The Encyclopedia Americana (1920)/Fertilizers

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The Encyclopedia Americana
Edition of 1920. See also Fertilizer on Wikipedia, and the disclaimer.

FERTILIZERS. Any substance applied to the soil to aid the growth of plants may properly be called a fertilizer. The name has, however, become associated with a class of materials manufactured and sold for this purpose, the handling of which now constitutes an important industry.

The constituents of the plant that it derives from the soil are nitrogen, phosphorus, sulphur, potassium, sodium, calcium, magnesium, iron, aluminum, manganese, silicon, chlorine. Of these nitrogen, phosphorus or phosphoric acid, and potassium or potash, are the substances most likely to be deficient in the soil and therefore the ones that are contained in fertilizers. A fertilizer may contain any one or more of these substances.

The chief distinction between the functions of farm manure and commercial fertilizers may be stated in a general way to be that farm manure increases crop production by improving the condition of the soil, as well as by furnishing plant nutrients, while commercial fertilizers may act directly as a plant-food material without materially affecting soil structure.

It is evident, therefore, that commercial fertilizers are not an adequate substitute for farm manure or green manures for producing permanent improvement. Their function consists in supplementing the available supply of plant-food in a soil which may be deficient in any one or more of the substances usually contained in fertilizers. On virgin soils commercial fertilizers are usually superfluous; but as loss of plant-food goes on under cropping, restitution must be made and, as this is usually not adequately done with farm manure, fertilizers are finally called upon.

Where there is large loss of plant-food from soil constituents due to leaching, there must be a large excess of potential fertility, in order that the growing crop shall at all times be supplied with available plant-food. A deficiency in any constituent will check growth by compelling the plant to depend upon a less readily available supply. Commercial fertilizers are useful by presenting readily available food to the plant when it is beginning growth and when a deficiency in the supply is likely to produce a permanent injury.

As the price of land increases, as the cost of labor becomes greater and as the value of the crop augments it becomes more important that maximum crops shall be raised, and for these reasons commercial fertilizers usually have greatest sale where agriculture is most intensive. There are also many special crops requiring more of one plant-food element than of another, and the needs of these can best be met by the use of commercial fertilizers.

Historical. — The value of animal excrements applied to soils on which crops were grown has been appreciated by the husbandman as far back as records go. Why this manure is beneficial, and what relation its constituents bear to those of plants, are matters which have only been worked out during the last century, and knowledge of which has led to the use of fertilizers composed of mineral salts, commonly known as commercial fertilizers.

The earliest record of the use of artificial manures for increasing the yield of crops is contained in a book entitled ‘A Discourse Concerning the Vegetation of Plants.’ The title page also contains the following announcements: “Spoken by Sir Kenelm Digby, at Gresham College, on the 23d of January 1660.” The author advocates the use of saltpetre to increase the yield of crops, and says: “By the help of plain saltpetre, dilated in water and mingled with some other fit earthly substance, that may familiarize it a little with the corn into which I endeavored to introduce it, I have made the barrenest ground far outgo the richest, in giving a prodigiously plentiful harvest.” His dissertation does not, however, show any true conception of the reason for the increase in the crop through the use of this fertilizer. The almost total absence of any knowledge of the composition of plants, and the crude state of chemistry at that time, made this quite impossible.

It was not until 1804 that any light was thrown upon the subject. In that year was published ‘Recherches Chimique sur la Végétation’ by Theodore de Saussure. This brilliant French investigator was the first to appreciate the significance of the ash ingredients of plants; to point out that without them plant life was impossible, and to show that only the ash of the plant was derived from the soil.

Justus von Liebig is commonly regarded as having laid the foundation upon which the commercial fertilizer industry has been built. Certainly his reports to the British Association in 1840 and again in 1842 made the British agriculturists regard as they never had done before the importance of a sufficient supply of certain readily soluble mineral substances in the soil. He supported the contention of De Saussure regarding the importance of mineral matter in the plant, and its extraction from the soil. He refuted the theory, at that time popular, that plants absorbed their carbon from humus, but made the mistake of attaching little importance to the presence of humus in the soil. He showed the importance of potash and phosphates in manures, and in his earlier writings spoke of the value of nitrogen for fertilizing crops, but afterward made the mistake of denying the usefulness of nitrogenous manures for plants, holding that the ammonia washed down by rain affords a sufficient supply.

By the middle of the 19th century it was well understood that potash, phosphoric acid and nitrogen were valuable constituents of fertilizers, and that the other mineral elements of plant-food, with the occasional exception of calcium, were always present in sufficient quantity in arable soils. It has since been shown that the family of plants known as the Leguminosæ have the ability to secure a large part, at least, of their nitrogen from the air, and that as these plants are very rich in nitrogen they leave a supply of nitrogen in organic matter in the soil when plowed under.

Complete and Incomplete Fertilizers. — Fertilizers, containing nitrogen, phosphoric acid and potash, mixed together so that an application of the preparation will result in bringing each of these substances in contact with the soil wherever applied, are called complete fertilizers. The proportions in which the constituents are mixed vary with the different brands and factories. Sometimes a fertilizer of this kind will be advertised for use on a certain crop, and will contain the nitrogen, phosphoric acid and potash in the proportion in which the manufacturer believes they will produce the best yields of that crop. He has, however, no means of knowing the requirements of the soils on which the fertilizer is to be used.

Incomplete fertilizers contain only one or two of the customary fertilizer ingredients.

Available and Unavailable Fertilizer Material. — The fertilizing constituents in a fertilizer may be present in a readily soluble or difficultly soluble condition, depending upon the chemical combinations and, to some extent, upon the physical condition in which they are found. Thus phosphoric acid when in the form of phosphate rock as it is taken from the deposits is a very difficultly soluble substance, and plants can avail themselves of it only in small quantities. After the same rock has been treated with sulphuric acid the phosphoric acid is in a form in which it can be readily used by plants. To a more limited extent the same is true of the phosphoric acid in basic slag, which, when the slag is in an unground condition, is not of much use to plants, but when the slag has been finely ground furnishes a valuable form of plant-food.

When the fertilizing material is in a condition in which it can be readily used by the plant it is said to be “available,” and when it can be used only with great difficulty it is said to be “unavailable.” It is important that the purchaser should know in what form the constituents are present in a fertilizer.

Nitrogenous Fertilizers. — Nitrogen is absorbed by most plants only in the form of a soluble salt; hence for the cereals and many other crops it is desirable either to have it in this form or to incorporate it in the soil in a condition in which it will be readily converted into a soluble condition. Plants like the clovers, alfalfa, peas, beans, etc., have the power of using the nitrogen of the air, and hence do not require nitrogenous fertilizers in such large amounts. These plants may be used to increase the supply of nitrogen in the soil. Nitrogen is the most expensive constituent of fertilizers, and is extremely important, as it is used in large amounts by plants and is likely to be deficient in soils. The form in which nitrogen is present in a fertilizer may make a great difference in its value and in the way in which it can be best applied. Nitrogenous fertilizers differ in having their nitrogen either in the form of a soluble salt or combined as organic material.

Nitrate of Soda. — This now constitutes one of the principal sources of inorganic nitrogen in artificial manures, having largely taken the place of Peruvian guano, which was used for many years, and the profitable use of which did much to promote the extensive use of commercial fertilizers. The salt occurs in the crude condition called caliche, in enormous deposits, principally in the province of Tarapaca in northern Chile. The earlier records of these deposits extend back to 1820, but it was not until several years later that they were exploited. The portion of the country in which the deposits lie is a sandy desert where rain never falls. The altitude is 3,000 to 4,000 feet above the sea-level. The deposits have been attributed to the action of minute organisms fixing in the soil, through countless ages, the nitrogen of the atmosphere, and leaving the product finally in the form of sodium nitrate that has crystallized out of solution in which it has at some time been held. Several other theories have been advanced but no wholly adequate explanation has yet been offered.

The raw product is found beneath a covering consisting of two layers, the upper one of sand and gypsum and the lower of baked clay and gravel. The thickness of the caliche varies from a few inches to 12 feet. It is extracted by boring through the upper layers and introducing a charge of blasting-powder, which, when fired, exposes a considerable quantity of the material. The nitrate is somewhat purified by crystallization before shipping. Iodine is a by-product in the purification process. The nitrate, when ready for shipment, contains about 96 per cent sodium nitrate, or about 16 per cent of nitrogen, 2 per cent of water, and small amounts of sodium chloride, sulphates, and insoluble matter.

The development of the nitrate industry may be appreciated from the fact that the product, which amounted in 1884 to 550,000 tons, increased to 1,000,000 tons in 1890, and to 1,660,000 tons in 1907. The output is exported almost entirely to Europe and the United States. It is held by persons who have examined these deposits that at the present rate of mining they will be exhausted by the year 1930, in which case a serious deficiency in fertilizer nitrogen will occur, unless a new supply shall meanwhile have been discovered.

Because of its easy availability, sodium nitrate acts quickly in inducing growth. For this reason it is used much by market gardeners, and for other purposes when a rapid growth is required. It is the most active form of nitrogen, with the possible exception of calcium nitrate. A light dressing on meadow land in early spring assists greatly in hastening growth by furnishing available nitrogen before the conditions are favorable for making available the more inert nitrogen of the soil. On small grain a similarly useful purpose is served where the soil is not rich.

Owing to the fact that nitrate is not absorbed by the soil in large quantities, it is easily lost in the drainage water; for this reason it should be applied only when crops are growing on the soil.

Ammonium Sulphate. — When coal is distilled a portion of the nitrogen is liberated as ammonia, and is found in the gas and in the ammoniacal liquor which condenses when the gas is cooled. Coal is distilled commercially in coal-gas plants and in by-product coke-oven works, the latter being the larger in tonnage. In either case the ammonia is recovered from the gas by washing it with water or with dilute sulphuric acid. Where water is used ammoniacal liquor is produced, this being a mixture of various ammonia compounds, among them the carbonate, sulphide, hydrosulphide, cyanide, sulphate, sulphite, thio-sulphate and chloride. The first four of them are classed as volatile because they give up their ammonia on boiling, whereas the remaining ones do not. The liquor is treated by steam distillation and the ammonia is driven off, the non-volatile portion being freed by adding lime. The ammonia gas then passes to a saturating box where it is bubbled through dilute sulphuric acid and sulphate of ammonia is formed. This is dipped from the box with long-handled copper ladles, drained and dried in a centrifugal, and is then ready to bag for shipment.

Where the washing of the gas is done with sulphuric acid, the sulphate is formed directly, and may be dried and bagged without further treatment. This process requires less apparatus and produces an equally high-grade of product.

Chemically pure sulphate of ammonia contains 21.2 per cent of nitrogen. The commercial product contains about 20 per cent of nitrogen. It is the most concentrated form in which nitrogen can be purchased for use as a fertilizer. Its effect on crops is not so rapid as that of sodium nitrate, but it is not so quickly carried from the soil by drainage water, as the ammonium salts are readily absorbed by the soil. A pound of nitrogen in the form of ammonium sulphate has about the same agricultural value as the same amount in the form of nitrate of soda if the soil on which it is used is abundantly supplied with lime; but on an acid soil ammonium sulphate has less value.

The long and extensive use of ammonium sulphate on a soil has a tendency to produce an acid condition, through the accumulation of sulphates which are not largely taken up by plants. Ammonium sulphate, like sodium nitrate, should not be applied in autumn, as the ammonia is converted into nitrates and leached from the soil.

Chemical Processes for the Fixation of Nitrogen. — There are now several chemical processes for the fixation of atmospheric nitrogen, two of which are well known and were in use on a commercial scale before the outbreak of the present war, the others are more recent, and exact knowledge regarding them is rather difficult to obtain.

Calcium Cyanamide. — The fertilizers produced by the older processes are calcium cyanamid and calcium nitrate. The former is sold in this country under the trade name “Cyanamid.” One process for its production consists in passing nitrogen into closed retorts containing powdered calcium carbide heated to a high temperature; the product being calcium cyanamide and free carbon: —


In preparing the calcium carbide for this process it must be ground out of contact with air. The nitrogen required for the process is obtained either by passing air over heated copper, or by the fractional distillation of liquid air.

The fertilizer, as placed on the market, is a heavy black powder or granulated material with a somewhat disagreeable odor, the dark color being due to the carbon set free in the process and which remains distributed in the cyanamid.

Cyanamid as manufactured in this country has about the following composition:

Per cent
Calcium cyanamide          CaCN2 45.92
Calcium carbonate CaCO3 4.04
Calcium sulphide CaS 1.73
Calcium phosphide Ca3P2 0.04
Calcium hydroxide Ca(OH)2 26.60
Free carbon C 13.14
Iron and aluminia R2O3 1.98
Silica SiO2 1.62
Magnesia MgO 0.15
Combined moisture 3.12
Free moisture H2O 0.35
Undetermined 1.31


According to this composition the material would contain 16 per cent of nitrogen. Lime in the form of carbonate and hydroxide would add something to its value, and the residue of calcium cyanamide, which upon decomposition becomes calcium hydroxide, is likewise beneficial to the soil.

Calcium cyanamide must decompose in the soil before its nitrogen becomes available to plants. There are several steps in the decomposition process by which the nitrogen finally emerges in the form of ammonia. The intermediate products formed during decomposition will vary somewhat with the condition of the soil. When, for instance, a soil is saturated with water, or when it is acid some more or less injurious substances may be formed. For this reason “cyanamid” is not likely to be so satisfactory on soils of this nature as on better soils. To very sandy soils it is not well suited. Under good conditions its fertilizing value is not greatly below that of sodium nitrate and is about equal to that of ammonium sulphate when not used in heavy applications.

It should be incorporated with the soil at least a week before planting, as it may injure the young plants if decomposition has not proceeded far enough to remove its somewhat toxic properties. As it must undergo this decomposition before its nitrogen becomes available to young plants, there is an added reason for this precaution. It does not give its best results as a top-dressing because it requires incorporation with the soil for its proper decomposition.

Other Fixation Processes. — The first commercial process for the fixation of nitrogen was the arc process. Its use, however, is confined to Norway where 250,000 kilowatts of electricity are employed in its use. By this process nitrogen and oxygen are directly burned to nitric acid under the influence of the electric arc. The resulting product is nitric acid, which is neutralized with lime forming calcium nitrate, a fertilizer somewhat similar to nitrate of soda. Owing to the enormous electrical power required the operation is an expensive one unless the electricity can be cheaply developed, as when water power is abundantly available, which is the case in Norway. It does not seem likely that the process will be extensively adopted in this country.

There are several very promising processes that have been brought forward since the beginning of the present war, but which on account of the great demand for explosive combinations of nitrogen have not been applied to the production of fertilizers.

Organic Carriers of Nitrogen. — Organic nitrogenous fertilizers include cotton-seed meal (7 per cent nitrogen when free from hulls), linseed meal (5.5 per cent nitrogen), castorpomace (6 per cent nitrogen), and a number of refuse products from packing houses, consisting of red-dried blood (13 per cent nitrogen), black-dried blood (6 to 12 per cent nitrogen), dried meal (13 per cent nitrogen), hoof-meal (12 per cent nitrogen), ground fish (8 per cent nitrogen), and tankage (concentrated 10 to 12 per cent, crushed, 4 to 9 per cent nitrogen), also leather-meal, and wool-and-hair waste, which last two on account of their physical condition are of little value.

The forms in which combined nitrogen is available to most agricultural plants are nitrates, ammonium salts and certain organic compounds. Of the latter the simpler compounds, like urea, appear to be most readily taken up by plants. Decomposition is therefore a necessary process for most of the organic fertilizers before their nitrogen becomes available, and their usefulness is, in general, proportional to the readiness with which aerobic decomposition proceeds, or to the proportion of available compounds which they contain in their original condition. Guano, for instance, apparently, contains much nitrogen that is available without further decomposition. Dried blood quickly decomposes and soon forms available substances. In fact, it produces results more quickly than any other form of organic nitrogen.

Dried meat contains a high percentage of nitrogen, but does not decompose so readily as dried blood, and is not so desirable a form of nitrogen. Hoof meal while high in nitrogen, decomposes slowly, being less active than dried blood. Ground fish is an excellent form of nitrogen, and is as readily available as blood but has a lower nitrogen content. Tankage is highly variable in composition, and the concentrated tankage being more finely ground, undergoes more readily the decomposition necessary for the utilization of the nitrogen. Crushed tankage contains from 3 to 12 per cent of phosphoric acid, in addition to its nitrogen. Leather meal and wool-and-hair waste when untreated are in such a tough and undecomposable condition that they may remain in the soil for years without losing their structure. They are therefore not to be recommended as fertilizers.

Phosphate Fertilizers. — By the term phosphate fertilizers is meant those substances that are used for manures chiefly because of the phosphorus they contain. The phosphorus is generally present in the form of a phosphate of lime, iron and alumina mixed with other substances. Some of these phosphates also contain organic matter, in which case they generally carry some nitrogen, which adds something to their value as fertilizers. Phosphates associated with organic matter decompose more quickly in the soil than purely mineral phosphates, and are therefore more readily available fertilizers.

Bone-Phosphate. — The bones of anirnals have been for a great many years an important form of phosphate manure. Formerly bones were used entirely in the raw condition either ground or unground. In the ground condition they are a more quickly acting fertilizer. Raw bones contain about 22 per cent phosphoric acid and 4 per cent nitrogen. The phosphoric acid is in the form of tricalcium phosphate.

At the present time most of the bone used as manure is first boiled or steamed. This frees it from fat and nitrogenous matter both of which are used in other ways. Steamed bone is a more valuable fertilizer than raw bone, as the fat in the latter retards decomposition and also because the steamed bone is in a better mechanical condition. The form of the phosphoric acid is the same as in the raw bone and varies from 28 to 30 per cent, while the nitrogen is reduced to about 1½ per cent. Bone tankage, which has already been spoken of as a nitrogen fertilizer, contains from 7 per cent to 9 per cent phosphoric acid, largely in the form of tricalcium phosphate. All of these bone-phosphates are slow-acting fertilizers and should be used finely ground and for the permanent benefit of the soil rather than as direct plant-food material.

Mineral Phosphates. — There are many natural deposits of mineral phosphates scattered over the earth, some of the most important of which are in America. The phosphorus in all of these is in the form of tricalcium phosphate, but the materials associated with it vary greatly.

Coprolites are concretionary nodules found in the chalk or other deposits in the south of England and in France, the name having been given them on the assumption that they consisted of fossilized animal excrement. They contain 25 to 30 per cent of phosphoric acid, the other constituents being calcium carbonate and silica.

Apatite is found in large quantities in the provinces of Quebec and Ontario, Canada. It occurs chiefly in crystalline form. The calcium phosphate of which it is composed is in one form associated with calcium fluoride and in the other with calcium chloride. The Canadian apatite contains about 40 per cent phosphoric acid, being richer in this ingredient than that found elsewhere. Phosphorite is another name for apatite, but is chiefly applied to the impure amorphous form.

South Carolina phosphate occurs in the form of lumps from the size of a pebble to a mass weighing a ton. These are distributed through a deposit varying from 1 to 20 feet in thickness, which occurs both on the land and in the river beds. It contains from 26 to 28 per cent of phosphoric acid and but a very small amount of iron and aluminum. As these substances interfere with the manufacture of superphosphates from the rock, their presence is very undesirable, rock containing more than 3 to 6 per cent being unsuitable.

Florida phosphates are found in the forms of soft phosphate, a whitish product somewhat resembling clay and largely contaminated with it; pebble phosphate, consisting of hard pebbles; rock or boulder phosphate, consisting as the name implies of rocks or boulders. Soft phosphate contains from 18 to 30 per cent of phosphoric acid and on account of its being more easily ground than most of these rocks is applied to the land without being first converted into a superphosphate. Pebble rock constitutes the major portion of the Florida phosphate. It contains from 20 to 40 per cent of the phosphoric acid, being very variable in composition. Rock or boulder phosphate is much more uniform in composition than the other kinds, but there is less of it. It contains about 40 per cent of phosphoric acid.

Tennessee phosphate differs from the Carolina and Florida phosphate in that it does not exist as nodules, pebbles or boulders, but in veins and pockets and does not need to be washed previous to its treatment. It contains from 30 to 35 per cent of phosphoric acid.

Utah, Wyoming and Montana contain deposits of mineral phosphate interbedded among limestones and shales with amount estimated at nearly six billion tons. At present they are not producing much, the output in 1916 being only 1,700 tons. This is because of the small local demand for fertilizer and high cost of transportation to distant markets. In Idaho there is high grade phosphate on both sides of Blackfoot River, in Fort Hall Indian Reservation, near Montpelier and north of Bear Lake. In Utah the deposits are in the Wasatch, Oquirrh and Uinta ranges and east of Bear Lake. In western Wyoming the deposits are mostly in Wind River, Gros Ventre, Salt River and Owl Creek ranges, and in Montana the deposits are near Ellison, Garrison, Philipsburg and Melrose. The material is generally an even-bedded gray to brown or black oolitic rock, closely resembling limestone, and emitting a fetid odor when struck with a hammer. Weathered surfaces have a light-bluish or white coating commonly in reticulated pattern. Many of these deposits carry 65 to 81 per cent of tricalcium phosphate and are four to six feet thick. On places there are two or more beds and they extend for many miles. Most of the areas underlain by the thicker beds are government land, parts of which have been withdrawn from entry in order to preserve them for a reserve which can be developed for the use of the people.

Basic slag, or as it is also called, phosphate slag, or Thomas phosphate, is a by-product in the manufacture of steel from pig iron rich in phosphorus. The phosphoric acid present is in the form of tetracalcium phosphate (CaO)4P2O5. It also contains calcium, magnesium, aluminum, iron manganese, silica and sulphur. On account of the presence of iron and aluminum and because its phosphoric acid is more readily soluble than the tricalcium phosphate, the ground slag is applied directly to the solid without treatment with acid. It is produced in large quantities in England, Germany and France.

Superphosphate Fertilizers. — In order to render more readily available to plants the phosphoric acid contained in bone and mineral phosphates, the raw material, purified by being washed and finely ground, is treated with sulphuric acid. This results in a replacement of phosphoric acid by sulphuric acid with the formation of monocalcium phosphate and calcium sulphate, with a smaller amount of dicalcium phosphate, according to the reactions:

Ca2(PO4) + 2H2SO4 = CaH4(PO4)2 + 2CaSO4


Ca3(PO4)3 + H2SO4 = Ca2H2(PO4)3 + CaSO4.

The tricalcium phosphate, being in excess of the sulphuric acid used, a part of it remains unchanged.

In the treatment of phosphate rock part of the sulphuric acid is consumed in acting upon the impurities present, which usually consist of calcium and magnesium carbonates, iron and aluminum phosphates and calcium chloride or fluoride, converting the bases into sulphates and freezing carbon dioxide, water, hydrochloric acid and hydrofluoric acid. The resulting superphosphate is therefore a mixture of monocalcium phosphate, dicalcium phosphate, tricalcium phosphate, calcium sulphate and iron and aluminum phosphates.

In the superphosphates made from bone the iron and aluminum sulphates do not exist in any considerable amounts. However, as long as the phosphoric acid remains in the form of monocalcium phosphate the value of a pound of available phosphoric acid in the two kinds of fertilizer is the same, but the remaining tricalcium phosphate in the bone superphosphates has a greater value as before explained.

The superphosphates made from animal bone contain about 12 per cent available phosphoric acid and 3 or 4 per cent of insoluble phosphoric acid. They also contain some nitrogen. Bone-ash and bone-black superphosphates contain practically all of their phosphoric acid in an available form, but they contain little or no nitrogen. South Carolina rock superphosphate contains from 12 to 14 per cent available phosphoric acid, including from 1 to 3 per cent reverted phosphoric acid. The best Florida superphosphates contain from 17 per cent downward of available phosphoric acid, part of which is reverted. The Tennessee superphosphates vary from 14 to 18 per cent available phosphoric acid.

Reverted Phosphoric Acid. — On standing, a change sometimes occurs in superphosphates by which a part of the phosphoric acid becomes less easily soluble and to that extent the value of the fertilizer is decreased. This change, known as “reversion,” is much more likely to occur in superphosphates made from rock than in those derived from bone. It also varies in different samples, a well-made article usually undergoing little change even after long standing. It is supposed to be caused by the presence of undecomposed tricalcium phosphate and of iron and aluminum sulphates.

The decrease in solubility of the superphosphate is due in part at least to the interaction of monocalcium phosphate and tricalcium phosphate leading to the formation of dicalcium phosphate and perhaps to the formation of ferric and aluminum phosphates by the action of monocalcium phosphate upon the iron and aluminum sulphates.

Double Superphosphates. — In making superphosphates, a material rich in phosphoric acid must be used, not less than 60 per cent tricalcium phosphate being necessary for their profitable production. The poorer materials are sometimes used in making what are known as double superphosphates. For this purpose they are treated with an excess of dilute sulphuric acid; the dissolved phosphoric acid and the excess of sulphuric acid are separated from the mass by filtering and are then used for treating phosphates, rich in calcium phosphate, forming superphosphates. The superphosphates so formed contain more than twice as much available phosphoric acid as those made in the ordinary way.

Relative Availability of Phosphate Fertilizers. — Superphosphates and double superphosphates contain their phosphoric acid in a form in which it can be taken up by the plant at once. They are therefore best applied at a time when the crop is planted or just before, or they may be applied when the crop is growing. Well-made superphosphates contain no free acids and therefore are not injurious to vegetation unless used in excess.

Reverted phosphoric acid, although not soluble in water, is readily soluble in dilute acids. Its value as a ready source of food for plants has been the subject of much discussion among agricultural chemists. It is now quite generally believed that it furnishes an available supply of phosphoric acid to the plant.

Crude phosphate rock is sometimes used as a fertilizer without being treated with acid. Owing to the slight solubility of this material it acts slowly and should be applied in very large quantities — enough to last several years. It should also be very finely ground. It appears to be used more profitably on the rich prairie soils of the Middle West than in the Eastern States. Experiments in Indiana, Ohio and several States further east have given results unfavorable to the use of ground phosphate rock, or “floats” as they are sometimes called.

Potash Fertilizers. — The production of potash fertilizers is largely confined to Germany, where are inexhaustible beds varying from 50 to 150 feet in thickness lying under a region of country extending from the Harz Mountains to the Elbe River and known as the Stassfurt deposits. These deposits of potash salts have been mined since 1862, during which time they have constituted the world's principal supply of potash. They are operated by a syndicate comprising all the large owners and maintaining a uniform price on the output. The principal salts placed on the market are sylvine (potassum chloride); sylvinite, a mixture of sylvine, rock-salt and kainite; carnallite (magnesium chloride and potassium chloride); schoenite (magnesium sulphate and potassium sulphate); kieserite (magnesium sulphate with carnallite); kainite (magnesium sulphate and potassium chloride, or magnesium and potassium sulphates and magnesium chloride); polyhalite (potassium, calcium and magnesium sulphates).

The potash fertilizers contain their potassium either as chloride or sulphate. The chloride has the advantage of being more diffusible in the soil but in most respects the sulphate is preferable. Potassium chloride has an injurious effect upon such crops as tobacco, sugar-beets and potatoes. In tobacco the potassium chloride in the ash prevents the proper burning of the tobacco. In the case of clover, corn and grass, however, potassium chloride appears to have no harmful effects.

Kainite. — The potash contained in kainite is in the form of sulphate, but because of the large quantities of other salts it contains, chiefly sodium and magnesium chloride, it has the same effect upon plants as has the chloride. It contains from 12 to 20 per cent of potash and 25 to 45 per cent of sodium chloride, with some chloride and sulphate of magnesium. Kainite should be applied to the soil a considerable time before the crop which it is intended to benefit is planted. It should not be drilled in with the seed. By this method of application the injurious properties of the chlorides will not affect the crop. Indirectly kainite serves to supply the plant with food other than potash. The salts associated with the potash, particularly the chlorides, aid plant-growth by making other substances in the soil, particularly phosphoric acid, more available to the plant, besides improving the physical condition of the soil.

Sylvinite. — This salt contains its potash both as chloride and as sulphate. It also contains sodium and magnesium chlorides. It contains about 16 per cent potash. Owing to the presence of chlorides it has the same effect as kainite.

Muriate (Chloride) of Potash. — This is a more concentrated form of potash than either of those just mentioned. It contains about 50 per cent of potash, making it 80 per cent potassium chloride. The impurities are largely sodium chloride and insoluble mineral matter. A purer form of muriate of potash may be obtained which contains 98 per cent pure potassium chloride.

Sulphate of Potash. — High-grade sulphate of potash contains from 49 to 51 per cent of potash. Its advantage over muriate is that it is not, like the latter, injurious to any crops; but it has the disadvantage of being more expensive.

Schoenite. — This is a double sulphate of potash and magnesium. It contains about 26 per cent of potash. The cost of the potash is greater than in the muriate. The sulphate of magnesium it contains is supposed to have a beneficial effect upon the soil.

Potash Production in the United States. — The potash resources of the United States are most inadequate to meet the present needs owing to the loss of the German supply from the autumn of 1914 to the present (1918). In 1911 this country imported, mainly from Germany, 672,639,518 pounds of potash salts valued at $11,826,106. There have recently been many efforts made to obtain potash in a form suitable for use as a fertilizer. Wood ashes constituted about the only domestic source of potash supply previous to 1914. At present the largest quantity comes from some of the alkali lakes of the arid or semi-arid regions, the waters of which carry enough potash in solution to make its recovery profitable at present prices.

Another source of potash is the kelp or giant sea weed that grows in the Pacific Ocean from Lower California to Alaska. The ash of the plants contain about 30 per cent of potash. Machinery has been built for harvesting the plants and the operation of recovering potash has been conducted on a small scale. It is not likely, however, that the quantity of potash secured in this way will ever constitute any large part of the total amount required by the fertilizer industry of this country.

Alunite, a hydrous sulphate of potash and alumina, is a possible source of potash and one that has been worked in a small way. Several processes for the extraction of potash from this mineral have been proposed and the recovery of potassium sulphate appears to be a rather simple matter.

Another enterprise that has been tried with some degree of success is the recovery of potash from the flue dust of Portland cement plants, some potash bearing rocks or minerals being added to the raw products used in the burning process.

The extraction of potash from silicate rocks has received a great deal of attention during the last few years. Among the materials used experimentally for this purpose are feldspar, of which there are small deposits scattered widely over the United States; leucite, occurring as one constituent of a rock of which immense quantities are found in Wyoming; sericite, in extensive beds in Georgia, and green sand, of which large deposits are found in New Jersey.

By application of heat and fluxes to lower the melting point of the slag, it has been found possible to convert almost all of the potash into a form soluble in 2 per cent citric acid or into a condition considered to be available to plants. One distinct disadvantage which these silicates possess is that their content of potash is low and much material must be handled in order to obtain the rather small percentage of potash.

By means of these various processes there were produced in the United States during 1916 a total quantity of 20,000,000 pounds of potash salts, valued at $3,500,000.

Other Fertilizing Materials. — There are a number of fertilizers not included in the classes already mentioned. Many of them are very valuable sources of fertility.

Wood Ashes. — For some time after the use of fertilizers became an important farm practice, wood ashes constituted a large portion of the supply of potash. They also contain a considerable quantity of lime and a small amount of phosphoric acid. The product known as unleached wood ashes contains 5 to 6 per cent of potash, 2 per cent of phosphoric acid and 30 per cent of lime. Leached wood ashes contain about 1 per cent of potash, 1.5 per cent of phosphoric acid and 28 to 29 per cent of lime. They contain the potash in a readily available form as potassium carbonate. The lime conduces to a good physical condition of the soil.

Guano. — This fertilizer has been formed from the excrement and carcasses of sea-fowl. The composition of guano depends upon the climate of the region in which it is found. Guano from an arid region contains nitrogen, phosphoric acid and sometimes potash, while that from a region where rains occur contains only phosphoric acid, the nitrogen and potash having been leached out. In a dry guano the nitrogen occurs as uric acid, urates and in small quantities ammonium salts. A damp guano contains more ammonia. The phosphoric acid is present as calcium phosphate, ammonium phosphate and as phosphates of other alkalies. A portion of the phosphate is readily soluble in water. All of the plant food is thus either directly available or becomes so soon after admixture with soil. The composition is extremely variable. Guano was formerly a very important fertilizing material but the supply has become so nearly exhausted that it is relatively unimportant at the present time. South America, South Africa, Australia and certain islands in the Pacific contribute to the supply. Other fertilizers of less value are powder-waste, agricultural salt, cotton-hull ashes, coal ashes, seaweed, king-crab, mussel and lobster shells, muck, peat, marl, tobacco stems and stalks, crude fish-scrap, wool and hair waste, sewage, street sweepings, etc.

Catalytic Fertilizers. — The term catalytic fertilizer has been used rather loosely to designate a class of substances that, when added to a soil, increase plant growth by apparently accelerating the processes that normally take place in soils. They do not really function as fertilizers because their value does not lie in the nutrients that they possess. The term catalysis is employed in a chemical sense to mean a change brought about in a compound by an agent that itself remains stable. When an attempt is made to study these phenomena in soils, it becomes difficult, owing to the multiplicity of factors and reactions, to determine whether the agent is acting in a purely catalytic manner.

A large number of substances have been found to act as catalytic fertilizers. Among these are various salts of manganese, iron, aluminum, zinc, lead, copper, nickel, cobalt, uranium, boron, cerium, lanthanum, and the like. These substances when used in small quantities stimulate plant growth, but are toxic in large amounts.

Of the various plant stimulants mentioned manganese is the only one that gives promise, at the present time, of usefulness on a commercial basis. The manganese salts that have been found to be effective as fertilizers are the sulphate, chloride, nitrate, carbonate and dioxide. Of these the first has been most often used, and in quantities up to 50 pounds an acre it has in most cases not been toxic. It is supposed to be most effective on poor soils that are not acid.

Lime. — This substance is not a fertilizer in the same sense as are the materials previously discussed. It is because of its effect on the soil rather than directly on the plant that lime is applied to soils. The forms in which it is used are (1) ground limestone, (2) marl, (3) air-slaked lime, (4) quick-lime, (5) water-slaked lime.

Lime is applied to soils to correct what is called acidity of the soil, which arises from a large removal of basic material by crops and drainage water and to other minor causes. This condition is widespread in soils and is increasing so that the agricultural lime industry is growing rapidly. The tendency in the business at the present time is to produce ground limestone rather than burnt lime. The ground limestone is somewhat more bulky to transport, but it is much easier for the farmer to handle and it may be kept without danger of heating. Owing to its bulk and the large quantities applied per acre lime is not usually transported long distances, and this is one reason why the grinding mills are scattered so widely over the regions in which limestone is found. Dolomitic limestone may be used on most soil with profit, but the trade usually prefers a lime with only a small percentage of magnesia.

Home Mixing of Fertilizers. — Fertilizers are ordinarily sold as (1) complete fertilizers containing nitrogen, phosphoric acid and potash, which in different mixtures are to be found in different relative amounts; and (2) incomplete fertilizers, or raw material containing only one of these ingredients. Authorities at a number of the experiment stations have urged that it is in many cases more profitable for the farmer to purchase the incomplete fertilizers and mix them himself. The arguments advanced in favor of this practice are that the materials can be purchased more cheaply in that form than when mixed by the manufacturer; that they can be used in the proportion desired, which may effect a saving in the amount of some ingredient, and that the form in which each ingredient is present is then known, which is not always the case when mixtures are purchased.

On the other hand, it is more difficult to obtain the raw materials from the small dealer, and when purchased they are not generally so finely ground as they are in the mixtures.

State Control of Fertilizer Sales. — In most States in which fertilizers are largely used laws are in effect compelling manufacturers or dealers in fertilizers to state the actual amounts of the fertilizing ingredients contained in the fertilizers offered for sale, and also to state in what form each of these ingredients exist. A chemical control is provided and a penalty imposed for any failure to comply with the law. The control authorities publish each year a statement of the commercial value per pound of nitrogen, phosphoric acid and potash in each of the various forms in which they occur in fertilizers. Fertilizer control is in most States attached to the State agricultural experiment stations, and information concerning the purchase and use of fertilizers may be obtained from these institutions

Statistical. — The quantity of fertilizers manufactured and consumed is increasing in all portions of the crop-producing world; and especially in the United States there has been a rapid increase from one decade to another. In 1859 the value of the entire output of the fertilizer factories was $891,344; in 1869, $5,815,118; in 1879, $23,650,795; in 1889, $39,180,844; in 1899, $44,657,385 and in 1909, $103,960,213. Not all of this is used on farms in this country, there being $8,700,640 worth exported in 1910. The imported fertilizer materials consisted largely of nitrate of soda and potash salts, while the exported fertilizers were animal refuse and phosphate fertilizers.

Although the commercial fertilizer industry if little more than half a century old, the sale of fertilizers in the United States in 1909 amounted to $114,883,000, an increase of 115 per cent as compared with the expenditure in 1899. This large quantity of fertilizer was largely used in a rather narrow strip of country lying along the Atlantic seaboard. The South Atlantic states show a higher rate of increase in expenditures for fertilizers between 1899 and 1909 than any other, such increase amounting to 162.3 per cent, but Ohio shows an increase of 155 per cent and Indiana of 141 per cent for the same period, which indicates a westward movement of the practice of using fertilizers.

As is naturally to be expected, the first settled and longest cultivated portion of the country is that using the greatest quantity of fertilizers. This region lies nearest to the large centres of population and hence of consumption; it produces crops of greatest acre value; and it is so situated that the distances which it is necessary to transport fertilizers are generally less than they would be for the non-fertilizer-using portion of the country. It is true in the United States, as it is in Europe, that a large consumption of fertilizers usually goes hand in hand with a highly developed and intensive system of agriculture.

T. Lyttleton Lyon,
Professor of Soil Technology, Cornell University.