212 THE PLAXT WORLD. 



gro\Aai in the solution gave off substances injurious to themselves, 

 and that these substances were removed bv the absorbing action 

 of the finely divided solids. 



Two recent bulletins (Kos. 36 and 40) from the Bureau of 

 Soils have contributed more information on this subject. In 

 Bulletin 36, Livingston and others give more evidence in favor 

 of the existence of toxic bodies in unproductive soils and add cer- 

 tain points as to the nature and origin of such substances. The 

 conclusion is that toxic material is present in certain unproduct- 

 ive soils, either in very minute quantities or in a very slightly 

 soluble form ; that this material is volatile in some cases and in 

 others non-volatile; that it is often destroyed by boiling the soil 

 extract in which it occurs ; that it is often accompanied by an 

 acid reaction of the extract, but that in such cases the toxicity is 

 not due to the acidity as such ; that it is probably organic in its 

 nature ; and that it is absorbed by finely divided solids. As to the 

 origin of such material, it is sho^vn that toxic properties appear 

 not only in nutrient solutions in which wheat is growing, but 

 also in pure sand when this is used as a medium for growth. 

 Similar substances appear to dift'use from soaking wheat seeds, 

 and a similar toxicitv is exhibited bv the washins'S from the 

 leaves and bark of certain trees. 



In Bulletin 40, Schreiner and Reed show that agar-agar, in 

 which the roots of wheat have been allowed to grow, soon becomes 

 injurious to these roots. Agar-agar in which maize roots have 

 grown is injurious to wheat, but not to so marked a degree as 

 that rendered injurious by the growth of wheat itself. In point- 

 ing out the logical conclusion that the physiological action of the 

 used agar-agar must be due to excretion from the first crop of 

 roots, the authors call attention to the analogy between this sup- 

 posed excretion by roots and the well-known excretion of toxic 

 substances by bacteria. 



The importance of all this to scientific agriculture is evident, 

 and the changes that these considerations may bring about in the 

 theorv of soil fertility mav be verv profound. The beneficial 



