io6 CHAPTER VI 



Micro-organisms in Relation to the Soil. — This subject, while only 

 indirectly connected with manuring, may be touched on here in some special 

 connections with the sugar cane. The flora of the soil is made up of bacteria, 

 protozoa, fungi and moulds. The fertihty of the soil is largely controlled 

 by the organic life therein, and it is the first-named class of its inhabitants 

 that have been most studied. Following on Stoklasa,^' soil bacteria in re- 

 lation to nitrogen may be classed as follows : — 



T. Bacteria which decompose organic nitrogen and produce ammonia. 



2. Bacteria which oxidize ammonia to nitrates. 



3. Bacteria which oxidize nitrites to nitrates. 



4. Bacteria which reduce nitrates to nitrites and then to ammonia. 



5. Bacteria which reduce nitrates to nitrites and eventually to nitrogen. 



6. Bacteria which change nitrates, nitrites and ammonia to protein 



compounds. This type includes members of all groups. 



7. Bacteria which fix atmospheric nitrogen. 



Of these forms those connected with the production of nitrates have been 

 most studied and the elucidation of the problem forms one of the world's 

 classics of research. Briefly, the formation of nitrates, whence plants 

 mainly obtain their nitrogen, takes place in a number of stages. First of all 

 organic nitrogen is broken down into ammonia salts. The ammonia salts 

 are then converted to nitrites by organisms, of which two types are known, 

 one, Nitrosomonas, peculiar to the Old, and the other, Nitrosococcus, occurring 

 in the New World. Following on the activities of these organisms, the nitrites 

 are converted into nitrates by an oiganism, Niirobacter, of which one type 

 only is known. Conversely, a reverse process takes place whereby the nitrates 

 are reduced to nitrite and finally to gaseous nitrogen. The conditions re- 

 quired for the development of the maximum activity of the nitrifying 

 organisms are : — 



1. The limits of activity are 5°-55° C, with an optimum of 37° C. 



2. Absence of a great excess of organic matter, of alkahne chlorides or 

 carbonates. 



3. A base is required to neutralize the acid formed, and for this purpose 

 calcium carbonate is most efficient, though the naturally occurring zeohtes 

 in soils may suffice. 



4. A supply of carbon and of oxygen is necessary, and the former may 

 be supplied by carbonate or by carbon dioxide. 



5. Water, but not an excess, is necessary. 



6. Absence of direct sunlight. 



The conditions which favour the development of the denitrifying organism 

 are the reverse of the above, and it hence follows that nitrification will 

 be at a maximum in well drained, well tilled, well aerated soils and in the 

 presence of calcium carbonate. Denitrification and infertility will be found 

 in unfilled, badly drained, water-logged soils, and in others where, for example, 

 an alkahne reaction results from the too long-continued application of sodium 

 nitrate, or where salty subsoil water rises to the surface, or where there is 

 a great excess of organic matter. 



To the action of direct sunlight is to be attributed one of the bad effects 

 of a naked fallow, which always results in a loss of nitrates, and this observa- 

 tion has been utilized by Eckart in U.S. patent 1,274,527. He proposes, 

 and the scheme has been put into operation at Olaa in the Hawaiian Islands, 

 to lay down strips of paper between the cane rows, whereby artificial nitrate 

 beds will be formed. Simultaneously the growth of weeds is prevented. 



