On the other hand, we are acquainted with no satisfactory evidence that the soluble organic matters of the soil and of peat, especially the crenates and apocrenates, are not actually appropriated by, and, so far as they go, are not directly serviceable as food to plants.
Be this as it may, practice has abundantly demonstrated the value of humus as an ingredient of the soil, and if not directly, yet indirectly, it furnishes the material out of which plants build up their parts.
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 subsoil of forest | 0.82 | 3,458 | 28 |
| Surface soil of forest | 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 |
| Soil of asparagus bed, newly manured | 1.54 | 10,948 | 172 |
| Sandy soil, six days after manuring, and three days of rain | 2.21 | 11,536 | 257 |
| Sandy soil, ten days after manuring, and three days of rain | 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 | |
From the above it is seen that in soils containing little decomposing organic matters—as the forest sub-soils—the quantity of carbonic acid is no greater than that contained in an equal bulk of the atmosphere. It is greater in loamy and clayey soils; but is still small. In the artichoke field (probably light soil not lately manured), and even in an asparagus bed unmanured for one year, the amount of carbonic acid is not greatly larger. In newly manured fields, and especially in a vegetable compost, the quantity is vastly greater.
The organic matters which come from manures, or from the roots and other residues of crops, are the source of the carbonic acid of the soil. These matters continually waste in yielding this gas, and must be supplied anew. Boussingault found that the rich soil of his kitchen garden (near Strasburg) which had been heavily manured from the barn-yard for many years, lost one-third of its carbon by exposure to the air for three months (July, August and September,) being daily watered. It originally contained 2.43 per cent. At the conclusion of the experiment it contained but 1.60 per cent., having lost 0.83 per cent.
Peat and swamp-muck, when properly prepared, furnish carbonic acid in large quantities during their slow oxidation in the soil.