SUMMARY OF THE CHANGES PRODUCED BY LIME. 407 



which are washed out by the rains, and escape from the soil, while 

 others minister to the growth of plants ; — and second, that very much 

 carbonic acid is produced as their final result — of which also part is 

 taken up by the roots of plants, and part escapes into the air. Thus 

 at every successive stage a portion of organic matter is lost to the 

 soil. If this quantity be greater than that which is yearly gained in 

 the form of roots or decayed leaves and stems of plants, or of manure 

 artificially added, the soil will be gradually exhausted — if less, it will 

 every year become more rich in vegetable matter. 



It is also to be borne in mind, that although, for the purpose of il- 

 lustration, I have supposea the carbonate of lime first formed in the 

 soil to be subsequently combined with other acids, which gradually 

 decompose and leave it again in the state of carbonate, — yet it will 

 rarely happen that the whole of the carbonate of lime in the soil 

 will be in any of these new states of combination. In general, a part 

 of it only is thus at any one time employed in working up the acid 

 substances produced. But it is necessary that it should be univer- 

 sally diffused through the soil in order that it may be everywhere at 

 hand to perform the important part of its functions above explained. 

 It is only where little lime is present, or where decaying vegetable 

 matter is in exceeding abundance, that the whole of the carbonate 

 can at one and the same time disappear (p. 380.) 



The changes, therefore, which lime and organic matter, supposed 

 to be free from nitrogen, respectively undergo, and their mutual ac- 

 tion in the soil, may be summed up as follows : — 



1°. The organic matter, under the influence of air and moisture, 

 spontaneously decomposes, and besides carbonic acid which escapes, 

 forms also other acid substances which linger in the soil. 



2°. With these acids the quick-lime combines, and, either by its 

 union with them or with carbonic acid from the air, soon (compara- 

 ratively) loses its caustic state. 



3 -•. The production of acid substances by the oxidation of the organ- 

 ic matter — goes on more rapidly under the disposing influence of the 

 lime, whether caustic or carbonated. These acids combine with the 

 lime, liberating from it, when in the state of carbonate, a slow but 

 constant current of carbonic acid, upon which plants at least partly 

 live. 



4°. The organic acid matter which thus unites with the lime con- 

 tinues itself to be acted upon by the air and water, aided by heat and 

 light — itself passes through a succession of stages of decomposition, 

 at each of which it gives off water or carbonic acid, retaining still 

 its hold of the lime, till at last being wholly decomposed it leaves the 

 lime again in the state of carbonate, ready to begin anew the same 

 round of change. 



Durmg this series of progressive decompositions, certain more so- 

 luble compounds of lime are formed, by which plants are in part at 

 least supplied with this earth, and which with the aid of the rains 

 carry off both lime and organic matter from the soil. 



And, again, the more rapid the production of the acid substances 



