22 CHAPTER III 



The finor the texture the narrower is this zone, i.e., the 

 shorter the period during which good ploughing can be done. 

 Granulation lowers cohesion and plasticity, thereby widening 

 the moisture zone for proper ploughing. It follows then that 

 the promotion of granulation tends not only to eliminate the 

 danger of bad structural relationships, but also to bring the 

 soil into condition for easier and more convenient tillage." 



Soil Class. — Soils are composed essentially of varying 

 proportions of sand, silt and clay, together with more or less 

 decayed and decaying organic matter. The leading types "of 

 farm soils are roughly classified according to the relative 

 proportions of these constituents present. The chief physical 

 difference between sand, silt and clay is in regard to size of 

 the particles. Thus, we may arbitrarily designate as sand all 

 particles the diametres of which lie betw^een *04 — 1 mm ; as 

 silt, those between 'OO^ — '04 mm ; and clay those smaller than 

 •002 mm. 



The clay properties are of great importance to fertility 

 from both chemical and physical points of view. No soil con- 

 stituent is more necessary in proper proportion ; at the same 

 time, however, no constituent is more harmful in excess. For 

 practical purposes the sand properties may be regarded simply 

 as the negatives of those which characterise clay, while silt 

 occupies an intermediary position. The percentage of clay 

 present is very generally the dominant consideration in deter- 

 mining the class of a soil. For our present purpose the follow- 

 ing classification of soils, based on their relative content of 

 the three " separates " as defined above will serve : — 



(a) Coarse sands — consisting almost entirely of sand alone. 

 (h) Sandy soils — about 75 per cent, or more sand and a 

 small amount of silt, but very little clay. 



(c) Sandy loams — about 50 to 75 per cent, sand, a fair 



amount of silt and about 10 per cent. clay. 



(d) Loams — about 40 to 50 per cent, sand, 10 to 15 per 



cent, clay, and the rest silt. 



(e) Clay loams — about 15 to 25 per cent. ; the rest sand 



and silt. 

 (/) Clays — about 25 to 35 per cent, clay ; the rest sand 



and silt. 

 (fj) Heavy clays — over 35 per cent, clay ; the rest mainly 



silt. 

 (h) Silt loams — about 50 to 75 per cent, silt ; 10 to 15 



per cent, clay ; the rest sand. 



