254 SOILS. 



soil except in combination with some acid, and this 

 is most commonly the carbonic; the former substance 

 is also not unfrequentlj found combined with sul- 

 phuric acid, constituting gypsum, or sulphate of lime 

 (237). 



661. The oxides of iron in the soil are usually 

 nncombined, as they have not sufficient attraction for 

 carbonic acid to combine with that gas, which is 

 always present in the air. They in great measure 

 occasion the variations of color observed among 

 soils; for according as the iron is in a state of prot- 

 oxide or peroxide, it gives to the soil a black or 

 brownish-red color. 



662. It must not be supposed, however, that the 

 color of soils is wholly dependent on the iron which 

 they contain, or that the blackness of any particular 

 soil is indicative of the presence of oxide of iron. 

 The decomposing vegetable substances, which all 

 soils contain in greater or less quantity, are usually 

 of a brown or black color, and therefore not unfre- 

 quently give a very dark color to a soil which only 

 contains a very small portion of oxide of iron. 



663. Sulphate of iron is also sometimes present in 

 soils in very small quantity, being formed by the 

 gradual oxidation of sulphuret of iron in the manner 

 previously mentioned (286). A very minute quantity 

 of this salt of iron confers upon the soil peculiar 

 properties, which render it appropriate for particular 

 plants; but a slight increase of its amount is at- 

 tended with bad results, for, unless in very minute 



