CHAP, x STRUCTURE OF SOIL 41 



position of rock there arises loose soil : the active agencies are, particularly, 

 changes of temperature, congelation of water as well as the chemical 

 action of water and of the oxygen and carbon dioxide of the atmosphere. 

 In certain cases lowly organized plants, such as lichens and bacteria, 

 also play a part. Chemical decomposition and mechanical disintegration 

 always go hand in hand. 



Secondary soil. This owes its origin to the transport, and partially 

 to the separation of the different constituents, of soil produced by weather- 

 ing ; the transporting agents are, particularly, currents of water, move- 

 ments of glaciers, and wind. Rivers (Po, Nile, Ganges, and others) heap 

 up at their mouths masses of loose substance that has been conveyed 

 from mountains ; glaciers during the Glacial Epoch transported vast 

 masses of soil to distant spots (for instance, from Norway and Sweden 

 to Denmark and North Germany) and continue to do so in the present ; 

 seas in their currents carry with them other masses of substance. Wind 

 deposits sand from the seashore and from inland sandy soil in the form 

 of dunes ; it also carries away fine particles from the surface of soil 

 and deposits them in sheltered places (as loess). 



The characters of loose soil depend upon many and divers features, 

 and particularly upon the fineness, chemical nature, arrangement, and 

 cohesion of its constituents, as will be detailed in the sequel. 



From loose soil there often arise new kinds of rock, for instance sand- 

 stone, shale, and conglomerate, which differ in character from the original 

 rock and play a different role in plant-economy. 



Loose soil has the following structure. It is a mixture of 



1. Solid constituents; 



2. Air (Chap. XI) ; 



3. Water (Chap. XII). 



SOLID CONSTITUENTS OF THE SOIL 



The solid constituents of soil are : 



(a) Larger mineralogical constituents, stones varying in quantity and 

 size down to extremely small grains of sand ; if the soil be shaken up in 

 water and allowed to stand, these constituents rapidly sink to the bottom. 

 In chemical composition they vary greatly, but quartz is most common. 



(b) Very minute, powder-like particles, which remain suspended in 

 water for a long time when the soil is shaken up in water and allowed to 

 stand. By this process they can be easily separated from sand. They 

 likewise vary greatly in chemical composition, but are mainly composed 

 of aluminium silicate, and compounds of iron and calcium ; they have 

 an essential influence on the amount of nutriment in the soil, on its 

 power of absorption, and on its physical characters. 



(c) Humus-substances arise from the corpses and by-products of 

 plants or animals. They are destroyed by combustion. Many humus- 

 substances clearly show their organic origin and mostly impart to the soil 

 a black or dark brown colour. 



These three kinds of constituents occur in nearly all soils. 

 All constituents that are too large to pass through a sieve with meshes 

 0-3 millimetre in width are termed by W. Knop the soil-skeleton (coarse 



