Envir:3 



of pore space distribution. On the vertical axis is expressed pore 

 space per 100 j^rams of soil. In the fine sandy loam, only a small por- 

 tion of the total pore space is less than 0.02 microns in diameter and 

 a large portion is greater than 20 microns in diameter. In the clays, 

 a large portion of the pore space is very sr.all in diameter. In many 

 of the clays, the total volume of pore space of large-sized pores will 

 be small. Thus, there are all gradations of the sizes of pore spaces. 



The size of the pore spaces and the other physical properties accompany- 

 ing pore size distribution have a lot to do with the kinds and types of 

 organisms that live in the soil and the environment they find to live in. 

 Let us consider the environment these microorganisms encounter in the 

 soil. Consider the water of the environment first. The pores of the 

 soils may fill up with water and then, as the water subsides, the pore 

 spaces fill with air. As water is added to the soil, the first result 

 is absorbed xrater as films on the soil particles. The first increments 

 of water that are added are absorbed very strongly. Succeeding incre- 

 ments of water are held less tightly to the soil particles, and, as 

 water is added, the surface films begin to fill the small capillaries. 

 ,-is more water is added, successively larr^-er pore spaces become filled 

 ■lith water vintil, perhaps, only a few of the larger pore spaces remain 

 unfilled. 



The soil physicist characterizes the water in the soil differently than 

 stating the total quantity of water present. He is probably more inter- 

 ested in the forces that hold water in the soil and finds a nmiber of 

 systems of measuring these forces. Physicists may characterize the 

 situation by detennining the amount of tension it tal<es to pull the 

 water out of the soil, and they may use a number of methods of ejrpress- 

 ing that valv^. They may use "atmospheres tension" which is a unit of 

 force expressed in centimeters of height of a column of water. Or, 

 they may use another term, "pF, " which is the logarithm of this force 

 in centimeters of water. Or, still another term is used here as tliis 

 force is related to the relative humidity of the soil air ^^rithin the 

 pore spaces. 



Figure 3 shows the relationships between several of these measures of 

 the force that holds water in the soils. The first column gives the 

 appearance of the soil. When the soil is drj^, the tensions are very 

 high; and when the soil is noist, the tensions may go nearly to zero. 

 The second column shows tensions in centimeters of water. The third 

 column is the log of this tension; and so on. The column for "pore 

 size filled" refers to the largest diameters of pore sizes which are 

 filled at particular tensions. The normal range of water in the soil 

 at which plants will grow lies somewhere between the wilting range and 

 what is called "field capacity" dovm to about a tension of 100 aa. of 

 water. 



You x-ri.ll note the pore sizes that remain filled with water. In general, 

 soils do not remain at field capacity very longj so you see, there are 

 not many large pores filled vriLth water to permit the microorganisms to 

 svjira freely about all the tijiie. Host likely, they are restricted to a 



