72 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



species of bacteria found in the interstices of the soil can be classified, 

 somewhat artificially, in five main groups. 



1. The denitrifying* bacteria which are held responsible for the 

 breaking down of the compound substances known as nitrates, 

 and the decomposition bacteria which break down complex 

 organic products other than nitrates into simpler bodies. 



2. The nitrifying" bacteria which do not reduce compound pro- 

 ducts like the denitrifying bacteria, but oxidise and change from 

 one form to another. 



3. The free nitrogen-fixing* bacteria to be found principally in 

 the nodules on the rootlets of certain plants. 



These three groups may be regarded as " economic " bacteria. There 

 are two other groups, but they are not of much direct importance to the 

 cultivator so far as at present known. 



The fourth group is a provisional one, and includes the common 

 saprophytic bacteria, which feed on decayed animal and vegetable 

 matter. This group is by far the most abundant as regards 

 number in the soil. They live on the dead organic matter of the 

 soil and break it into simpler constituents ; but their function is at 

 present but imperfectly known, and when known they will probably 

 be classified otherwise. The denitrifying, decomposition, nitrify- 

 ing, and nitrogen-fixing organisms are all saprophytes that have, in 

 a sense, been brought more or less into daylight. The business of 

 the other saprophytes in the soii world is at present unknown. 



The fifth group of bacteria found in the soil are the pathogenic, or 

 disease-producing organisms. The three chief members of this 

 group being the bacillus of lockjaw, the bacillus of quarter evil or 

 symptomatic anthrax, and the bacillus of malignant oedema, or 

 gangrenous septicemia, the last two being similar in some respects 

 to anthrax itself. 



The "new soil science " deals chiefly with the •'economic" bacteria 

 present in the soil ; but it must be borne in mind that though a consider- 

 able amount of knowledge has been obtained, there is still a great deal to 

 be discovered before the micro-biology of the soil is complete. It must 

 also be admitted that at present our knowledge is rather a heterogeneous 

 collection of isolated facts and theories, some of which still require more 

 ample confirmation. Yet with the limitations that this implies, an 

 advance has been made sufficient to relegate many chemical theories in 

 respect to soil cultivation kc. to a secondary place. 



Some general factors affect all soil microbic life. Heat, for instance, 

 is an important condition. A sweltering hot summer's day — 90° F. — 

 most favours their development, which will cease at 50° higher, or if it 

 lulls to freezing. In the tropics, where the mean temperature is about 

 1)0°, soil bacteria, as was to be expected, are far more abundant than in 

 temperate regions ; in fact, the enormous rate at which these germs can 

 increase under suitable heat conditions is simply astounding. Soil germs 

 further want plenty of air, and they also need moisture, while the absence 

 or presence of an excess of water is bad for their development. Sunlight, 



