124 



Physics of the Soil. 



145. The Effective Diameters of Soil Grains and the 

 Amount of Surface Computed From Them. We have no 



means of knowing yet how accurately the computed sur- 

 face of soil grains in a given weight of sample compares 

 with that which is possessed by it. We do know, however, 

 that the comparison is accurate enough to furnish a valua- 

 ble basis for comparing different types of soils, and in the 

 table which follows is given the effective diameters of sev- 

 eral kinds of soils, together with the pore space and the 

 computed amount of soil surface per cubic foot of dry soil. 



Table of computed surface of soil grains in different types 



of soil. 



It will be seen from this table that the amount of surface 

 in the true soils is indeed very great, ranging from a little 

 less than a quarter to more than a third of an acre in the 

 sandy soils, through more than an acre in the loams to as 

 much as four acres per cubic foot in the finest clay soils. 

 The amount of soil surface in the upper four feet of every 

 cultivated field ranges from not less than one acre to more 

 than 16 acres per each square foot of surface cultivated. 



146. Relation of the Surface of Soil Grains to the Water 

 Capacity. A large portion of the water held by a soil is 

 spread out as a thin film surrounding the soil grains and it 



