12 



THE SOIL AND THE PLANT 



>^ 



tegration of the plant materials. To this is added a large quan- 

 tity of organic matter in the form of bodies of microorganisms 

 and their various decomposition products. 



In addition to the mineral soils, which are predominantly 

 inorganic (95 per cent), there are soils which are largely organic 

 in nature. Here belong the peat soils, which originate from bogs. 

 These soils contain 30 to 98 per cent organic matter and only 2 

 to 70 per cent inorganic material. The surface layers of certain 

 types of forest soils are also predominantly organic in nature. 

 These soils are characteristically organic, since the accumulation 

 of plant materials takes place more rapidly than the decomposition 

 of these residues by microorganisms, due either to saturation 

 with water (as in peat bogs) or high acidity of soil combined with 

 other factors unfavorable to the activities of the organisms. 



Soil Phases. — The soil thus consists of three definite phases 



(see Fig. 4): (1) The 

 solid phase, which, in 

 the case of mineral 

 soils, is composed 

 largely (up to 99.5 

 per cent) of inorganic 

 rock constituents and 

 only to a limited ex- 

 tent (0.5 to 10 per 

 cent) of organic ma- 

 terials; in the case of 

 peat soils, the organic 

 matter may make up 

 30 to 99 per cent of 

 the solid phase of the 

 soil constituents on 

 a water-free basis. 

 (2) The liquid phase, 

 consisting of water 

 which contains in 

 solution various or- 

 ganic and inorganic 



<S45 



35 



25 



15 



Air 



Water 



Organic Mineral 

 Matter 

 Fig. 4. — The relative distribution of materials 

 which compose a soil under conditions favorable 

 to plant growth. 



materials. This water may be combined with the soil colloids 

 as hygroscopic water which is held very tenaciously or it may be 

 present as capillary film water or as free gravitational water. 



