Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions p. 274-276 (1911)
Eugene Woldemar Hilgard

Absorption of gases by dry soils

Perfectly dry soils are powerful absorbers of ammonia, and their absorption of this gas, as well as of carbonic gas, can readily be shown by the arrangement shown on the page opposite.

The two tubes shown to the left are filled with carbonic gas, those to the right with ammonia gas. After being immersed in a mercurial trough, there are introduced into each tube through the mercury small cylinders (conveniently one cubic centimeter in volume) consisting respectively of a very sandy soil or loose hardpan, a gray plastic clay, a gray clay soil or adobe, a very black "adobe" clay, and a highly ferruginous and humous soil (from Hawaii), which gives the highest absorption of all; next brown peat, and pine charcoal. The latter, and the ferruginous soil, were also exposed for the absorption of carbonic gas. All the absorbing cylinders are first heated for an hour to 110°C. (218°F) for the purpose of expelling from them moisture, air, and other absorbed gases. They are then quickly introduced into the tubes through the mercury and allowed to absorb the gases enclosed until the mercury columns cease to show any farther rise; in which condition they are shown in the figure.

It will be seen that this absorption is a different one, not only for each of the different substances used, but is also differently proportioned for the two gases. For it will be noted that while the clay soil has absorbed a very much larger amount of ammonia than the charcoal, and the sandy soil has remained far behind both: yet the charcoal has absorbed a considerably larger proportion of carbonic gas than either the clay or the sandy soil, proving that charcoal has a strong specific absorptive power for carbonic gas, independently of the relative size of clay and charcoal particles respectively. The sandy soil shows, by its low absorption even of ammonia gas, the coarseness of its particles and the scarcity of clay in its composition. The highest absorption of all is shown by the ferruginous soil from Hawaii, containing nearly 40 % of ferric oxid together with 3 1-3% of humus. The moisture-absorption of this soil at the ordinary temperature is 19.7 per cent. The difference in the absorbing power of the (nonhumous) gray clay and gray adobe soil indicates the strong influence of humus upon the absorption; which is still farther emphasized by the difference between the gray and black adobe, the latter containing 1.2% of humus. As to the peat, since its weight was only .5 grams against an average of 2 grams for the soils employed, its absorptive power by weight doubtless exceeds all other substances.

Fig. 51.—Absorption of Carbonic and Ammonia Gases by different Soils.

While the experiment shown in the figure serves as a convenient and striking demonstration for lecture purposes, it is of course not adapted to a direct comparison of the absorbing powers of the several substances, because of different heights of the mercurial columns counteracting the atmospheric pressure. For direct comparative measurement the tubes must be sunk in mercury so as to equalize the levels inside and outside, since the corrected volumes obtained by calculation would not serve the purpose.

According to special measurements made under normal atmospheric pressure, the writer found that a black clay soil ("adobe") absorbed (at 60°F) over two hundred times its bulk of ammonia gas, while under the pressure of one-fifth of an atmosphere (as shown in the photograph) the absorption was one hundred and twenty-three times its bulk. This energetic absorption of ammonia and related gases explains the marked disinfecting effects which a covering of dry earth exerts in the case of cemeteries, manure piles, and earth closets. But the difference between the sandy soil and the clay soil in the amount of absorption admonishes us that in all these cases, to secure disinfection the earth to be used should contain as much clay as possible, and should not be mere sand, as is sometimes the case. It also shows that the addition of charcoal to such materials does not increase their efficacy, as has been supposed, but that an equal bulk of clay would be more efficient.

Of course, so soon as the absorbing cylinders used for this experiment are exposed to the atmosphere, the principle above stated in regard to "partial pressure" asserts itself. The absorbed gases quickly begin to be given off, and in some hours the equilibrium with the ordinary conditions of the atmosphere is reestablished. That the strong absorptive power of soils for ammonia is to some extent effective in maintaining the supply of this substance by absorption from the atmosphere, cannot be doubted.

Boussingault, and later Stenhouse, determined the absorptive power of wood charcoal for ammonia to be 90 and 98 volumes respectively.