LECT. V.] THE HOOT. SOILS. 235 



injurious to plants ; as proved by the experiments 

 of Mr. Tennant. When, however, even the best 

 quicklime is too freely used, it becomes hurtful by 

 over-stimulating the growing plants ; and, there- 

 fore, the more frequent and small application of 

 it is preferable. 



It would be out of place in this Lecture to 

 notice all the substances used as manures; the 

 object of all of them is either to alter the re- 

 tentive quality of the soil, or immediately to 

 supply carbonaceous matter to the plants. For 

 these purposes, as occasion has required, clay, 

 brick rubbish, limestone, marl, chalk, sand, 

 gravel, have been employed as mechanical 

 means * ; salts of various kinds as stimulants ; 

 and soot, ashes, and dung, a* affording the pro- 

 per nutriment of plants. That salts are taken 

 up ready formed from the soil by vegetables is 

 pretty certain ; Du Hamel and Cadet having 

 established the fact, that, if the marine plants, 

 which yield soda while they grow near the sea, 

 be removed to inland situations, they gradually 

 cease to yield soda, and at length potash only 

 is obtained from their ashes -\-. We shall merely 



* Wet clay can be burnt to powder if put into the fire un- 

 compressed, and consequently prove useful as a manure. Me- 

 moirs of Lord Kames. Appendix, p. 224. 



f Even earths, although they cannot be converted into or- 

 ganized matter, yet, when in a state of extreme division, are 

 taken up by the roots of plants, and deposited in various parti 

 of the vegetable system. Thus silica is so abundant in the 



