- 



. Q the middle jar were added 



not exhausted of neces- all of the mineral salts needed by the plant ; 



Sary mineral Salts, as 



J } 



to that on the left all except potassium ; 



to that on the right, all except iron. In 

 Commonly Supposed, but the latter case the upper, less shaded, 

 arp noi^rmpd hv thp ao leaves are white, not green, in the plant. 

 by 3 ac- (Originally from works of Pfeffer.) 



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 



