Home › Agriculture › Fertilizers
Fertilizers
think that the temporary diminution of fertility | the purpose of raising manure for itself; and in a field is chiefly owing to the loss of the decaying vegetable matter it previously contained: it is principally the consequence of the exhaustion of potash and soda, which are restored by the slow process of the more complete disintegration of the materials of the soil. It is evident that the careful tilling of fallow land must accelerate and increase this further breaking up of its mineral ingredients. Nor is this repose of the soil always necessary. A field, which has become unfitted for a certain kind of produce, may not, on that account, be unsuitable for another; and upon this observation a system of agriculture has been gradually formed, the principal object of which is to obtain the greatest possible produce in a succession of years, with the least outlay for manure. Because plants require for their growth different constituents of soil, changing the crop from year to year will maintain the fertility of that soil (provided it be done with judgment) quite as well as leaving it at rest or fallow. In this we but imitate nature. The oak, after thriving for long generations on a particular spot, gradually sickens; its entire race dies out; other trees and shrubs succeed it, till, at length, the surface becomes so charged with an excess of dead vegetable matter, that the forest becomes a peat moss, or a surface upon which no large tree will grow.
Generally long before this can occur, the operation of natural causes has gradually removed from the soil substances, essential to the growth of oak, leaving others favorable and necessary to the growth of beech or pine. So, in practical farming, one crop, in artificial rotation with others, extracts from the soil a certain quantity of necessary materials; a second carries off, in preference, those which the former has left.
One hundred parts of wheat straw yield 15t of ashes; the same quantity of barley straw, 8; of oat straw, only 4; and the ashes of the three are, chemically, of about the same composition. Upon the same field, which will yield only one harvest of wheat, two successive crops of barley may be raised, and three of oats. We have in these facts a clear proof of what is abstracted from the soil, and the key to the rational mode of supplying the deficiency.
Since wheat consumes a large amount of silicate of potassa from the soil, the plants which should succeed or alternate with it must be such as require but little potassa, as potatoes or turnips. After three or four years the same lands may well bear wheat; because, during the interval, the soil will have been, by the action of the atmosphere, and the solution of vegetable and animal substances decaying upon or in it, again rendered capable of yielding what the wheat requires.
Whether this process can be artificially anticipated, by supplying the exhausted ingredient to the soil, is a further and most interesting and important inquiry.
We could keep our fields in a constant state of fertility by replacing, every year, as much as is removed from them by their produce. An increase of fertility may be expected, of course, only when more is added of the proper material to the soil than is taken away. Any soil will partially regain its strength by lying fallow. But any soil, under cultivation, must at length (without help) lose those constituents which are removed in the seeds, roots and leaves of the plants raised upon it. To remedy this loss, and also increase the productiveness of the land, is the object of the use of proper manures. Land, when not employed in raising food for animals or man, should, at least, be applied to this, to a certain exten, may be effected by means of green crops, which, by their decomposition, not only add to the amount of vegetable mould contained in the soil, but supply the alkalies that would be found in their ashes. That the soil should become richer by this burial of a crop, than it was before the seed of that crop was sown, will be understood by recollecting that threefourths of the whole organic matter we bury has been derived from the air: that by this process of ploughing in, the vegetable matter is more equally diffused through the whole soil, and therefore more easily and rapidly decomposed; and that by its gradual decomposition, ammonia and nitric acid are certainly generated, though not so largely as when animal matters are employed. He who neglects the green sods, and crops of weeds that flourish by his hedgerows and ditches, overlooks an important natural means of wealth. Left to themselves, they ripen their seeds, exhausting the soil, and sowing them annually in his fields: collected in compost heaps, they add materially to his yearly crops of corn.
Organic Manures.
The following conclusions may be regarded as scientifically sustained, as well as confirmed by practical experience: 1. That fresh human urine yields nitrogen in greater abundance to vegetation than any other material of easy acquisition; and that the urine of animals is valuable for the same purpose, but not equally so. 2. That the mixed excrements of man and animals yield (if carefully preserved from further decomposition), not only nitrogen, but other invaluable saline and earthy matters that have been already extracted in food from the soil. 3. That animal substances which, like urine, flesh, and blood, decompose rapidly, are fitted to operate immediately and powerfully on vegetation. 4. That dry animal substances, as horn, hair, or woollen rags, decompose slowly, and (weight for weight) contain a greater quantity of organized as well as unorganized materials, manifesting their influence it may be for several seasons. 5. That bones, acting like horn, in so far as their animal matter is concerned, and like it for a number of seasons more or less, according as they have been more or less finely crushed, may ameliorate the soil by their earthy matter for a long period (even if the jelly they contain have been injuriously removed by the size maker), permanently improving the condition and adding to the natural capabilities of the land.
Uses of Guano.
This manure is a powerful stimulant to vegetable development generally; it is especially available in raising wheat, corn, potatoes, garden vegetables, and tobacco. If the land needs it, it may be put on as often as a crop is to be raised; though not, it is said, as a top dressing. For wheat, 150 to 200 pounds of guano may be used to the acre; for Indian corn, 300 to 400 pounds; unless it is put directly in the hills, when 100 pounds per acre will do. For potatoes, 300 to 400 pounds, in a drill, with bone dust. The addition of the latter makes the good effects of the guano more durable.
Mineral Fertilizers.
Simple lime, although an important constituent of plants, is rarely suitable as an application to them in its pure state. Carbonate of lime (renresented by chalk, &c.) is a natural ingredient in very many soils. The sulphate of lime (gypsum, plaster of Paris) is often used for fertilizing pur- | rivers and rains, or which have been added by the poses. It is less easily decomposed than the carbonate. The precise conditions which make it most advantageous, are not positively determined yet. Phosphate of lime is a very important constituent of plants; and, as it exists also in the bones of animals, a double relation follows: namely, that it should be abundant in soil on which plants are raised for food of men and animals; and, on the other hand, that animal bones contribute it to the soil when they decay upon it.
Wood ashes contain a large amount of carbonate of potassa, with also the sulphate and silicate of that alkali. Peat ashes vary in different regions, but always are found useful as manure.
Kelp, or the ashes of sea-weeds, are often employed in the same way; they contain soda in considerable amount. Nitrate of potassa (nitre, or saltpetre) is said to quicken vegetable action when added to the soil, and to give the leaves a deeper green. A hundred pounds to the acre of grass or young corn, have been reported to produce a beneficial effect. In localities far inland, common salt, chloride of sodium, is indispensable to the soil, although a small amount of it will suffice. Animal manures contain it. An excess of salt will render land barren; as was well known to the ancients.
Conclusions.
We may take it for granted that every thinking, practical mind, will admit it as proved, that there must be an exact adaptation and fitness between the condition of any given soil and the plants intended to be raised upon it; and, further, that if this mutual fitness does not naturally exist, a knowledge of its requirements will enable us to supply it artificially. The great difficulty is, to obtain this knowledge fully and accurately. It must be confessed that, at present, much is wanting to render it complete and directly available. Industrious observation and experiment may, hereafter, make it so; and thus give us a system of truly scientific agriculture.
A few statements only remain to be added to what has been said. The best natural soils are those where the materials have been derived from the breaking up and decomposition, not of one stratum or layer, but of many-divided minutely by air and water, and minutely blended together: and in improving soils by artificial additions, the farmer cannot do better than imitate the processes of nature.
We have spoken of soils as consisting mostly of sand, lime, and clay, with certain saline and organic substances in smaller and varying proportions; but the examination of the ashes of plants shows that a fertile soil must of necessity contain an appreciable quantity of at least eleven different substances, which in most cases exist in greater or less relative abundance in the ash of cultivated plants; and of these the proportions are not by any means immaterial. În general, the soils which are made up of the most various materials are called alluvial; having been formed from the depositions of floods and rivers. Many of them are extremely fertile. Soils consist of two parts; of an organic part, which can readily be burned away when the surface-soil is heated to redness; and of an inorganic part, which remains fixed in the fire, consisting of earthy and saline substances; from which, if carbonic acid or any elastic gas be present, it may, however, be driven by the heat. The organic part of soils is derived chiefly from the remains of vegetables and animals which have lived and died in and upon the soil, which have been spread over it by industry of man for the purposes of increased fertility.
This organic part varies much in quantity, a. well as quality, in different soils. In peaty soils it is very abundant, as well as in some rich, long cultivated lands. In general, it rarely amounts to one-fourth, or 25 per cent., even in our best arable lands. Good wheat soils contain often as little as eight parts in the hundred of organic animal or vegetable matter; oats and rye will grow in a soil containing only 1½ per cent.; and barley when only two or three parts per cent. are present.
The inorganic portion of any given soil, again, is divisible into two portions; that part which is soluble in water, and thus easily taken up by plants, and a much more bulky portion which is insoluble.
Sir Humphrey Davy found the following to be the composition of a good productive soil. In every 9 parts, 8 consisted of siliceous sand; the remaining (one-ninth) part was composed, in 100 parts, as follows:
Carbonate of lime (chalk), .
Pure silex, Pure alumina, or the earth of clay, Oxide (rust) of iron, Vegetable and other saline matter, Moisture and loss, . 63 grains. 15 grains. 11 grains. 3 grains. 5 grains. 3 grains. 100 Thus the whole amount of organic matter in this instance is only 1 part in 200, or one-half of one per cent.; a fact which, in itself, would demonstrate the fallacy of supposing that decomposed animal and vegetable matter in the soil form the exclusive supply to growing plants.
In another instance, soil was taken from a field in Sussex, remarkable for its growth of flourishing oak trees. It consisted of 6 parts of sand, and 1 part of clay and finely-divided matter. One hundred grains of it yielded, in chemical language- Of silica (or silex), Of alumina, Carbonate of lime, Oxide of iron, Vegetable matter in a state of decomposition, Moisture and loss, 54 grains. 28 grains. 3 grains. 5 grains. 4 grains. 6 grains. 100 To wheat soils, the attention of the practical farmer will be most strongly directed. An excellent wheat soil from West Drayton, in England, yielded 3 parts in 5 of silicious sand; and the remaining two parts consisted of carbonate of lime, silex, alumina, and a minute proportion of decomposing animal and vegetable remains.
Of these soils, the last was by far the most, and the first the least, coherent in texture. In all cases, the constituent parts of the soil which give tenacity and stiffness, are the finely-divided portions; and they possess this quality in proportion to the quantity of alumina (or earth of clay) they contain.
The varying power of soils to absorb and retain water from the air, is much connected with their fertility. This absorbent power is always greatest in the most fertile lands. Their productiveness is also much influenced by the nature of the subsoil on which they rest; for, when soils are situated immediately upon a bed of rock or stone, they dry sooner by the sun's agency than when the subsoil is clay or marl.
A great deal more might be said upon other