74 The Substratum 



soil into grains, clumps, and flakes. Soil particles are classified ac- 

 cording to size in the accompanying table. 



Sand 1 . 00-0 . 05 mm in diameter 



Silt . 05-0 . 002 



Clay < . 002 



In a good loam all three of the categories are well represented. 

 The type of structure into which the particles are arranged affects 

 profoundly the porosity of the soil. It also controls the amount of 

 surface which is presented on the one hand to the air and water 

 moving through the soil, and on the other to the hairs of roots grow- 

 ing in the soil. The relative proportions of the soil constituents and 

 of the air and water present are extremely variable. In an average 

 good soil about half the volume is commonly represented by pore 

 space of which half may be occupied by air and half by water. The 

 solid material of such a soil may consist of 95 per cent mineral par- 

 ticles and 5 per cent organic matter. In tropical soils, however, 

 organic matter may be less than 1 per cent, and in peaty soil it may 

 approach 100 per cent of the dry material. In addition to differences 

 in texture and structure, soils vary physically in the type of layering 

 that they develop as they mature under biological and climatic in- 

 fluences as will be discussed in the next section. 



The chemical nature of soils is even more diverse and variable. 

 Upon the disintegration of the parent rocks the whole spectrum of 

 minerals present becomes available for incorporation into the soil. 

 Added to these are a wide varrety of organic substances derived from 

 animals and plants and other materials introduced from the air and 

 ground water. Further chemical changes take place within the soil 

 as climatic and biological agents work on it. As a consequence, soils 

 and soil water differ widely in chemical composition, organic content, 

 and total salinity, as well as in degree of acidity, oxidation-reduction 

 potential, and other physicochemical characteristics. Some of these 

 features of the soil are intimately interrelated with the physical char- 

 acteristics. For example, the smallest particles involved in the texture 

 of a soil are colloids, and their behavior and reactions are also in- 

 volved in the chemistry and physical chemistry of the soil. The abun- 

 dance and type of the colloids present affect the amount of water 

 retained by the soil and its availability to plants. At the same time 

 the colloids influence the chemical composition of the soil water. 



In the present section we are concerned with soil primarily in its 

 physical nature as a substratum for land organisms. The foregoing 

 brief sketch of soil has indicated the extremely complicated nature 



