2. _The organic matters of peat as indirect food to plants._ Very nearly one-half, by weight, of our common crops, when perfectly dry, consists of _carbon_. The substance which supplies this element to plants is the gas, carbonic acid. Plants derive this gas mostly from the atmosphere, absorbing it by means of their leaves. But the free atmosphere, at only a little space above the soil, contains on the average but 1/2500 of its bulk of this gas, whereas plants flourish in air containing a larger quantity, and, in fact, their other wants being supplied, they grow better as the quantity is increased to 1/12 the bulk of the air. These considerations make sufficiently obvious how important it is that the soil have in itself a constant and abundant source of carbonic acid gas. As before said, _organic matter, in a state of decay_, is the single material which the farmer can incorporate with his soil in order to make the latter a supply of this most indispensable form of plant-food.
When organic matters decay in the soil, their carbon ultimately assumes the form of Carbonic acid. This gas, constantly exhaling from the soil, is taken up by the foliage of the crops, and to some extent is absorbed likewise by their roots.
Boussingault & Lewy have examined the air inclosed in the interstices of various soils, and invariably found it much richer (10 to 400 times) than that of the atmosphere above. Here follow some of their results:
CARBONIC ACID IN SOILS. -------------------------------------------------------------------------- Key: A - _Volumes of Carbonic acid in 100 of air in pores of Soil._ B - _Cubic feet of air in acre to depth of 14 inches._ C - _Cubic feet of Carbonic acid in acre to depth of 14 inches._ D - _Volumes of Carbonic acid to 100 of air above the soil._ E - _Cubic feet of air over one acre to height of 14 inches._ F - _Cubic feet of Carbonic acid over one acre to a height of 14 inches._
--------------------------------------------------------+-----+------+---- _Designation and Condition of Soil._ | A | B | C --------------------------------------------------------+-----+------+---- Sandy subsoil of forest |0.24 | 4,326| 14 Loamy " " " |0.82 | 3,458| 28 Surface soil " " |0.86 | 5,768| 56 Clayey soil of artichoke field |0.66 |10,094| 71 Soil of asparagus bed, unmanured for one year |0.79 |10,948| 86 " " " " newly manured |1.54 |10,948| 172 Sandy soil, six days after manuring, and three | | | days of rain.|2.21 |11,536| 257 " " ten " " " " " | | | " " " |9.74 |11,536|1144 Compost of vegetable mold |3.64 |20,608| 772 | | | _Carbonic Acid in Atmosphere_ | D | E | F |-----+------+---- |0.025|50,820| 14 --------------------------------------------------------+-----+------+----