Relation of the Plant to the Soil 



81 



that this is so, for, otherwise, the valuable soil constitu- 

 ents would be washed off to the sea by the percolating 

 water. It is the great solubility of some substances, like 

 nitrates, that explains their scarcity in the soil. 



MINERAL MATTER DISSOLVED IN 100,000 PARTS OF 

 DRAINAGE WATER. 



113. Chemical Change in the Soil. The soil is the seat 

 of constant changes, and these changes have great in- 

 fluence on the productiveness of the soil. When the 

 soil is plowed, the particles are exposed more to the 

 action of the air, water, frost, etc. When humus is put 

 into the soil, acids are formed as the humus decomposes, 

 and these tend to dissolve the substances in the soil. 



114. Soil-Bacteria. Humus also encourages the growth 

 of soil bacteria, because they live on plant and animal 

 remains. These bacteria decompose the humus, and, 

 in doing so, set free carbonic acid, which aids in dis- 

 solving the particles of soil. Thus it is that the bacteria 

 of decay act beneficially on the soil. Other species of 

 bacteria cause the formation of nitrates from ammonia 

 or other nitrogen compounds or the free nitrogen of the 

 air. No soil will long remain fertile unless the supply of 

 organic matter is kept up. 



