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 
