CARBON AND NITROGEN CYCLES IN THE SOIL 213 



Green manuring is, however, commonly adopted and con- 

 siderably benefits the crop : the decompositions taking place 

 when the green crop is ploughed in have been studied by 

 W. H. Harrison and Aiyer (126). In the body of the soil marsh 

 gas, hydrogen, and CO 2 are evolved, as would be expected 

 from the anaerobic decomposition of cellulose. But at the 

 surface of the soil the change is entirely different and the 

 gases consist of oxygen and nitrogen only. The difference 

 was traced to a film of organisms which have the power of 

 converting the marsh gas into COg 1 and this into oxygen. 

 The oxygen is directly beneficial to the plant by providing 

 for the aeration of the root. The production of oxygen was 

 suppressed when the film was killed by adding copper sul- 

 phate : marsh gas and hydrogen then appeared at the surface. 



It is only so long as the film is working that green manur- 

 ing is beneficial to the crop. The plant roots must have 

 oxygen and the film supplies it. The green manure, there- 

 fore, not only supplies ammonia, but also dissolved oxygen. 



It would be interesting to have studies of the conditions 

 obtaining in water cress-beds. 



The Effect of Adding Plant Residues to the Soil. 



The addition of plant residues to the soil is a normal oc- 

 currence and a recognised method of manuring land. The 

 effects produced depend on the proportions of carbohydrate 

 and protein present in the residues. 2 



If the conditions are favourable to the activity of micro- 

 organisms the result of the addition is to cause an increase in 

 numbers and in activity of the micro-organisms : this is shown 

 by increases in oxygen absorption and CO 2 evolution. The 

 increased numbers may have any of the following effects : 



1 The biological oxidation of marsh gas has been described by Kaserer and 

 Sohngen and by I. Giglioli and G. Masoni (Pisa: Chim. Agric. Stud. e. Ri- 

 cherche, 1909-14, Part 22, 76-108). 



2 For further discussion see H. B. Hutchinson (1400); P. E. Brown and 

 F. E. Allison (6oe) ; P. Felber, Mitt. Landw. Hochschule f. Bodenkulture in 

 Wien, 1916, 3, 23. 



