SAND. 49 



The constituents of a soil are popularly divided into four 

 groups, an arrangement which, though perhaps not scientific, 

 is often convenient. 



These proximate constituents are : 



(i.) Sand, consisting mainly of silica, but containing small 



fragments of felspar, mica, or even of limestone, 

 (ii.) Clay, mainly kaolin, but also containing finely divided 



felspar, &c. 



(iii.) Finely divided limestone, 

 (iv.) Humus, the somewhat indefinite product resulting 



from the decay of organic matter in the soil. 



Sand, from a chemical standpoint, is one of the least impor- 

 tant constituents of a soil, furnishing as it does only very little 

 of the food of plants. In fact, the essential constituents, the 

 grains of quartz, are probably of no value whatever as plant 

 food. The little plant food " sand " does provide is furnished 

 by the fragments of felspar, rnica, or calcium carbonate which 

 may be present in it. From a practical point of view, how- 

 ever, sand is of the greatest value, because of the effect of its 

 presence upon the physical texture of the soil. The impor- 

 tance of the physical condition of a soil is apt to be overlooked 

 by a student of chemistry, but is often even greater than that 

 of the chemical composition. Eecently much attention has 

 been paid to a study of the physical properties of soils, and 

 valuable information is rapidly being accumulated, especially 

 in America and Germany. * 



The terms "light" and " heavy" as applied to soils possess 

 a meaning well known to agriculturists, referring to the ease 

 of working and really depending upon the tenacity or cohesion 

 and not upon relative weight. As a matter of fact, the specific 

 gravities of " light" and " heavy" soils are just the opposite to 

 what the terms would seem to imply, the specific gravity of 

 quartz being 2 -62, that of clay 2*50, that of humus 1-3. How- 

 ever, the numbers quoted are the true specific gravities, i.e., 

 they represent the weights of a volume of solid quartz or solid 

 clay compared to that of an equal volume of water. The 



vol V : Indlffng ' "S?ISl ' iCal Pr perties of 8oils " ; Wile y> " Agricultural Analysis," 



