

THE SOIL. 67 



The first class includes very many bacteria which are the 

 micro-organisms of diseases. Parasitic, putrefying, and 

 denitrifying organisms do more or less harm in the soil. 

 The good bacteria, those of decay and nitrification, and 

 the fixers of free nitrogen are those which must be 

 favoured in soil cultivation. The good bacteria are helped 

 by having the soil in a good state of cultivation, by having 

 the soil well ventilated and well drained, and by supplying 

 organic manures. Wet land, or soil containing salt, is 

 unsuited to the requirements of the good micro-organisms. 

 The activity of bacteria is greatest when the medium in 

 which they are found has a temperature of about 38 C. and 

 generally decreases with a higher or lower temperature. 



The good bacteria act directly or indirectly in preparing 

 plant food. The decay, nitrifying, and symbiotic (nitrogen 

 fixing) bacteria act directly on the raw materials and pre- 

 pare them for use by the plant. A soil containing much 

 humus but none of the above micro-organisms is absolutely 

 infertile. Thus salt, wet, or sour soil, may be rich in raw 

 plant food, but if it does not contain the good bacteria, 

 it is incapable of producing a crop. Applying sulphate 

 of ammonia to the soil is only of use when the soil contains 

 nitrifying bacteria to change it into available plant food. 

 Without the nitrogen-fixing bacteria the supply of nitro- 

 genous food would become less and less and the fertility 

 of the soil would gradually be exhausted. The bacteria 

 preparing plant food are ever active, and hence on bare 

 land there is an accumulation of soluble plant food Avhich 

 may be washed away if the land is heavily flooded. 



Indirectly, the bacteria assist the growth of crops by 



