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ORGANIC SUBSTANCES IN SOILS.

decaying they present a constant source of carbonicacid, and likewise of ammonia, if they contain nitrogen;whilst, by rendering the soil open and porous, and atthe same time augmenting its absorbent powers, theyassist in keeping it uniformly moist. They also increasethe means which the soil possesses of absorbing andcondensing ammonia and carbonic acid from the air.

312. It is a remarkable fact, that a mixture of twoor three earths, such as lime, silica, and alumina, is bet-ter fitted to absorb moisture and gaseous matter, thaneither of the earths taken singly; and in the same way,the addition of organic matters to the soil has the effectof increasing this power still further ; and, therefore, aswas before said, the best soils are those containing aquantity of each of the three earths, mixed with amoderate proportion of decaying organic matters.

313. Some soils contain a very large quantity oforganic substances, and indeed not unfrequently consistalmost wholly of the remains of vegetables : the differentvarieties of peat are of this character. In such soils weobserve the had effects of the presence of too much or-ganic matter in a soil. It is thus rendered tough, porousand spongy, so that it is soon saturated with water, orcompletely dried, by changes of weather. Besides allthis, it frequently contains a peculiar acid analogousto humic acid, produced during the decay of vegetablematter, which is hurtful to the growth of plants.

314. Another use of the organic constituents of thesoil is, that they furnish to the plants growing on it the