i86 FARMYARD MANURE [chap. 



bacteria which break down the proteins into simpler 

 compounds such as amino-acids, amides, and finally 

 ammonia. Some of these bacteria, like B. coli connnutiis, 

 are abundant in the large intestine of herbivorous 

 animals, and of course continue their work in the excreta 

 after ejection. Without discussing them individually, 

 their function is to convert the insoluble nitrogenous 

 bodies of the straw (those of the faeces are more difficult 

 of attack because they have already resisted the actions 

 of digestion) into soluble bodies akin to ammonia and 

 therefore more nearly utilisable by the plant. Thus, 

 with a certain amount of loss as free nitrogen, the trend 

 of the bacterial actions taking place in the fresh farm- 

 yard manure is to break down the complex insoluble 

 compounds of nitrogen to more and more simple ones, 

 ammonia being the final term. At the same time, there 

 is always a reverse change going on ; as the bacteria 

 themselves multiply, they seize upon the active soluble 

 forms of nitrogen and convert them into insoluble 

 proteins in their body tissues. Which action is pre- 

 dominant will depend on the stage that has been 

 reached in the dung-making process — i.e., on the supply 

 of carboh)drate, air, water, and other variable factors — 

 but after the first rapid production of ammonium com- 

 pounds, the longer the dung is stored the more the 

 ammonia returns to a protein form. 



So far we have been considering only changes in 

 the nitrogenous material of the excreta and the litter, 

 since nitrogen is the chief fertilising constituent of the 

 manure, but the most characteristic change in dung- 

 making is the destruction of the straw and its con- 

 version into dark brown " humus," which in the end 

 retains none of the structure of the original straw. 

 There are a number of organisms to be found commonly 

 in the air and dust which readily attack such carbo- 



