74 



NATUEE x\ND PEOPEETIES OF SOILS 



conversant with these various values, a mechanical analysis 

 should reveal at a glance certain soil conditions, which may- 

 or may not be conductive to the best plant growth. 



The sands and the gravel, because of their sizes, function 

 as separate particles. They are irregular and rounded, the 

 continual rubbing that they have received being sufficient in 



many cases to have ef- 

 faced their angular char- 

 acter. They exhibit very 

 low plasticity and cohe- 

 sion, and as a consequence 

 are little influenced by 

 changes in water content. 

 Their water-holding ca- 

 pacity is low, and because 

 of the large size of the 

 spaces between each sep- 

 arate particle the passage 

 of percolating water is 

 rapid. They, therefore, 

 facilitate drainage and 

 encourage good air move- 

 ment. In all the grades 

 of sand, the separate par- 

 ticles are visible to the 

 naked eye, a condition 

 impossible with the silt 

 and clay groups. Soil containing much sand or gravel, there- 

 fore, is of open character, possessing good drainage and 

 aeration, and is usually in a loose friable condition. 



The clay and silt particles are very minute, many of ^ the 

 former being so small as to be invisible under the ultra-micro- 

 scope. Both groups are really shreds and fragments of min- 

 erals often rather gelatinous in nature. The clay particles 

 are highly plastic and when kneaded with just the correct 



a 



Pig. 15. — Apparatus for making^ 

 mechanical analysis of soils. 

 Shaker-bottle (A), shaking-rack (B), 

 sieves (0), centrifuge (D) and cen- 

 trifuge-tube (S). 



