NEW SOIL STUDIES. 211 



dered along in agricultural science ever since, flickering up occa- 

 sionally only to be smothered for lack of experimental proof. 



During the last few years, however, a great deal of new evi- 

 dence has been presented in favor of DeCandolle's idea. In 

 1897, and subsequently, investigations at the Woburn Experi- 

 mental Fruit Farm, in England, have shown that the presence 

 of grass in the soil about apple trees has a marked deleterious 

 effect upon the growth of the trees. It was shown experiment- 

 ally that this effect could not be due to removal of nutrient mate- 

 rials nor of water, nor to the exclusion of air, and it was sug- 

 gested that it must be caused by poisonous bodies emanating from 

 the grass roots. A similar antagonism has been shown to exist 

 between butternut trees and cinquefoil, and between peach trees 

 and several herbaceous plants. In 1904, Livingston published 

 evidence to the effect that bog water exhibits properties of a toxic 

 nature and suggested that the xerophilous character of bog plants 

 may be due to these properties. 



For our knowledge of the presence of toxic substances in 

 agricultural soils we are largely indebted to the work of the U. S. 

 Bureau of Soils. In Bulletin 23 of this bureau, it was shown 

 that the unproductiveness of certain soils examined could not be 

 attributed to any lack of available mineral matter, and that the 

 injurious properties of the soil could be transmitted to its aque- 

 ous extract, independent of the salt content. In later publica- 

 tions from the same bureau, evidence was presented in favor of 

 the idea that certain poor soils contain toxic substances which act 

 to retard the growth of roots. Further evidence favored the con- 

 clusion that wheat roots give off su.bstances toxic to themselves, 

 and that this toxicity, as well as that of the soils mentioned above, 

 can be removed from nutrient solutions or soil extracts by the ab- 

 sorbent action of carbon black, ferric hvdrate, and other finelv 

 divided inert solids. In Bulletin 28 it was shown that when a 

 nutrient solution becomes "exhausted," so that plants grow but 

 poorly in it, it is greatly improved by treatment with carbon 

 black, etc., the suggested explanation being that the roots first 



