THE SOIL 83 



Silt consists of fragments of quartz, felspar, mica, 

 zircon, iron-oxides and other minerals which have resisted 

 weathering action. 



Sand consists mainly of quartz, but the other minerals 

 found in silt are also present to some extent. A funda- 

 mental character of sand is that it falls to powder when 

 dry and is not plastic like clay. 



Chalk is an amorphous form of carbonate of lime; 

 when crystalline, it is limestone. Soils derived from 

 chalk or limestone are naturally alkaline and rich in 

 lime. They usually contain some sand, derived either 

 from the sUiceous skeletons of organisms from which the 

 rocks are built up, or from overlying deposits which have 

 been removed by denudation. 



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 

 souaring 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 xock-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 bum as fuel the compacted and miaeralized humus 

 of past ages ; in the form of vmnure, 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, whilst in forests large quantities of mild humus 

 are formed from rotting leaves and wood. 



