INFLUENCE OF ALKALINE SUBSTANCES. 403 



introduced, by which the elements ofthe organic mattermay again be 

 set in motion. Lime is the agent which for this purpose is most large- 

 ly employed in practical agriculture. 



§ 25. General action of alkaline substances upon organic matter. 



It is this action of alkaline matters upon the organic substances of the 

 soil in the presence of air and water that we are principally to investigate. 



When organic matter undergoes decay in the presence of air and 

 water only, it first rots, as it is called, and blackens, giving off water 

 or its elements chiefly, and forming humus — a mixture of humic, ulmic, 

 and some other acids, (Lee. XIII., § 1.) with decaying vegetable fibre. 

 It tlien commences, at the expense of the oxygen of the air and of 

 water, to form other more soluble acids (malic, acetic, lactic, crenic, 

 mudesic, &c.;) among which is a portion of carbonic — and, by the aid 

 of the hydrogen of the water which it decomposes, one or more of 

 the many compounds of carbon and hydrogen, which often rise up, 

 as the marsh-gas does, and escape into the air, (Lee. VIII., § 3.) 



Thus there is a tendency towards the accumulation of acid substances 

 of vegetable origin in the soil, and this is more especially the case when 

 t'le soil is moist, and where much vegetable matter abounds. The effect 

 of this saper-abundance of acid matter is, on the one hand, to arrest the 

 further natural decay of the organic matter, and, on the other, to render 

 the soil unfavorable to the healthy growth of young or tender plants. 



The general effect of the presence of alkaline substances in the soil 

 is to counteract these two evils. They combine with and thus remove 

 the sourness of the acid bodies as they are formed. In consequence of 

 tliis the soil becomes sweeter or more propitious to vegetation, while the 

 natural tendency of the vegetable matter to decay is no longer arrested. 



It is thus clear that an immediate good effect upon the land must fol- 

 low either from the artificial application or from the natural presence of 

 alicaline matter in the soil — while at the same time it will cause the 

 vegetable matter to disappear more rapidly than would otherwise be 

 the case. But the effect of such substances does not end here. They 

 actually dispose or provoke — pre-dispose0\\Gm\s,i& call it — the vegeta- 

 ble matter to continue forming acid substances, in order that they may 

 combme with them, and thus cause the organic matters to disappear 

 more rapidly than they otherwise would do — in other words, they 

 hasten forward the exhaustion of the vegetable matter of the soil. 



Such is the general action of all alkaline substances. This action 

 they exhibit even in close vessels. Thus a solution of grape sugar, 

 mixed with potash, and left in a warm place, slowly forms melassic 

 acid — while in cold lime-water the same sugar is gradually converted 

 into another acid called the glucic. But in the air other acids arc 

 formed in the same mixtures, and the changes proceed more rapidly. 

 Such is the case also in the soil, where the elements of the air and 

 of water are generally at hand to favor the decomposition. 



But the nature of the alkaline matter which is present determines 

 also the rapidity with which such changes are produced. The most 

 powerful alkaline substances — potash and soda — produce all the above 

 effects most quickly ; lime and magnesia are next in order ; and the 

 alumina of the clay soils, though much inferior to all of these, is far 

 from being without an important influence. 



