IV. THE EEACTIONS OCCURRING IN SOILS . 71 



added, even to the extent of 1 per cent of the soil, only about one third of 

 the nitric nitrogen was lost. He suggests that the injurious effects of 

 large additions of farm-yard manure to a soil may be largely due to 

 the nitrification being checked, rather than to actual destruction of 

 nitrates. 



Beyerinck and Minkman l in 1909, confirmed the presence in soils 

 of the denitrifying organisms isolated by Gayon and Dupetit and also 

 found two other organisms destructive to nitrates. One of these was 

 able to produce large quantities of nitrous oxide from nitrates. Other 

 organisms present in soils are able to remove oxygen from nitrous 

 oxide, setting free nitrogen. They also found an organism which could 

 cause the interaction of a mixture of free hydrogen and nitrous oxide 

 and utilise the energy evolved in decomposing carbon dioxide and 

 building up complex organic nitrogenous compounds. 



This has, to a great extent, been confirmed by Suzuki 2 and 

 Lebedeff. 3 



Toxic Substances in Soils. Enough has been said to show how 

 highly complex are the processes occurring in soils, but in addition to 

 the factors already mentioned, there are, according to American in- 

 vestigators (e.g., Whitney and Cameron, Schreiner and Shorey), sub- 

 stances which are toxic to plants, produced in soils, either by the 

 growth of the plants themselves or by bacteria, and upon the relative 

 scarcity or abundance of these substances, the fertility or non-fertility 

 of a soil mainly depends. 



Fletcher 4 in India, supports the view that toxic substances are 

 produced by the growth of plants in soils, while other investigators 

 ascribe to bacteria the production of toxins, to which the non-fertility 

 of some soils is attributed. 



Soil gases. The interstices between the particles composing a soil 

 are usually occupied by air except when heavy rain or other cause fills 

 them with water. This enclosed air must not be regarded as confined, 

 but is constantly undergoing renewal by diffusion from the air above. 

 Inasmuch as the processes going on in a soil are accompanied by, and 

 indeed largely consist of, oxidation, it is obvious that the air within a 

 soil will be poorer in oxygen than that of the atmosphere above. 

 Schloesing 5 in 1890 published the results of a number of analyses of 

 air sucked out from soil from various depths. This was always found 

 to contain only the gases of the atmosphere, no measurable amount of 

 marsh gas or other combustible gas being detected. The general 

 results of these determinations were 



1. The sum of the percentage amounts of carbon dioxide and 

 oxygen is equal to 21. 



2. The amount of carbon dioxide varies very much, from about 1 

 per cent to as high as 8 or 10 per cent, the oxygen from 10 to 20 per 

 cent. 



1 Jour. Chem. Soc., 1909, Abstracts, ii. 1043. 



2 Ibid., 1911, Abstracts, ii. 916. Ibid., 917. 



4 Jour. Chem. Soc., 1908, Abstracts, ii. 617. 5 Compt. Rend., 109, 173. 



