162 



E. J. RUSSELL. Factors which determine Fertility in Soils. 



(Rothamsted Experimental Station Harpenden). Science Pro- 

 gress, No. 15, January 1910. -- London. 



The profound effect on the fertility of soils which is exercised 

 by their organic contents is not yet sufficiently appreciated; some 

 of the products, the simpler ones in particular, are of great value 

 as plant food whilst others may be deleterious. The effect of the 

 decomposition of organic matter in the soil has always interested 

 agricultural chemists and in early years it was thought to be purely 

 chemical. 



Soil bacteriology may be said to date from 1878 when Schlcesing 

 and Miintz observed a delay of twenty days in the commencement 

 of nitrification in an artificial soil in contact with sewage, and argued 

 that nitrification must therefore be a biological, not a chemical pro- 

 cess. The new hypothesis was put beyond question by Warington 

 and in course of time the suspected organisms were actually isolated. 

 In a not unusual case the soil consists of about 80% of inert 

 mineral matter, 15 % of water and 5 / of organic matter only part 

 of which, however, can be readily decomposed. 



Bacterial counts show that some millions are present in each 

 gram of soil. 



The micro-organisms show great diversity in their food require- 

 ments, their mode of life, and in the way they are influenced by 

 external conditions. 



The organic matter is finally resolved into carbon dioxide, water, 

 nitrogen, ammonia, calcium carbonate and other mineral matter, as 

 well as into more or less stable organic substances which tend to 

 accumulate in the soil. 



Normally ammonia does not remain as such in the soil, but is 

 either absorbed by some of the clay constituents to form a curious 

 compound not yet investigated, or it is oxidised by bacteria to ni- 

 trite, and finally to nitrate. 



The nitrifying organisms derive their carbon not from organic 

 matter, which indeed is rather injurious to them, but from carbonic 

 acid, which they assimilate and convert into complex cell sub- 

 stances, without the aid of sunlight or the intervention of chloro- 

 phyll ; apparently they utilise the energy set free by the oxidation 

 of ammonia. 



Another set of organisms possesses the remarkable property 

 of absorbing gaseous nitrogen from the air and converting it into 

 protein. 



