SOIL FORMATION 



Soil Formation. — In the initial processes of formation of 

 soils, lichens, mosses and other small organisms attack and weaken 

 the rock constituents by re- 

 moving certain of their more 

 soluble elements or com- 

 pounds. Changes in tempera- 

 ture cause expansion and 

 contraction which open seams 

 and gradually form small frag- 

 ments which, when mixed 

 with the decomposing organic 

 remains of the first invaders, 

 supply footholds for the de- 

 velopment of larger plants. Fig. 3.— Physical structure of soil in rela- 

 In turn they further disinte- ^ion to root development. Schematic 



, , V 1 J. • 1 1 representation of the sohd soil particles, 



grate the rock materials by r\.u ■ i r ^ i i I 



° ''or the au- spaces and of root development 



physical and chemical forces. (from France). 



Gradually the organic residues 



of these plants become mixed with the coarse and fine rock 

 materials, thus giving rise to the beginnings of agricultural soils 

 (Fig. 3). 



The abrasion of soil particles is carried out further by the 

 action of air, water and ice, thus adding to the disintegrated 

 material. IVIuch of this fine substance reaches the lowlands by 

 water and ice removal and tends to fill in the valleys. Here, 

 temperature conditions being more conducive to plant growth, a 

 vegetation develops, the abundance of which is determined 

 principally by the available moisture. The repeated develop- 

 ment of plants and the incorporation of their remains with the 

 soil, where they undergo partial disintegration, finally produces 

 a material having very few characteristics in common with the 

 rocks from which it originated. The greatest modification takes 

 place near the surface, where pronounced disintegration and 

 chemical changes have been produced. This region is exposed to 

 the most marked processes of leaching and receives also the largest 

 amount of plant residues. The deeper layers of soil, or the 

 so-called subsoils or B horizons, contain much less organic matter 

 and consequently consist almost entirely of mineral substances 

 some of which may have originated from the surface and accumu- 

 lated at deeper zones as the result of leaching. Below this 



