PHYSICAL PROPERTIES OF SOILS 3 



up what is known as soil organic matter or, more often, soil humus. The 

 soil as a medium for the development of microorganisms is thus mark- 

 edly different from the common artificial laboratory media, whether 

 these be synthetic or consist of products of animal or plant life, upon 

 which these organisms are grown. 



The inorganic soil particles are surrounded by films of colloidal ma- 

 terials, which are both inorganic and organic in nature. As a rule, the 

 microorganisms inhabiting the soil adhere to these films, although some 

 move freely in the water surrounding the particles. Water and air play 

 essential roles in the soil system and control the nature and extent of the 

 soil population. The nature and size of the mineral and organic soil 

 fractions, as well as the phenomena of adsorption, also influence the 

 abundance, nature, and distribution of microorganisms in the soil. Sandy 

 soils are better aerated than heavy clay soils j they are, therefore, more 

 favorable for the growth of aerobic bacteria and fungi. However, since 

 such soils lack the high water-holding capacity of the heavier soils, they 

 are more readily subject to the process of drying out, which may result 

 in a reduction in microbial activities. 



Oxygen, another important factor in microbial development in the 

 soil, becomes available to microorganisms by gaseous diffusion. The 

 oxygen supply diminishes with increase in depth of the soil. When an 

 excess of free water is present in the soil, gaseous oxygen cannot pene- 

 trate very deeply and soil organisms then become dependent upon the 

 dissolved oxygen which diffuses into the soil solution. Since the rate of 

 oxygen diffusion is extremely slow, waterlogged soils tend to become 

 depleted of oxygen. Under these conditions, there is marked change in 

 the microbiological population of the soil : the fungi and actinomycetes 

 tend to decrease, and the bacteria, especially the anaerobic types, pre- 

 dominate. Peat bogs are examples of soils in a perpetual anaerobic state j 

 the microbial population is quite distinct from that of mineral soils. 

 Semiarid soils, with a much greater diffusion of oxygen into the deeper 

 soil layers, possess a population which is largely aerobic j in these and 

 other mineral soils the abundance and nature of the organic matter exert 

 a decided influence upon the abundance and nature of the microorgan- - ^ ^^^ 

 isms present. X \3\>» ^'^/^S^ 



