400 ACTION OF LIME UPON THE SOIL, AND AS THE FOOD OF PLAN a k.. 



The ultimate resolution of all vegetable matter in the soil into carbo- 

 nic acid and water (Lee. VIII., § 3,) likewise aids the removal of the 

 lime. For if the soil be everywhere impregnated with carbonic acid, 

 the rain and spring waters that flow through it will also become charg- 

 ed with this gas, and thus be enabled to dissolve a larger portion of the 

 carbonate of lime than they could otherwise do. Thus theory indi- 

 cates, what I believe experience confirms, that a given quantity of lime 

 will disappear the sooner from»a field, the more abundant the animal 

 and vegetable matter it contains. 



§21. Theory of the action of lime. 



Lime acts in two ways upon the soil. It produces a mechanical al- 

 teration which is simple and easily understood, and is the cause of a 

 series o^ chemical changes, which are really obscure, and are as yet 

 susceptible of only partial explanation. 



In the finely divided state of quick-lime, of slaked lime, or of soft 

 and crumbling chalk, it stiffens very loose soils, and opens the stifTer 

 clays, — while in the form of hmestone gravel or of shell-sand, it may 

 be employed either for opening a clay soil or for giving body and firm- 

 ness to boggy land. These effects, and their explanation, are so obvi- 

 ous to you, that it is unnecessary to dwell upon them. 



The purposes served by lime as a chemical constituent of the soil are 

 at least of four distinct kinds. 



1°. It supplies a kind of inorganic food which appears to be necessa- 

 ry to the healthy growth of all our cultivated plants. 



2°. It neutralizes acid substances which are naturally formed in the 

 soil, and decomposes or renders harmless other noxious compounds 

 which are not unfrequently within reach of the roots of plants. 



3°. It changes the inert vegetable matter in the soil, so as gradual- 

 ly to render it useful to vegetation. 



4°. It causes, facilitates, or enables other useful compounds, both 

 organic and inorganic, to be produced in the soil, — or so promotes 

 the decomposition of existing compounds as to prepare them more 

 speedily for entering into the circulation of plants. 



These several modes of action it will be necessary to illustrate in 

 eome detail. 



§ 22. Of lime as the food of plants. 



In considering the chemical nature of the ash of plants (Lee. X., 

 § 3 and 4), we have seen that lime in all cases forms a considerable 

 proportion of its whole weight. Hence the reason why lime is re- 

 garded as a necessary food of plants, and hence also one cause of its 

 beneficial influence in general agricultural practice. 



The quantity of pure lime contained in the crops produced upon one 

 acre during a four years' rotation amounts, on an average, to 242 lbs. 

 which are equal to about 430 lbs. (say 4 cwt. ) of carbonate of lime, m 

 the state of marl, shell-sand, or hme-stone gravel. (See Lee. X., § 3.) 

 It is obvious, therefore, that one of the most intelligible purposes served 

 by lime, as a chemical constituent of the soil, is to supply this compara- 

 tively large quantity of lime, which in some form or other must enter 

 into the roots of plants. 



