TENACITY OF SOIL 23 



it is possible to calculate approximately the number of 

 particles contained in any given weight of soil, and to estimate 

 their total surface area ; this has been done by Whitney in the 

 case of the soils mentioned in Table II. The number of parti- 

 cles in one gram of dry soil, and their surface area were as follows. 



Number of Particles Surface Area, of 



in one gram. Particles. 



Soil 1 ... 1958 millions . . . 760 square centimetres 



2 ... 3955 ... 1008 



3 ... 6786 ... 1902 



,, ^ ... 10229 ... 2493 



5 ... 14736 ... 3593 



6 ... 19638 ... 4575 



The average diameter assumed for the particles of clay is 

 of course very uncertain. 



The above calculation assumes that all the ultimate particles 

 in the soil are free, and not arranged so as to form compound 

 particles, the existence of which would necessarily dimmish 

 both the number of particles and their available surface area. 

 The figures given above are thus probably maxima, which 

 are not actually reached in the respective instances. The 

 calculation, however, shows in a striking manner the character- 

 istic differences which exist between soils composed of coarse 

 and those composed of fine particles. 



The Tenacity of Soil. A fertile soil must possess a sufficient 

 solidity, one of its functions being to afford a firm support 

 to the plant, and enable it to withstand the force of wind and 

 rain. It must not. on the other hand, offer too great an 

 obstacle to the spread of roots. An open texture of soil is 

 especially needed during the early growth of a seedling plant. 

 A good arable soil should also allow of the easy use of tillage 

 implements, and should break down readily when they are 

 skilfully employed. 



