SOILS. 255 



quantity, it is poisonous to plants ; and hence such 

 soils as contain much sulphate of iron are invariably 

 barren, or incapable of supporting healthy vegeta- 

 tion. 



664. The saline matters in the soil are principally 

 the sulphates, muriates, nitrates, and phosphates of 

 the alkaline and earthy bases. The nature and 

 quantity of these substances vary considerably, but 

 they never constitute more than a very small portion 

 of the soil; their influence upon vegetation is, how- 

 ever, considerable, for certain plants seem to require 

 particular saline substances, and do not grow well in 

 soils not containing them. 



665. Thus all kinds of grass and corn contain silica 

 and phosphoric acid ; the former substance requires 

 the presence of an alkali, either potash or soda, to 

 render it soluble, and to enable it to enter the roots 

 when it is dissolved in water ; whilst the latter is also 

 always associated with a base, which is usually either 

 lime or magnesia. In the same way, all plants are 

 found to contain small quantities of similar saline and 

 earthy matters. 



666. The relative proportion of the different ingre- 

 dients of soils vary very greatly. The chief consti- 

 tuent of nearly all soils is generally silica, in the 

 form of sand ; and in fact there are comparatively 

 but few good fertile soils in which this is not the 

 case. The sandy matter of most silicious soils con- 

 sists of nearly pure silica, but in some cases it con- 

 tains alumina, lime, &c. (625). The following ana- 



