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The organic matter in the soil, consisting of the dead 

 bodies of animals and plants, contains all the elements 

 required to. build up new plants. Hence, the humus is 

 a most valuable, in fact indispensable, part of the soil. 

 As humus, it cannot be taken by plants and hence the 

 preparation of available plant food from it is one of the 

 most necessary and important processes taking place in 

 the soil. The importance of humus is evident when it is 

 remembered that it is the only part of the soil which can 

 yield the very valuable food material, Nitrogen. Neither 

 chemical nor physical agents have under ordinary cir- 

 cumstances much effect on organic matter. For example, a 

 piece of wood kept in air or pure water remains unchanged 

 w r hereas when it is buried in the moist soil it quickly 

 decays. In dry desert soil, which contains no micro-or- 

 ganisms, the wood remains unchanged. Even the juices 

 excreted by the plant roots have an inappreciable effect 

 on organic matter. In short, the micro-organisms alone 

 may be regarded as the preparers of useful material from 

 the soil humus, and a soil without micro-organisms is not 

 fertile, however much humus it may contain. These small 

 organisms 'are to the plant what the digestive juices are 

 to the animal ; both prepare food, one for absorption by 

 the root hair into the plant, the other for the passage of 

 the food from the outside into the animal system. 



Different soils have naturally different numbers of bac- 

 teria living in them. In any soil the number of bacteria 

 is greatest a short distance under the surface, where they 

 find the most favourable conditions for life. Here they 

 find a good supply of organic matter to serve as food, 



