W'^ 



406 ACTION OF CARBONATE OF LIME UPON VEGETABLE MATTER. 



In answering this question, it is of importance to observe that all 

 the effects produced by alkahne matter in general — whether by lime 

 or by potash — in the caustic state, are produced in kind also by the 

 same substances in the state of carbonate. The carbonic acid with 

 which they are united is retained by a comparatively feeble affinity 

 and is displaced with greater or less ease by almost every other acid 

 compound which is produced in the soil. With this displacement is 

 connected an interesting series of beautiful reactions, which it is of 

 consequence to understand. 



You will recollect that the great end which nature, so to speak, has 

 in view, in all the changes to which she subjects organic matter in the 

 soil, is to convert it— with the exception of its nitrogen — into carbonic 

 acid and water. For this purpose it combines at one time, with the 

 oxygen of the air, while at another it decomposes water and unites with 

 the oxygen or the hydrogen which are liberated, or with both, to form 

 new chemical combinations. Each of these new combinations is either 

 immediately preliminary to or is attended by the conversion of a por- 

 tion to the elements of the organic matter into one or other of those 

 simpler forms of matter on which plants live. Now during these pre- 

 liminary or preparatory steps, acid substances, as I have already ex- 

 plained, are among others constantly produced. With these acids, the 

 carbonate of lime, when present in the soil, is ever ready to combine. 

 But in so combining, it gives off the carbonic acid with which it is al- 

 ready united, and thus a continual, slow evolution of carbonic acid is 

 kept up as long as any undecomposed carbonate remains in the soil. 



I do not attempt to specify by name the various acid substances 

 which are thus formed during the oxidation of the organic matter, and 

 which successively unite with the hme, because the entire series of 

 interesting and highly important changes, which organic substances 

 undergo in the soil, has as yet been too little investigated, to permit 

 us to do more than speak in general terms of the nature of the che- 

 mical compounds which are most abundantly produced. Of two facts, 

 however, in regard to them, we are certain — that they are simpler in 

 their constitution than the original organic matter itself^ from which 

 they are derived — and that they have a tendency to assume still 

 simpler forms, if they continue to be exposed to the same united action 

 of air, water, and alkaline substances. 



Hence the compounds which lime has formed with the acid sub- 

 stances of the soil, themselves hasten forward to new decompositions, 

 — unite with more oxygen, liberate sloAvly portion after portion of 

 their carbon in the form of carbonic acid, and of their hydrogen in the 

 form of water, till at length the lime itself is left again in the state 

 of carbonate, or in union with carbonic acid only. This residual car- 

 bonate begins again the same round of changes through which it had 

 pre v/:ously passed. It gives up its carbonic acid at the bidding of 

 some more powerful organic acid produced in its neighborhood, while 

 this acid, by exposure to the due influences, undergoes new altera- 

 tions till it also is finally resolved into carbonic acid and water. 



Two circumstances are deserving to be borne in mind in reference 

 to these successive decompositions— ^r^^, that in the course of them 

 more soluble compounds of lime are now and then formed, some of 



