184 



SOIL PHYSICS AND MANAGEMENT 



with columnar arrangement there will be 625 pores per square inch 

 and with clay 625,000,000. The former will be one million times 

 as large as the latter. The large size of the pores permits certain 

 physical processes, such as percolation and aeration, to take place 

 so readily that they may be detrimental to the crop. On the other 

 hand, fine-grained soils, as clays and clay loams, with their very 

 minute pores, but large total pore space, may so retard percolation 

 and aeration as to be equally detrimental to the crop. Medium- 

 grained soils, as silt loamls or fine sandy loams possessing an inter- 

 mediate pore space, fere best suited to most crops, although both the 

 coarse and very fine soils have their advantages under certain con- 

 ditions. The size of the pores in fine-grained soils is increased by 

 granulation. 



The following table shows the porosity of different types of soils : 



Porosity in Soils of Varied Physical Composition * 



The total pore space of soils is rarely less than 30 per cent. 

 Coarse, clean sand has about this amount. This means that a cubic 

 foot of such soil will contain 70 per cent of solid material. In case 

 of sandy loams and silt loams, the pore space amounts to about 50 

 per cent, or half of the volume of the soil, or, in a cubic foot, there 

 are approximately 862 cubic inches of air space. The increase in 

 porosity, within certain limits, is beneficial because of the increase 

 in aeration. Extremely large air spaces in soils are detrimental 

 because they permit of excessive evaporation. 



QUESTIONS 



1. What is specific gravity? 



2. Give specific gravity of a few common soil-forming minerals. 



3. What is the apparent specific gravity? 



4. How is it determined? 



5. How does porosity affect it? Tilth? 



6. How is the weight of a cubic foot of soil determined? 



