402 UPON THE DECOMPOSITION OF ORGANIC MATTER. 



2^. In that of dry, and still undecomposed, vegetable mat ter, such 

 as straw. 



3°. In a more or less decayed or decaying state, generally black or 

 brown in colour — and often in some degree soluble in water. 



4°. In what is called the inert state, when spontaneous decay ceases 

 to be sensibly observed. And 



5°. In the state of chemical combination with the earthy substances 

 — with the alumina for example, and with the lime or magnesia — al- 

 ready existing in the soil. 



Upon these several varieties of organic matter lime acts with differ- 

 ent degrees of rapidity. 



II. — The final result of the decomposition of these several forms of 

 organic matter, when they contain no nitrogen, is their conversion into 

 carbonic acid and water only (Lee. VIII., §3). They pass, however, 

 through several intermediate stages before they reach this point — the 

 number and rapidity of which, 'and the kind of changes they undergo 

 at each stage, depend upon the circumstances under which the decom- 

 position is effected. Thus the substance may decompose — 



1°. Alo7ie, in which case the changes that occur proceed slowly, and 

 arise solely from a new arrangement of its own particles. This kind of 

 decomposition rarely occurs to any extent in the soil. 



2°. In the presence of water only. — This also seldom takes place in 

 the soil. Trees long buried in moist clays impervious to air exhibit the 

 kind of slow alteration which results from the presence of water alone. 

 In the bottoms of lakes, ditches, and boggy places also, from which in- 

 flammable gases arise, water is the principal cause ofthe more rapid 

 decomposition. 



3°. In the presence of air only. — In nature organic matter is never 

 placed in this condition, the air of our atmosphere being always largely 

 mixed with moisture. In dry air decomposition is exceedingly slow, 

 and the changes which dry organic substances undergo in it are often 

 scarcely perceptible. 



4°. In the presence of both water a7id air. — This is the almost uni- 

 versal condition of the organic matter in our fields and farm-yards. 

 The joint action of air and water, and the tendency ofthe elements of 

 the organic matter to enter into new combinations, cause new chem- 

 ical changes to succeed each other with much rapidity. It will of 

 course be understood that moderate Avarmth is necessary to the pro- 

 duction of these effects.* 



5°. In the presence of lime, or of some other alkaline substance (pot- 

 ash, soda, or magnesia). — Organic matter is often found in the soil in 

 such a state that the conjoined action of both air and water are unable 

 to hasten on its decomposition. A new chemical agency must then be 



' A familiar illustration of tlie conjoined efficacy of air and water in producing oxidation is 

 exhibited in their action upon iron. If a piece of polished iron be kept in perfectly dry air 

 it will not rust. Or if it be completely covered over with pure water in a well stoppered 

 bottle, from which air is excluded, it will remain bright and untarnished. Dut if a polished 

 rod of iron be put into an open vessel half full of water, so that one part of its lengih only 

 is under water— then the rod will begin very soon to rust at the surface of tlie water, and a 

 brown ochrey ring of oxide will form around it, exactly where the air and water meet. 

 From this point the rust will gradually spread upwards and downwards. So it is with tha 

 organic matter of the soil. Wherever the air and water meet, their decomposing action 

 upon it, in ordinary temperatures, soon becomes perceptible. 



