112 BACTERIA AND SOIL MINERALS. 



But there are also constant losses of lime from the soil. A small 

 quantity is carried away by the crops taken from the land, but a far 

 larger quantity is lost to the soil by drainage. The soluble lime 

 salts are dissolved by the soil waters and pass off with the drainage. 

 Very large amounts are thus removed so that a more or less frequent 

 liming is necessary to maintain in the soil a quantity sufficient to 

 keep the proper condition for bacterial action. Different soils show 

 wide differences in the amount of lime needed. Soils containing 

 limestone rock have an abundant natural supply, while soils without 

 limestone need to be furnished with it in varying amounts. The 

 lime thus drained away is a permanent loss, for it finds its way into 

 the ocean whence it is not easily returned to the soil. But this 

 loss is not serious, since limestone rocks are practically unlimited 

 and there need be no lack in the supply of available lime. Lime is 

 rendered available chiefly, if not wholly, through the action of bac- 

 teria. Limestone consists mainly of carbonate of lime which is 

 only very slightly soluble in water, and cannot be utilized directly, 

 for this reason. But water containing carbonic dioxid in solution 

 readily dissolves the carbonate of lime. We have seen that by the 

 constant decomposition processes going on in the soil, carbon 

 dioxid gas is being set free from the decomposing organic com- 

 pounds, such as proteids, sugar, cellulose, etc. This gas is taken up 

 by the water, which is then able to dissolve the limestone. The 

 greater the extent of the bacterial action, the greater will be the 

 amount of carbon dioxid eliminated, and the amount of lime 

 brought into solution; the more effectually also will the soil be main- 

 tained in proper condition for bacterial growth. Hence, as the 

 amount of lime in the soil increases, the bacterial action will become 

 greater, more lime will be dissolved, and consequently more will be 

 lost by drainage. 



In this way the limestones on the earth's crust are being dis- 

 solved and carried away. The extent to which this is possible is 

 indicated by the huge limestone caves whose great spaces show how 

 the limestone has been dissolved by waters which held carbon di- 

 oxid in solution. All such dissolved lime finds its way to the ocean 

 where it supplies marine animals and plants with the lime for their 



