216 CONSERVATIVE ORGANS. [LECT. V. 



vegetables in a state of decay, animal matter and 

 a large proportion of salts, which are chiefly com- 

 mon salt, sulphates of magnesia and of potash, 

 nitrates of lime, and carbonates of potash and of 

 soda. 



Such are the earths generally contained in 

 soils : when any one of them abounds, the com- 

 pound earth is named after this component ; as 

 for instance, a calcareous soil, an argillaceous 

 soil, &c. 



The principal difference which characterizes 

 these various kinds of earths, is their power of 

 retaining the next component of soils, WATER. 

 Water, as forming a part of soils, is either che- 

 mically combined with the earth, or merely me- 

 chanically mixed with it, and retained in combi- 

 nation by cohesive attraction. In the former 

 state it is of no use to vegetables, in the latter it 

 is essentially necessary for their support. If the 

 soil be not sufficiently retentive, the plant is 

 starved, for nothing can be taken up from the 

 earth which is insoluble ; and, as we shall show 

 afterwards, water itself is a principal part of the 

 food of plants. If the soil be too stiff and re- 

 tentive, the water remains upon its surface, and 

 does not percolate to a sufficient depth to be ap- 

 plied to the roots : and if the vegetable be of a 

 succulent kind, the herbaceous part remaining 

 constantly surrounded with moisture has its vege- 



