George E. Waring

What proportion of organic matter is required for fertility?

How does the soil obtain its organic matter?

How does the growth of clover, etc., affect the soil?

It will be recollected that, in addition to its mineral portions, the soil contains organic matter in varied quantities. It may be fertile with but one and a half per cent. of organic matter, and some peaty soils contain more than fifty per cent. or more than one half of the whole.

The precise amount necessary cannot be fixed at any particular sum; perhaps five parts in a hundred would be as good a quantity as could be recommended.

The soil obtains its organic matter in two ways. First, by the decay of roots and dead plants, also of leaves, which have been brought to it by wind, etc. Second, by the application of organic manures.

When a crop of clover is raised, it obtains its carbon from the atmosphere; and, if it be plowed under, and allowed to decay, a portion of this carbon is deposited in the soil. Carbon constitutes nearly the whole of the dry weight of the clover, aside from the constituents of water; and, when we calculate the immense quantity of hay, and roots grown on an acre of soil in a single season, we shall find that the amount of carbon thus deposited is immense. If the clover had been removed, and the roots only left to decay, the amount of carbon deposited would still have been very great. The same is true in all cases where the crop is removed, and the roots remain to form the organic or vegetable part of the soil. While undergoing decomposition, a portion of this matter escapes in the form of gas, and the remainder chiefly assumes the form of carbon (or charcoal), in which form it will always remain, without loss, unless driven out by fire. If a bushel of charcoal be mixed with the soil now, it will be the same bushel of charcoal, neither more nor less, a thousand years hence, unless some influence is brought to bear on it aside from the growth of plants. It is true that, in the case of the decomposition of organic matter in the soil, certain compounds are formed, known under the general names of humus and humic acid, which may, in a slight degree, affect the growth of plants, but their practical importance is of too doubtful a character to justify us in considering them. The application of manures, containing organic matter, such as peat, muck, animal manure, etc., supplies the soil with carbon on the same principle, and the decomposing matters also generate[Q] carbonic acid gas while being decomposed. The agricultural value of carbon in the soil depends (as we have stated), not on the fact that it enters into the composition of plants, but on certain other important offices which it performs, as follows:—

1. It makes the soil more retentive of manures.

2. It causes it to appropriate larger quantities of the fertilizing gases of the atmosphere.

3. It gives it greater power to absorb moisture.

4. It renders it warmer.

1. Carbon (or charcoal) makes the soil retentive of manures, because it has in itself a strong power to absorb, and retain[R] fertilizing matters. There is a simple experiment by which this power can be shown.

Ex.—Take two barrels of pure beach sand, and mix with the sand in one barrel a few handfuls of charcoal dust, leaving that in the other pure. Pour the brown liquor of the barn-yard through the pure sand, and it will pass out at the bottom unaltered. Pour the same liquor through the barrel, containing the charcoal, and pure water will be obtained as a result. The reason for this is that the charcoal retains all of the impurities of the liquor, and allows only the water to pass through. Charcoal is often employed to purify water for drinking, or for manufacturing purposes.

A rich garden-soil contains large quantities of carbonaceous matter; and, if we bury in such a soil a piece of tainted meat or a fishy duck, it will, in a short time, be deprived of its odor, because the charcoal in the soil will entirely absorb it.

Carbon absorbs gases as well as the impurities of water; and, if a little charcoal be sprinkled over manure, or any other substance, emitting offensive odors, the gases escaping will be taken up by the charcoal, and the odor will cease.

It has also the power of absorbing mineral matters, which are contained in water. If a quantity of salt water be filtered through charcoal, the salt will be retained, and the water will pass through pure.

We are now able to see how carbon renders the soil retentive of manures.

1st. Manures, which resemble the brown liquor of barn-yards, have their fertilizing matters taken out, and retained by it.

2d. The gases arising from the decomposition (rotting) of manure are absorbed by it.

3d. The soluble mineral portions of manure, which might in some soils leach down with water, are arrested and retained at a point at which they can be made use of by the roots of plants.

2. Charcoal in the soil causes it to appropriate larger quantities of the fertilizing gases of the atmosphere, on account of its power, as just named, to absorb gases.

The atmosphere contains results, which have been produced by the breathing of animals and by the decomposition of various kinds of organic matter, which are exposed to atmospheric influences. These gases are chiefly ammonia and carbonic acid, both of which are largely absorbed by water, and consequently are contained in rain, snow, etc., which, as they enter the soil, give up these gases to the charcoal, and they there remain until required by plants. Even the air itself, in circulating through the soil, gives up fertilizing gases to the carbon, which it may contain.

3. Charcoal gives to the soil power to absorb moisture, because it is itself one of the best absorbents in nature; and it has been proved by accurate experiment that peaty soils absorb moisture with greater rapidity, and part with it more slowly than any other kind.

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.

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:—

Organic matter thoroughly decomposed is carbon, and has the various effects ascribed to this substance on p. 79.

Organic matter in process of decay produces carbonic acid, and sometimes ammonia in the soil; also its decay causes heat.

Organic matter containing nitrogen, such as animal substances, etc., furnish ammonia, and other nitrogenous substances to the roots of plants.