[ 46 ] 



and impalpable matter. Soils which have the physical pro- 

 perty of clay may contain no clay in the chemical sense i.e., 

 silicate of aluminum. The composition of chemically pure 

 clay may be represented by the formula A1 2 O 3 .2 SiO 2 .2H 2 O. 



48. A more rough and ready method of mechanical analysis 

 consists in taking an ounce of soil, mixing it up with a pint 

 of water, leaving it in the water for 24 hours, then shaking it 

 up and allowing the heavier particles to settle for 5 minutes. 

 The supernatant liquid can then be poured into another 

 vessel which may be allowed to stand for another 24 hours. 

 The sandy part will be seen settled in one vessel and the 

 clayey part in the other. These may be dried and weighed 

 separately. 



49. If 100 grains of dry soil, not peaty or unusually rich 

 in vegetable matter, leave no more than 10 of clay treated in 

 this manner, it is called sandy soil ; if from 10 to 40 sandy 

 loam ; if from 40 to 70 a loamy soil ; if from 70 to 85, a clay 

 loam ; from, 85 to 95, a strong clay soil ; and when no sand 

 is separated at all by this process, it is a pure agricultural 

 clay. This pure clay contains silica and alumina in the pro- 

 portion of about 60 of the former to 40 of the latter but the 

 composition of agricultural clay is very uncertain. It rarely 

 happens, however, that arable land should contain more than 

 3 to 35 P er cent of alumina. Soil containing more than 5% 

 of carbonate of lime is called marl } and more than 2O/ > calca- 

 reous soil. Peaty soils contain more than 5/ of humus or 

 vegetable mould. Ferrugenous soils contain over 5% of 

 iron. 



50. For practical purposes, however, the systems of classi- 

 fication of soils in vogue in Bengal and in the other Presidencies 

 are numerous. They are based on various fundamenta 

 divisiones. Land is classified, for instance, as irrigated, 

 irrigable and non-irrigable ; also as ek-phashli and do-phashlt 

 or single-cropped and double-cropped ; also as cultivated, 

 culturable and non-culturable. The cultivated land may be 



