CHEMISTRY OF AGRICULTURE. 135 



These soluble or saline ingredients constitute the chief parts of the 

 plant which are left after burning it. If but a single grain of this 

 saline matter be found in a pound of soil a foot deep, it is equal to 500 

 Ibs. to the acre, which is thought to be more than is taken from the 

 soil by the rotation of crops during 40 years, where the straw and 

 herbage is uniformly returned to the soil in a state of manure. The 

 soil in some places forms an incrustation on the surface, especially 

 when the sub-soil contains much water. Water evaporated from the 

 soil leaves behind the salts which it always contains, and is the cause 

 of the saline incrustation. This, falling with the rain and descending 

 to the sub-soil, deposits its salts when evaporated. Hence the surface 

 is better supplied with them in dry weather, early in the season, for 

 the growth of young plants, and when the roots have penetrated fur- 

 ther, they bring it up from the sub-soil by decomposing the water. 



Insoluble or earthy parts form, generally, about 90 in the 100 parts 

 by weight, of soils. These are principally silica, or sand, alumina 

 or clay, and lime, or carbonate of lime, with 1 or 2 per cent, of oxide 

 of iron, or even much more in red soils. Magnesia and phosphate 

 of lime are also found. Plants are classified according to the quan- 

 tities of these substances they contain. The proportions of sand and 

 clay may be found by boiling, say an ounce of the soil in a pint of 

 water, till it is quite softened and diffused, then shaking it and allow- 

 ing it to stand for a few minutes, when the sand will subside while 

 the finer clay will float, and it may then be turned off into another 

 vessel. This, when perfectly clear, may be turned off, and thus the 

 sand will be in one and the clay in the other vessel, and they may 

 then be dried and weighed. If a 100 parts of the soil contain not 

 over 10 per cent, of clay, it is a sandy soil ; if from 10 to 40 it is a 

 sandy loam ; if from 70 to 85, a clay loam, and if from 85 to 95 it is 

 a strong day soil ; and if no sand is found, it is a pure agricultural 

 clay. Strong clay soils are used to make bricks, and agricultural clay 

 is pipe-clay, or used in making pipes, etc. Pure clay consists of 

 about 60 parts of silica to 40 alumina, chemically combined. Arable 

 soils rarely contain over 35 per cent, of alumina. Soils containing 

 but from 5 to 15 per cent, of sand, cannot well be brought into a state 

 of cultivation. 



Soils containing more than 5 per cent, of carbonate of lime are 

 called a marl ; if more than 20 per cent., a calcarious soil. Soils in 

 which vegetable matter greatly predominates are peaty soils. The 

 quantity of carbonate of lime in a soil is estimated by burning the 

 soil to redness in the air, and weighing, say 100 grains, stirring it well 

 in half a pint of cold water, mixed with half a wine-glass full of mu- 

 riatic acid, or spirit of salt, allowing it to stand for 2 hours, occasion- 

 ally stirring it then pour off the water, dry the soil, heated again to 

 redness, and weigh the product, when the loss will be the amount of 



