I20 BACTERIA AND SOIL MINERALS 
soil, but its chief use is to keep the bacterial activities in the soil 
at a high state of activity. 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 re- 
moved so that a more or less frequent liming is necessary to main- 
tain 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 avail- 
able chiefly, if not wholly, through the action of bacteria. Lime- 
stone 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 con- 
stant decomposition processes going on in the soil, carbon dioxid 
gas is being set free from the decomposing organic compounds, 
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 maintained 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 
