118 



Physics of the Soil. 



in the table, then to find the total number in a gram or 

 pound it would be necessary to multiply those numbers 

 by the per cent of each size found in a gram of the soil 

 and add the several products. If the soil were made up 

 of 20 per cent, of each size in the table the number would 

 be as follows: 



140. Amount of Soil Surface Possessed by a Gram of Soil. 

 Much of the water retained by soils is held there in the 

 form of thin films surrounding the grains and the larger 

 this surface is the more water may be retained. So, too, 

 the solution of plant food from the grains takes place at 

 their surfaces and the larger the amount of surface the 

 more rapidly the solution may take place. 



The extent of soil-surface in a gram of soil can be found 

 by multiplying the number of grains by the surface of one 

 grain or by introducing Trd 2 into the equation of 

 thus: 



Weight X tfd 8 6 



d x a*. ,,. = a 



S011 sur(aoe 



expressed in square c. m. 



Using this formula to compute the surface in one gram 

 of soil grains having the sizes given in the table of (139) 

 the results below are obtained: 



Surface per gram Surface per pound 



