CHAPTER V 



BIOLOGY OF SOILS 



The organisms. The organisms of the soil, with which 

 the agriculturist is chiefly concerned, belong to the 

 simplest order of plants. They are known to botanists as 

 Thallophytae, and are divided by them into two groups, 

 called respectively Algae and Fungi. The former contain 

 chlorophyll and, often, other colouring matters as well. 

 The latter contain no chlorophyll, and are usually white or 

 colourless, but, when seen in mass, some of them exhibit 

 red, yellow, green or bluish colours. Both groups are 

 subject to minute botanical classification, but for pre- 

 sent purposes the algse need not be subdivided at all, and 

 it will be sufficient to divide the fungi into moulds and 

 bacteria. From this p.oint of view the bacteria are by far 

 the most important both numerically and functionally. 



Alga. These are simple cellular plants adapted for 

 growth in moist places or in water. The sea-weeds belong 

 to this group, but the forms present in the soil are of 

 microscopic size. They are distinguished from the fungi 

 by the presence of chlorophyll, which enables them to 

 decompose carbon dioxide and obtain their carbon from 

 the air. Sunlight is, therefore, necessary for their growth, 

 and the fact implies that their sphere of action must be 

 confined to the surface of the soil. 



It is believed that certain forms are able to utilise the 

 free nitrogen of the air to form organic compounds, by 

 the decomposition of which, after the death of the 

 organism, the nitrogen is rendered available to higher 

 plants. This is probably one of the sources of the com- 



