182 SOIL PHYSICS AND MANAGEMENT 



From the preceding table it will be seen that the total surface 

 area of the particles of a cubic foot of very fine soil is about four 

 acres, while a clayey loam contains an area of 1.23 acres per cubic foot 

 and a coarse sandy soil about one-fifth of an acre. Careful calcula- 

 tions show that a silt loam contains from 05,000 to 75,000 square feet 

 per cubic foot. Taking a soil with an internal surface of 70,000 

 square feet per cubic foot, the area of the particles in an acre to a 

 depth of one foot would be about 109 square miles, or three town- 

 ships. To a depth of four feet, from which plants take most of 

 their moisture, the total area of the soil particles in the silt loams 

 is not far from 436 square miles per acre, or twelve townships. 

 The capillary water is distributed over this surface. If the roots of 

 a corn plant did not pass beyond the middle of the rows, and no 

 more than four feet deep, each hill would have approximately 51.3 

 acres of soil particle surface from which to draw its supply of 

 moisture and food. 



The following table shows the variations in the internal surface 

 of some of the common soil types in the loessial areas of the middle 

 west: 



Internal Area of Soil Types, Calculated from Their Physical Composition 4 



11. Porosity of Soils. Porosity is the total amount of air 

 space in soils and is usually expressed in per cent of volume. It 

 depends upon the relation of solid particles to the interstitial space. 

 This is modified to a large extent by the size and shape of the par- 

 ticles, granulation, tillage and amount of organic matter. Porosity 

 or total pore space varies inversely as the size of the soil particles 

 and increases with their irregularity of form. For this reason 

 volcanic ash soils possess great porosity. Granulation and good 

 tilth increase the porosity of soils, and puddling diminishes it. 

 Tillage exerts a favorable influence on porosity, but sometimes in- 

 creases it to an injurious extent. Porosity varies directly as the 

 amount of organic matter. It is usually lessened by the rearrange- 



