146 LIVING ORGANISMS OF THE SOIL [chap. 



verted into proteins or substances akin to them, require 

 to go through the routine of decay and oxidation before 

 they can reach the plant. Next come the group of 

 denitrifying organisms which deal wastefully with the 

 nitrogenous reserves of the soil, converting them into 

 nitrogen gas, which possesses no value. These become 

 active when the soil is waterlogged and the supply of 

 oxygen cut off; their action is also promoted by an 

 abundance of organic matter. Then we have groups of 

 organisms working in a contrary sense by fixing nitrogen 

 gas and so enriching the soil ; some of these organisms 

 live free in the soil, others enter into symbiotic partner- 

 ship with the leguminous and a few other kinds of 

 plants. Similarly, for the non-nitrogenous organic 

 matter we have one group of oxidising or decay 

 bacteria, which burn up the carbohydrates to carbon 

 dioxide and water in the presence of air, and another 

 group working under anaerobic conditions, which 

 produce carbon dioxide and marsh-gas or hydrogen ; 

 both groups, but especially the latter, giving rise to 

 humus as an intermediary product. The tendency of 

 these various organisms, which are in many cases 

 working in opposite directions, is to arrive at some 

 state of equilibrium appropriate to each given soil and 

 to the treatment it receives; for example, in an 

 exhausted soil such as prevails on the unmanured plot 

 on the Rothamsted wheat field, the activity of the 

 ammonia-making organisms has been reduced to the 

 point that the nitrogen thus rendered available for the 

 crop is practically balanced by the nitrogen gained by 

 the fixation organisms. On the other hand, on the plot 

 receiving farmyard manure every year, to which about 

 four times as much nitrogen is being applied as is being 

 removed in the crop, there is no longer any increase of 

 nitrogen in the soil ; the activity of the denitrifying 



