There is still another quality of a soil on which its temperature depends, and that is its colour. This may seem at first sight to be scarcely worth taking into account, and yet it has been shown to have a very striking influence on the temperature of a soil. This naturally is best seen in climates where there is a good deal of sun. Dark-coloured soils have a greater heat-absorbing capacity than light-coloured soils; and experiments carried out for the purpose of determining the extent of this influence have shown that under certain conditions the difference between a soil covered with a black substance, and one covered with a white substance, amounted to from 13° to 14° Fahr. Other things being equal, a crop on a dark-coloured soil will be sooner ripened than one on a light-coloured soil. A soil covered by a crop is cooler than one without any crop.
The Power Soils have for absorbing Gases.
We have just seen that one cause of the heat of soils is the oxidation which is constantly going on in all soils, but more rapidly in soils containing a large quantity of vegetable matter. This suggests a word or two on the power soils have of absorbing gases.
The chief gases in the atmosphere are oxygen and nitrogen. Both these gases are absorbed by soils, although not in similar proportions.[46] With regard to the former, it is well known that a plentiful supply of oxygen in the pores of the soil is a necessary condition of fertility. This was long ago experimentally proved by de Saussure, who showed that plants absorbed oxygen through their roots. At certain periods of their growth this demand for oxygen on the part of the plant is greater than at other times. For example, seeds in the process of germination require to have free access to a plentiful supply of oxygen. This fact emphasises the enormous importance of providing a good seed-bed, and of seeing that the seed is not buried too deeply.
Carbonic Acid and Ammonia.
In addition to oxygen and nitrogen, the air contains other gases which are absorbed by the soil. Of these, carbonic acid is the most abundant. By far the largest portion of the carbonic acid which the soil obtains from the air, is washed down in solution in the rain.[47] Of the other constituents of the atmosphere, the combined forms of nitrogen—viz., ammonia, nitric, and nitrous acids—are the most important. These are all absorbed by the soil, but, like carbonic acid, they are chiefly washed down by the rain. The amount of ammonia which may be absorbed by a soil from the air, is very much greater than was formerly supposed. Some recent experiments by Schloesing, referred to in a following chapter,[48] show this. A damp soil may in the course of a year absorb far more ammonia than that washed down in rain.
Gas-absorbing Power of Soils varies.
The power of different soils to absorb these gases varies. This variation depends not only on their physical properties, but also on their chemical as well. Soils containing much organic matter have a greater capacity for absorbing gases than the more purely mineral ones.
Absorption of Nitrogen.
The absorption of nitrogen by the soil is a question of considerable importance. It will be referred to later on under the heading of the biological properties of soils, as it is fixed by the agency of micro-organisms.[49]