Humus and the Colloidal Complex 79 



the deciduous forest helps to produce an entirely different soil profile 

 from that found under coniferous trees. Because of the lack of the 

 earthworm population in the typical coniferous forest as well as the 

 slower rate of decay the fallen needles tend to accumulate year after 

 year as successive layers on the surface of the groimd. 



We have seen that living agents aid in the breakdown of the parent 

 rock material, take part in the vertical mixing and the horizontal dis- 

 tribution of soil substances, and add organic matter to the soil. The 

 by-products of animals and plants, and their own bodies when they 

 die, are the only source of organic compounds for the soil. These 

 organic substances provide necessary soil components, modify the 

 soil into many different types, and make possible the growth of a 

 varied fauna and flora that would not otherwise be able to exist. 

 The organic matter contributed to the soil contains a larger amount 

 of energy than the inorganic substances from which they were formed. 

 Living organisms therefore provide both potential and kinetic energy 

 as well as materials in helping to build the soil. 



Humus and the Colloidal Complex. Soils are far more than piles 

 of material derived from rocks and biological sources. Soils have 

 organization. Structure, layering, and other aspects of soil arrange- 

 ment are influenced by the animals and plants present, and, recipro- 

 cally, the activities of the soil organisms are frequently controlled by 

 the organization of the soil material. One way in which animal life 

 and vegetation affect soil organization is through their contribution 

 to the colloidal complex. The partially decomposed organic matter 

 added to the soil by living components is known as Inimus, and this 

 material combines with the finest clay particles to form the colloidal 

 complex. Often referred to as the 'lieart and soul" of the soil, the 

 colloidal complex plays many essential roles in its dynamic activity 

 (Waksman, 1936). In the first place the presence of colloids de- 

 rived from humus and other sources influences the water-holding 

 capacity of the soil and the rates at which air and ground water can 

 circulate through it. Water tends to move too freely through sandv 

 soil. The addition of humus to such soil tends to bind the grains to- 

 gether, to reduce pore size, and to increase the amount of water held. 

 A contrasting situation is found in soil that is too dense because of 

 an excessive amount of clay. Increasing the amount of humus present 

 has the effect here of separating the soil material into clumps, and 

 thus allowing better aeration, increased percolation, and easier root 

 penetration. In both instances the structure of the soil has been im- 

 proved. 



The colloidal complex also acts as a source of plant nutrients and 



