The soil is a storehouse of food for crops; the stores it contains are, however, only partly available for immediate use. In fact, by far the larger share is locked up, as it were, in insoluble combinations, and only by a slow and gradual change can it become accessible to the plant. This change is largely brought about by the united action of _water_ and _carbonic acid gas_. Nearly all the rocks and minerals out of which fertile soils are formed,--which therefore contain those inorganic matters that are essential to vegetable growth,--though very slowly acted on by pure water, are decomposed and dissolved to a much greater extent by water, charged with carbonic acid gas. It is by these solvents that the formation of soil from broken rocks is to a great extent due. Clay is invariably a result of their direct action upon rocks. The efficiency of the soil depends greatly upon their chemical influence. _The only abundant source of carbonic acid in the soil, is decaying vegetable matter._ Hungry, leachy soils, from their deficiency of vegetable matter and of moisture, do not adequately yield their own native resources to the support of crops, because the conditions for converting their fixed into floating capital are wanting. Such soils dressed with peat or green manured, at once acquire the power of retaining water, and keep that water ever charged with carbonic acid: thus not only the extraneous manures which the farmer applies are fully economized; but the soil becomes more productive from its own stores of fertility which now begin to be unlocked and available. Dr. Peters, of Saxony, has made some instructive experiments that are here in point. He filled several large glass jars, (2-1/2 feet high and 5-1/2 inches wide) with a rather poor loamy sand, containing considerable humus, and planted in each one, June 14, 1857, an equal number of seeds of oats and peas. Jar No. 2 had daily passed into it through a tube, adapted to the bottom, about 3-1/4 pints of common air. No. 3 received daily the same bulk of a mixture of air and carbonic acid gas, of which the latter amounted to one-fourth. No. 1 remained without any treatment of this kind, _i. e._: in just the condition of the soil in an open field, having no air in its pores, save that penetrating it from the atmosphere. On October 3, the plants were removed from the soil, and after drying at the boiling point of water, were weighed. The crops from the pots into which air and carbonic acid were daily forced, were about _twice as heavy_ as No. 1, which remained in the ordinary condition. Examination of the soil further demonstrated, that in the last two soils, a considerably greater quantity of mineral and organic matters had become soluble in water, than in the soil that was not artificially aërated. The actual results are given in the table below in grammes, and refer to 6000 grammes of soil in each case:--