IRRIGATION.

[On what does the benefit arising from irrigation chiefly depend?

What kind of water is best for irrigation?

How do under-drains increase the benefits of irrigation?]

_Irrigation_ does not come strictly under the head of inorganic manures, as it often supplies ammonia to the soil. Its chief value, however, in most cases, must depend on the amount of mineral matter which it furnishes.

The word "irrigation" means simply _watering_. In many districts water is in various ways made to overflow the land, and is removed when necessary for the purposes of cultivation. All river and spring water contains some impurities, many of which are beneficial to vegetation.

These are derived from the earth over, or through which, the water has pa.s.sed, and ammonia absorbed from the atmosphere. When water is made to cover the earth, especially if its rapid motion be arrested, much of this fertilizing matter settles, and is deposited on the soil. The water which sinks into the soil carries its impurities to be retained for the uses of plants. When, by the aid of under-drains, or in open soils, the water pa.s.ses _through_ the soil, its impurities are arrested, and become available in vegetable growth. It is, of course, impossible to say exactly what kind of mineral matter is supplied by water, as that depends on the kind of rock or soil from which the impurities are derived; but, whatever it may be, it is generally soluble and ready for immediate use by plants.

[What is the difference between water which only runs over the surface of the earth, and that which runs out of the earth?

Why should strong currents of water not be allowed to traverse the soil?]

Water which has run over the surface of the earth contains both ammonia and mineral matter, while that which has arisen out of the earth, contains usually only mineral matter. The direct use of the water of irrigation as a solvent for the mineral ingredients of the soil, is one of its main benefits.

To describe the many modes of irrigation would be too long a task for our limited s.p.a.ce. It may be applied in any way in which it is possible to cover the land with water, at stated times. Care is necessary, however, that it do not wash more fertilizing matter from the soil than it deposits on it, as would often be the case, if a strong current of water were run over it. Brooks may be dammed up, and thus made to cover a large quant.i.ty of land. In such a case the rapid current would be destroyed, and the fertilizing matter would settle; but, if the course of the brook were turned, so that it would run in a current over any part of the soil, it might carry away more than it deposited, and thus prove injurious. Small streams turned on to land, from the washing of roads, or from elevated springs, are good means of irrigation, and produce increased fertility, except where the soil is of such a character as to prevent the water from pa.s.sing away, in which case it should be under-drained.

Irrigation was one of the oldest means of fertility ever used by man, and still continues in great favor wherever its effects have been witnessed.

MIXING SOILS.

[How are soils improved by mixing?]

The _mixing of soils_ is often all that is necessary to render them fertile, and to improve their _mechanical_ condition. For instance, soils deficient in potash, or any other const.i.tuent, may have that deficiency supplied, by mixing with them soil containing this const.i.tuent in excess.

It is very frequently the case, that such means of improvement are easily availed of. While these chemical effects are being produced, there may be an equal improvement in the mechanical character of the soil. Thus stiff clay soils are rendered lighter, and more easily workable, by an admixture of sand, while light blowy sands are compacted, and made more retentive of manure, by a dressing of clay or of muck.

[Why may the same effect sometimes be produced by deep plowing?

What is absolutely necessary to economical manuring?]

Of course, this cannot be depended on as a sure means of chemical improvement, unless the soils are previously a.n.a.lyzed, so as to know their requirements; but, in a majority of cases, the soil will be benefited, by mixing with it soil of a different character. It is not always necessary to go to other locations to procure the soil to be applied, as the subsoil is often very different from the surface soil, and simple deep plowing will suffice, in such cases, to produce the required admixture, by bringing up the earth from below to mingle it with that of a different character at the surface.

In the foregoing remarks on the subject of mineral manures, the writer has endeavored to point out such a course as would produce the "greatest good to the greatest number," and, consequently, has neglected much which might discourage the farmer with the idea, that the whole system of scientific agriculture is too expensive for his adoption. Still, while he has confined his remarks to the more simple improvements on the present system of management, he would say, briefly, that _no manuring can be strictly economical that is not based on an a.n.a.lysis of the soil, and a knowledge of the best means of overcoming the deficiencies indicated, together with the most scrupulous care of every ounce of evaporating or soluble manure_.

FOOTNOTES:

[AG] Marl is earth containing lime, but its use is not to be recommended in this country, except where it can be obtained at little cost, as the expenses of carting the _earth_ would often be more than the value of the _lime_.

[AH] The straw producing the grain and the turnip and potato tops contain more lime than the grain and roots.

[AI] See Working Farmer, vol. 2, p. 278.

[AJ] Glycerine, etc.

CHAPTER X.

ATMOSPHERIC FERTILIZERS.

[Are the gases in the atmosphere manures?

What would be the result if they were not so?]

It is not common to look on the gases in the atmosphere in the light of manures, but they are decidedly so. Indeed, they are almost the only organic manure ever received by the uncultivated parts of the earth, as well as a large portion of that which is occupied in the production of food for man.

If these gases were not manures; if there were no means by which they could be used by plants, the fertility of the soil would long since have ceased, and the earth would now be in an unfertile condition. That this must be true, will be proved by a few moments' reflection on the facts stated in the first part of this book. The fertilizing gases in the atmosphere being composed of the const.i.tuents of decayed plants and animals, it is as necessary that they should be again returned to the form of organized matter, as it is that const.i.tuents taken from the _soil_ should not be put out of existence.

AMMONIA.

[How is ammonia used by plants?

How may it be carried to the soil?

How may the value of organic manures be estimated?

What effects has ammonia beside supplying food to plants?]

The _ammonia_ in the atmosphere probably cannot be appropriated by the leaves of plants, and must, therefore, enter the soil to be a.s.similated by roots. It reaches the soil in two ways. It is either arrested from the air circulating through the soil, or it is absorbed by rains in the atmosphere, and thus carried to the earth, where it is retained by clay and carbon, for the uses of plants. In the soil, ammonia is the most important of all organic manures. In fact, the value of organic manure may be estimated, either by the amount of ammonia which it will yield, or by its power of absorbing ammonia from other sources.

The most important action of ammonia in the soil is the supply of _nitrogen_ to plants; but it has other offices which are of consequence.

It a.s.sists in some of the chemical changes necessary to prepare the matters in the soil for a.s.similation. Some argue that ammonia _stimulates_ the roots of plants, and causes them to take up increased quant.i.ties of inorganic matter. The discussion of this question would be out of place here, and we will simply say, that it gives them such vigor that they require increased amounts of ashy matter, and enables them to take this from the soil.

[To how great a degree can the farmer control atmospheric fertilizers?

What should be the condition of the soil?

What substances are good absorbents in the soil?

How may sandy soils be made retentive of ammonia?]

Although, in the course of nature, the atmospheric fertilizers are plentifully supplied to the soil, without the immediate attention of the farmer, it is not beyond his power to manage them in such a manner as to arrest a greater quant.i.ty. The precautions necessary have been repeatedly given in the preceding pages, but it may be well to name them again in this chapter.

The condition of the soil is the main point to be considered. It must be such as to absorb and retain ammonia--to allow water to pa.s.s _through_ it, and be discharged _below_ the point to which the roots of crops are searching for food--and to admit of a free circulation of air.

The power of absorbing and retaining ammonia is not possessed by sand, but it is a prominent property of clay, charcoal, and some other matters named as absorbents. Hence, if the soil consists of nearly pure sand, it will not make use of the ammonia brought to it from the atmosphere, but will allow it to evaporate immediately after a shower. Soils in this condition require additions of absorbent matters, to enable them to use the ammonia received from the atmosphere. Soils already containing a sufficient amount of clay or charcoal, are thus far prepared to receive benefit from this source.

[Why does under-draining increase the absorptive power of the soil?