[238] Chemical Division, U. S. Department of Agriculture, Bulletin No. 38, p. 57.
PART FOURTH.
MISCELLANEOUS FERTILIZERS.
278. Classification.—Nitrogen, phosphoric acid, and potash are the most important of the plant foods both from a commercial and physiological point of view. They are the chief constituents of the most important fertilizers and manures, but are by no means the sole essential elements of plant nutrition. Lime, magnesia, soda, sulfur, chlorin, and many other elements are found constantly in plants and must be regarded as normal constituents thereof. It is the purpose here, however, to speak only of those substances which are used as fertilizers and which constantly or occasionally are subjected to chemical examination for determining their commercial or agronomic value. These bodies may be conveniently divided into two classes; viz., mineral and organic. Among those of mineral nature may be mentioned lime, gypsum, marls, wood-ashes, common salt, and ferrous sulfate; among those of organic nature may be included guano, hen manure, stall manure, composts, and muck.
279. Forms of Lime.—By the term lime is meant the product obtained by subjecting limestone or other lime carbonates to the action of heat until the carbon dioxid contained therein is expelled. The resulting lime, CaO, when exposed for some time to the air, or at once on the addition of water, is converted into the hydrate CaO₂H₂, known as slaked lime. On longer exposure to the air, the hydrate gradually absorbs carbon dioxid and becomes converted into carbonate. In whatever form, therefore, lime is applied to the soil, it is found in the end, as carbonate. A distinction should also be made between lime obtained from mineral substances and that got from organic products such as shells. Strictly speaking this is not a definite matter, inasmuch as limestones are sometimes but little more than aggregations of fossil shells. Practically, however, the distinction is made, and some farmers prefer shell lime to that of any other kind. Gas lime, that is lime which has been used for the purification of illuminating gas made from coal, is hardly to be considered in this connection, since it may contain very little even of the hydrate. In this case the lime has been converted largely into carbonate and sulfid.
280. Application of Lime.—For many reasons it is important that the lime be transported to the field before it has had time to be converted into hydrate. The transportation costs less in this state and it can be handled with far less inconvenience than when slaked. The lime should be placed in small piles and left thus, best covered with a little earth, until thoroughly slaked. It is then spread evenly over the surface. The quantity used per acre depends largely on the nature of the soil. Stiff clays and sour marsh lands require a larger dressing than loams or well aerated soils. From three to six thousand pounds per acre are the quantities usually employed. When the lime is once thoroughly incorporated in the soil it is rapidly converted into carbonate, but while in the caustic state it may act vigorously in promoting the decay of organic matter and may prove injurious in promoting the decomposition of ammonium salts with attendant loss of nitrogen.
281. Action of Lime.—The benefits arising from the application of lime to agricultural lands, although in many cases great, do not arise from any distinct fertilizing action of its own. Plants need lime for growth and need plenty of it, but as a rule any soil which is good enough to grow crops will contain enough lime to furnish that constituent of the crops for many years. Its action is both mechanical and chemical. By virtue of the latter property it renders available for plant food bodies already existing in the soil but existing in such shape as to be unavailable for plants. The supply of plant food available for the crops of one year is increased but this increase is at the expense of the following years. Lime is a stimulant. There is an European proverb that “lime enriches the father but beggars the son.” Nevertheless, a limestone country is usually a fertile one and soils containing plenty of lime naturally, are nearly always rich soils. It is said that the trees and plants which farmers pick out as indicative of rich land are nearly always those which prefer lime soils.
The mechanical action of lime on soils tends to lighten heavy clays and loams and to render firmer and more consistent the light and shifting sandy soils. When a lump of clay is stirred up in a bucket of rain water the water becomes muddy and remains that way for many days. If, however, to the bucket of muddy water a little lime water be added the suspended particles of clay begin to flocculate and soon the water is clear and the clay falls to the bottom, nor does it again make the water muddy for a long time when stirred up with it. The flocky character of the precipitate is tenaciously retained and it is necessary to knead the clay for some time to induce it to reassume its original heavy character. An action like this takes place when lime is added to heavy soils so that the soil becomes more porous and assumes a better tilth on plowing. With sandy soils an altogether different action takes place. In making mortar, as is well known, sand is stirred in with milk of lime and after being exposed to air for a while the mixture becomes hard and firm, the firmness increasing with age. This is due to the fact that when the mortar dries the lime begins to absorb carbon dioxid from the air and is converted into grains of carbonate which adhere strongly to neighboring sand grains and to each other so that the whole soon gets to be a solid mass. Something like this takes place in the soil and the sand grains are to some extent bound together. The increased firmness of the soil thus gained is often of considerable advantage.
Besides these actions, which are more or less mechanical, lime exerts a chemical action on many soil constituents. Feldspar and other common rocks contain potash, and this potash is in such a form as to be inaccessible to plants. These rocks exist in the shape of small particles in many soils and on them lime exerts a decomposing action, setting the potash free. Lime also hastens the decomposition of the nitrogenous organic matter and at the same time renders the soil more retentive of the products formed. The conversion of ammonia, resulting from the decomposition of such organic matter into nitrites and nitrates, is not easily accomplished without a proper amount of calcium carbonate. The microorganisms producing this change, which is known as nitrification, apparently require its presence for neutralizing the acid formed. In general, it may be said that the presence of lime hastens the putrefactive process. This is the reason it is so largely used in making composts.
It is difficult to say just what soils will be improved by liming and what will not, and it is a matter which must be settled by experiment in each case. As a rule heavy clays and loams are benefited, yet of two such soils, apparently identical, one may not be affected in any marked degree while the other may readily respond to treatment. Sandy soils are often improved but sometimes not. Sour, boggy lands, are usually improved by the addition of enough lime to neutralize their undue acidity. Marsh grasses and plants are more tolerant of acid in the soil than tame grasses are, so that in unlimed soil the former run out the latter. The application of lime alone to a very poor soil does not pay.
The particles of lime resting in the soil are partially dissolved by the next rainfall after application, or by the soil moisture, forming lime water, and the lime is distributed in this form through the soil to some extent. It all probably soon becomes converted into carbonate as ground air is usually quite rich in carbon dioxid. Indeed, for many soils, it is immaterial whether lime be applied as lime or as carbonate, granting, of course, that the latter be ground to a fine powder. Economy is in favor of the lime, however, not only because it needs no grinding, but because it is lighter than the corresponding amount of carbonate, making a saving in transportation. The difference is quite considerable, fifty-six pounds of lime being equivalent in effect to 100 pounds of carbonate. For these reasons as well as because it possesses some valuable properties not shared by the carbonate, it is possible that for most localities lime is to be preferred to any form of ground oyster shells, ground limestone, marble dust or the like.