HOFFMANN: SOIL PRODUCTIVITY. 85 



Bolley as it were, explains the matter on the basis of an 

 accumulation of disease organisms; Cameron and Whitney on 

 the accumulation of deleterious waste products. 



But to return to the work of the Bureau of Soils, more par- 

 ticularly the work of Schreiner. It is further claimed that the 

 above deleterious substances are toxic and are the result of root 

 excretions. Statements are thus made that "continuous crop- 

 ping tends towards an accumulation of the waste products of 

 plants, resulting in reduced crop yields" ; that "these waste 

 products are evidently of a soluble nature for when soil ex- 

 tracts are filtered through a Berkefeld filter, the toxic, retarda- 

 tive action of the soil persists in the filtrates" ; evidence which 

 would seem to indicate that the reduced yields are not directly 

 due to the bacterial flora but rather to the chemical nature of 

 the soil solution. The addition of lampblack and occasion- 

 ally of CaCO. removes this toxic property, in which case seed- 

 lings grown in extracts so treated apparently make good 

 growth. This evidence is indicative at least, that the retarda- 

 tive action is not due to a lack of plant food but rather to the 

 presence of some chemical substance of a toxic nature which 

 is removed from the sphere of action by the lampblack, and in 

 some cases even by CaCO ,. 



Considerable adverse criticism, notably that of King 10 has 

 been directed against the methods employed by the Bureau of 

 Soils for the attainment of these results. King claims that 

 the quantities of the various toxic substances added to the soil 

 filtrates in the Bureau's experiments are of such proportions 

 that even the smallest amount used would be equivalent to 70 

 pounds per acre foot. If present in such quantities, he argues 

 it should be an easy matter to isolate these substances from 

 the soil, whereas, as a matter of fact, it is necessary to resort 

 to the most elaborate methods to find even traces of these 

 substances in the soils. 



At this point reference should again be made to Bolley's 

 hypothesis. If his explanation is correct, then filtration 

 through a Berkefeld should remove the disease germs or their 

 spores and thus render the filtrates innocuous so that they 

 should support plant growth as efficiently as a sterilized fil- 

 trate from the same soil. As far as the author is aware this 

 has not been tried out, but deserves further consideration be- 

 fore final acceptance of Bolley's hypothesis can be warranted. 



10. King — Science, vol. 27, 1908, p. 694. 



