THE SOIL 83 



The relation between the various sedimentary rocks 

 may be illustrated by considering a river flowing into the 

 sea. When the river meets the sea, the speed of its current 

 is checked, and the sediment which it bears or rolls along 

 is deposited upon its bed or along its banks. The heaviest 

 particles come to rest as soon as the current can move 

 them along no farther. As the velocity of the stream 

 decreases, the lighter particles fall first, the coarse sand ; 

 then the finer sand and silt ; and, last of all, an almost 

 impalpable mud, which may be borne miles out to sea, 

 and which, when deposited, forms clay. 



Humus. This is one of the most important constituents 

 of fertile soil ; when it is absent, the ground is sterile and 

 unproductive. More or less of it is present in every soil 

 which supports vegetation, and sometimes the soil con- 

 sists of little or nothing else. Where plants grow, humus 

 accumulates. The conversion of dead vegetation or 

 animal remains into humus is brought about by germs 

 (bacteria) and fungi living in the soil. Where air is abun- 

 dant and the ground not too cold, the organic matter in 

 the soil is completely destroyed, the final products of its 

 decomposition being carbonic acid gas, water, and mineral 

 salts substances valuable to plants as sources of food. 

 When, through any cause, decomposition is checked, 

 dark-coloured " earth "-acids are formed, which have a 

 souring effect upon the soil. If lime, however, is present, 

 it combines with these acids, rendering the humus mild 

 and alkaline, and consequently fertile (p. 94). Enor- 

 mous masses of humus in the form of peat have accumu- 

 lated upon the rock-surfaces of the land, where, through 

 lack of air, abundance of water, or extreme cold, decom- 

 position has been arrested. In the form of coal, we dig 

 up and burn as fuel the compacted and mineralized humus 

 of past ages ; in the form of manure, the agriculturalist 

 renews the fertility of the fields with " artificial " humus. 

 At the present day peaty matter is constantly being 

 formed on wet, cold moors, on dry, cold heaths, in mossy 

 bogs, and in forests from rotting leaves and wood. 



