60 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



contain complex silicates of aluminium, iron, potassium, sodium, 

 lime, etc. The tendency of weathering is to reduce these to 

 simple compounds, such as silica, hydrated oxides of iron, 

 hydrated silicates of aluminium, carbonates or sulphates of lime 

 and magnesia, chlorides of sodium and potassium, and sili- 

 cates of magnesia. The complex silicates are not changed directly 

 to these simple bodies, but various intermediate products are 

 formed. A long period is required for this process to become 

 complete, so that in many soils all stages of the change may be 

 present, from particles of the original minerals, through various 

 hydrated silicates derived therefrom, down to the simpler com- 

 pounds. The conditions under which the weathering occurs 

 determine the degree of decomposition. If the weathering agen- 

 cies are chiefly mechanical, the rock may be reduced to a powder 

 with little chemical change. 



Loss of Material by Weathering. In every case of weathering, 

 a greater or less portion of the constituents of the rock have been 

 carried away. An estimate of the loss may be made where the 

 soil rests directly upon the rock from which it is derived. 

 Samples of the soil, and of the unchanged rock beneath it, are 

 subjected to analysis. We assume one ingredient of the rock 

 has lost nothing in weathering, and calculate the quantity of the 

 original rock containing the amount of this ingredient found in 

 100 parts of the soil. This gives us the quantity of original rock 

 from which 100 parts of soil was secured. When the composition 

 of both is known, it is a simple matter to calculate the loss of each 

 ingredient. 



Suppose, for example, the original rock contained 30 per 

 cent, alumina, and the weathered product contains 45 per cent, 

 alumina. It being assumed that no loss of alumina took place, 

 150 pounds of the original rock would contain 45 pounds of 

 alumina; that is, 150 pounds has weathered to 100 pounds. If 

 the original rock contained 2 per cent, magnesia, and the weath- 

 ered product 0.5 per cent., then 150 pounds contained three 

 pounds, and we have 0.5 pound left, giving a loss of five-sixths 

 or 83^ per cent, of magnesia. Two assumptions are made in 

 this procedure ; one being that some constituent has not been lost 



