SOIL E1WIR0»',^;NT AW\ IlICROBIOLOOKmT. ACTIVITY 

 iJ. V. Bartholomew 



It has been announced that you would be meeting someone to properly 

 introduce you to the subject of soil organic matter and physical factors 

 in the soil. With your permission, I would like to deal particularly 

 with the factors in the soil environment that influence microbiolofical 

 activity. I want to admit that there is a dearth of information, but 

 there is some which may be of interest. 



To begin, I want to say that the soil properties and the soil environ- 

 ment determine to a large extent the kinds of organisms which will 

 inhabit the soil and also, in turn, the activities of these organisms. 

 I would like to use some very simple illustrations in explaining some 

 of the things occurring in the soil." 



The soils are a mixture of things: solid materials, liquids, and gases. 

 The solids are very often of a mineral natiire. One also finds all kinds 

 and shapes of pore spaces, and it is chiefly the pore spaces in which we 

 shall be interested. Some pores are large; some, verj^ small. The kind 

 of pore space is mainly detennined by the kind of solid particles pres- 

 ent. If there are large, solid particles like sandy materials, in general, 

 the pore spaces will be of the larger kind. In clay soils, where the 

 solid particles are very small, there is a different kind of pore space 

 distribution and a different type of space in which the microorganisms 

 live. The solid particles have a density, in general, of about 2.6-2.75, 

 and \je may presume there are deviations on both ends of the scale. The 

 soil unit density per unit volume is a good deal less. Table 1 illus- 

 trates some of the ranges in bulk density of the soil. There is not 

 much deviation in the specific gravity of the solid materials; they are 

 usually limited to quite a narrow range. The differences one finds in 

 the bulk density, or the weight per unit volume of soil, arises chiefly 

 from the differences in packing of the soil particles. 



You will note that the bulk density of soils range in this table all the 

 way from the neighborhood of 1 (one gram per cc.) up to perhaps 1.7. 

 This last value would be for a rather dense soil. VJhat all this means 

 is that you have a volume of soil with about S0% of it being made of 

 solid particles and another S0% being space. When the soil is drj--, this 

 pore space would be filled with air. I-Jhen the soil is wet, the spaces 

 would be filled to various degrees with water. As the moisture fluctu- 

 ates in the soil, these pores are filled and drained again. 



■}!• Editor's note: Because of the large number of figures, they are 

 printed as a group at the end of Dr. Bartholomew's talk. Tables are 

 presented in the body of the text. 



