How does it render it warmer?
Is the heat produced by the decomposition of organic matter perceptible to our senses?
Is it so to the growing plant?
What is another important part of the organic matter in the soil?
4. Carbon in the soil renders it warmer, because it darkens its color. Black surfaces absorb more heat than light ones, and a black coat, when worn in the sun, is warmer than one of a lighter color. By mixing carbon with the soil, we darken its color, and render it capable of absorbing a greater amount of heat from the sun's rays.
It will be recollected that, when vegetable matter decomposes in the soil, it produces certain gases (carbonic acid, etc.), which either escape into the atmosphere, or are retained in the soil for the use of plants. The production of these gases is always accompanied by heat, which, though scarcely perceptible to our senses, is perfectly so to the growing plant, and is of much practical importance. This will be examined more fully in speaking of manures.
How is it obtained by the soil?
What offices does the organic matter in the soil perform?
Another important part of the organic matter in the soil is that which contains nitrogen. This forms but a very small portion of the soil, but it is of the greatest importance to vegetables. As the nitrogen in food is of absolute necessity to the growth of animals, so the nitrogen in the soil is indispensable to the growth of cultivated plants. It is obtained by the soil in the form of ammonia (or nitric acid), from the atmosphere, or by the application of animal matter. In some cases, manures called nitrates[S] are used; and, in this manner, nitrogen is given to the soil.
We have now learned that the organic matter in the soil performs the following offices:—