PHYSICAL PROPERTIES OF SOILS 



The apparent specific gravity of soils taken in the field appears 

 to increase as we go downward. This is in part due to the 

 pressure of the overlying stratum ; in part to the action of rain 

 carrying the finest particles of the soil into the open spaces of the 

 subsoil ; in part to the loosening action of tillage and plant and 

 animal life on the surface soil and to the presence of their 

 residues. 



The weights of the soil per acre were studied 1 at Rothamsted 

 and Woburn, England, by driving down an iron frame 6 inches 

 square and 9 inches deep. The core of soil was removed, dried, 

 and weighed. 



WEIGHT OF SOIL PER ACRE. 



The Rothamsted soil is a heavy loam or clay subsoil beneath 

 which is the chalk. The Woburn soil is a light sand. 



Air Space in Soils. The quantity of air space in soils may be 

 calculated from the apparent and the real specific gravity. To 

 say that the soil has an apparent specific gravity of 1.20 means 

 that i cc. of the loose soil weighs 1.20 grams. If the soil material 

 has the real specific gravity of 2.5, then I cc. weighs 2.5 grams, 

 and i. 20 grams of it occupies 1.20 divided by 2.50 equals 0.48 cc. 

 Thus 0.52 cc. or 52 per cent., is air space in this particular 

 instance. 



Adhesion and Cohesion. Cohesion is the force with which the 

 soil particles adhere to one another. It varies with the amount 

 of water present and the nature of the soil, from zero in some 

 sands, to a high degree in some clays when dry. Soils with little 

 cohesion when dry are liable to be blown by winds unless pro- 

 tected by vegetation. The larger the particles of soil, the less is 



1 Warington, Physical Properties of Soil, p. 46. 



