^2 Forestry Quarterly. 



sition products of proteins, nucleic acids, lecithins and similar 

 complex compounds of biological origin. 



The authors in the beginning elucidate a fact long understood 

 by the plant physiologist but too often not comprehended by the 

 silviculturist, namely that the intake of mineral food materials in 

 the soil water is not regulated by the amount of transpiration, but 

 rather by the life processes and their requirements, within the 

 plant. For example, in one of their culture solutions the loss of 

 water in a three day period was about io%, while .the mineral 

 elements were decreased by 70% in the same period, or, in other 

 words, the plants absorbed the mineral food elements much faster 

 than the water in which they were dissolved. 



The authors then show that plant roots have the power of 

 directly absorbing certain nitrogenous compounds without their 

 first being changed to ammonia or nitrates, and, indeed, that these 

 com.pounds may serve as substitutes for nitrates. For example, 

 when combined with phosphate and potash in a culture solution 

 with nitrates absent, they increased the growth of plants from 30 

 to 74%. Here the compounds were the only source of nitrogen. 

 When nitrates were also present, the addition of the compounds 

 produced a further increase in growth, but this was not so marked 

 as when the compounds served as a substitute for nitrates. Nine 

 compounds of this nature beneficial to growth have been isolated ; 

 others are injurious. 



If these cultural experiments are an index of what happens in 

 the nature, then a mere nitrate determination is erroneous and 

 misleading as to the nitrogen content of the soil readily available 

 to plants. Indeed, it may be that the formation of nitrates is 

 harmful rather than beneficial, since they are readily leached 

 away, do not last from season to season and so result in an actual 

 loss of soil nitrogen. On the other hand, the organic nitrogen 

 bearing compounds are leached out with the greatest difficulty; 

 they remain in the soil from season to season, ready for absorp- 

 tion and use by the plants at any time, and so conserve the soil 

 nitrogen. 



Thus we have to make another adjustment in our knowledge of 

 soil chemistry. 



C. D. H. 



