THE SOIL. 59 



food is brought about largely by the direct or indirect 

 action of the soil organisms the animals living in the 

 soil and chiefly insects and worms. These, boring through 

 the soil, open it up to the action of weathering agents and 

 facilitate the passage of the plant roots. Many of these 

 animals derive their food from the organic part of the soil 

 to obtain which they eat the soil, and passing it through 

 their body, give excrements richer in available plant food 

 than the original soil. Further, the physical properties are 

 also improved, as the soil leaves the worms or insects in 

 a finely pulverised state. Of course, many of the insects 

 usually found in a soil are enemies of the crop, and the 

 <r< >( xl they do to the soil is far more than counterbalanced 

 by the harm done to the crops. It is worthy of note 

 that whereas the plants prepare food for animals, the 

 animals by giving manure assist in the preparation of 

 plant food. 



Of the plant life in the soil, the micro-organisms or 

 bacteria and the crop roots are of greatest importance. 

 Every living root, while it is taking in food, is also active in 

 giving out waste products of the plant cells or excrements. 

 The acids excreted by the root attack the insoluble soil 

 particles and dissolve matter from them. Different plants 

 excrete juices of different dissolving power and therefore 

 some plants can do much more than others in the prepar- 

 ation of their own food. It is partly on this account that 

 no two crops can be grown on the same soil with equal 

 success; while lupins can derive sufficient mineral food 

 from a very poor sandy soil, many other plants would be 

 quite unable to prepare their food from the scanty material 

 at their disposal. 



