Emil Truog 31 



the pore space may approximate 3$ per cent in sands, 50 per cent in 

 silt loams, 60 per cent in clays, and 75 per cent or more in peats. The 

 remainder, making up the total of 100 per cent, represents, of course, 

 the solid phase in each case. 



The solid phase is made up of inorganic and organic material. The 

 proportion of each varies greatly in different soils. Well-drained upland 

 mineral soils seldom contain more than 10 per cent of organic matter, 

 and when they contain 5 per cent they are usually considered to 

 be well supplied. Poor sands may contain less than 1 per cent; mucks 

 contain 20 to 50 per cent; and peats contain over 50 per cent. Organic 

 matter, which consists chiefly of plant residues in various stages of 

 decomposition, plays a very important function in soils by favoring 

 desirable physical, chemical, and biological processes. The maintenance 

 of an adequate supply of organic matter in cultivated soils is of para- 

 mount importance and represents one of the major problems in practi- 

 cal soil management. 



The inorganic portion consists of mineral particles varying in size 

 from coarse sand grains, which can be seen easily with the naked 

 eye, to clay particles which are too fine to be seen individually, even 

 with the aid of a powerful microscope. The coarser particles consist 

 largely of primary minerals, that is, minerals which formed or crystal- 

 lized out when molten masses cooled to form the igneous (original) 

 rocks. As found in soils, they represent the product of physical weather- 

 ing solely : they have not been altered as regards composition by chemi- 

 cal weathering. For the most part, they usually consist of shiny crystals 

 of quartz, mica, and feldspar. The finer particles consist almost entirely 

 of secondary minerals — the new compounds which form and remain 

 when primary minerals undergo chemical weathering. They consist 

 largely of iron oxides, silica, and particularly the clay minerals which 

 are tremendously important in soils because they have the property 

 of holding plant nutrient cations in an exchangeable and readily avail- 

 able form. Details regarding the nature of these clay minerals and 

 associated exchange phenomena are given in another paper of this 

 symposium. Let it suffice to say at this point that the available plant 

 nutrients are held largely in the finer clay particles and in the organic 

 matter. 



