52 THE SOIL. III. 



brought about by liming, also the quick settling of the clay par- 

 ticles when a muddy river flows into the sea, with the con- 

 sequent production of deltas and bars. The waters of rivers 

 remain muddy for a longer time if deficient in lime compounds, 

 while rivers containing very hard waters soon clarify. Frost 

 also produces to some extent the shrinkage and coagulation 

 of colloidal clay. 



Limestone. This term in connection with the constituents 

 of a soil must be taken to mean the finely divided particles of 

 calcium carbonate, c., which are present, acting, perhaps, in 

 some cases partly as a cementing material to the quartz grains. 

 As already stated, it furnishes plant food by virtue of the 

 calcium, magnesium, and phosphoric acid which are always 

 present in it. Its action in the soil, however, is more 

 important than as a mere source of plant food. It acts upon 

 the colloidal clay in a manner already described as charac- 

 teristic of lime and other salts and thus modifies the physical 

 texture of the soil. Perhaps its most important function in a 

 soil, however, is to act as a weak base, with which acid pro- 

 ducts, formed by decomposition of the organic matter in the 

 soil, can readily unite and by w r hich their harmful acidity is 

 destroyed. If such basic material be absent, the soil becomes 

 " sour," as it is called, and unfitted for the growth of most 

 crops. This sourness is generally due to the production of 

 free organic acids of the humic acid type and is possessed by 

 many peaty soils where the amount of organic matter is ex- 

 cessive. The acidity is often apparent in such soils by the 

 bleaching effect they have upon the sand and gravel upon 

 wbich they rest. In many cases all the iron, &c., to which 

 the gravels and sands owe their colour is found to be washed 

 out by the acid drainage from the peaty soil.* Then still 

 more important is the part played by calcium carbonate in the 

 process of nitrification (c. Chap. IV.). Some basic material 

 is essential for the continuance of this process, and the base is 



* Another possible explanation of this bleaching action is that the organic matter 

 draining from the peat reduces the ferric oxide to ferrous oxide, which is converted 

 first into carbonate and then into the soluble bicarbonate by the carbon dioxide also 

 abundant in the drainage water. 



