Chapter 30

THE POSITION OF PHOSPHORIC ACID.

We now come to consider the position of phosphoric acid in agriculture. The question is, however, very much simpler in its nature than that of nitrogen, and may be consequently discussed in a much shorter space.

Most soils, as we have already had occasion to point out, are better supplied with available ash-plant ingredients than available nitrogen compounds. The quantity of phosphoric acid absorbed by the plant is also less than that of nitrogen; and lastly, the different chemical compounds of phosphoric acid occurring in the soil are not nearly so numerous as those of nitrogen. Phosphoric acid, however, must be regarded as ranking next to nitrogen in its importance as a soil-constituent.

_Occurrence of Phosphoric Acid in Nature._

That phosphoric acid is of universal occurrence may be assumed from the fact of the almost universal occurrence of vegetable life on the earth's surface; for plants are unable to grow without it. While thus of practically universal occurrence, its amount in most soils is very trifling. As the only source of it in the soil is from the disintegration of the different rocks, a short description of its occurrence in the mineral kingdom may first be given.

_Mineral Sources of Phosphoric Acid._

It was first discovered in the mineral kingdom towards the close of last century; but we have only of late years ascertained any exact knowledge of its percentage in the different rocks out of which soils are formed. This has been shown in many cases to be very trifling. It most abundantly occurs as _apatite_, a mineral consisting of calcium phosphate, with small quantities of calcium fluoride or calcium chloride. This apatite, or phosphorite, is found in certain parts of the world in large masses; but as a rule, it only occurs in small quantities in most rocks. It may be stated that the older rocks are, as a general rule, richer in it than those of more recent formation; and Daubeny has drawn attention to this fact as furnishing a useful guide in estimating the probable richness of a soil in phosphoric acid. The older, therefore, a rock is, the richer it is likely to be in phosphoric acid.

_Apatite and Phosphorite._

Of apatite there are a variety of kinds, which differ in their appearance as well as in their composition. It occurs chiefly in a crystalline form, and is found sometimes in regular crystals, but it also occurs in the amorphous form. In colour it may be white, yellow, brown, red, green, grey, or blue. Two classes of apatite are found. The first consists of calcium phosphate along with calcium fluoride; and in other kinds of apatite the calcium fluoride is replaced by calcium chloride. Phosphorite is another name for apatite, but is chiefly applied to impure amorphous apatite. The percentage of phosphate of lime in different kinds of apatite may be stated at from 70 to 90 per cent. It occurs in very large quantities in Canada, the Canadian apatite being very rich in phosphate of lime--80 to 90 per cent. In many parts of the world it forms portions of mountain-masses, and is quarried, crushed, and used for artificial manurial purposes. Further details of its occurrence and chemical composition will be found in the Appendix.[123]

_Coprolites._

In many parts of the world round nodules, largely consisting of phosphate of lime, have been found, to which the name "coprolites" has been given, on the assumption that they consisted of fossilised animal excrements. These coprolites, or osteolites as they have also been called, vary in the percentage of phosphate of lime they contain. Sometimes this amounts to 80 per cent, but as a rule it is very much less. They also in the past have formed an important source of manure, and will be referred to subsequently.

_Guano._

We have, lastly, phosphoric acid occurring in large quantities in guano-deposits, chiefly found on the west coast of South America. These deposits, which have been of enormous importance as a source of artificial manure, are of animal origin, and will be discussed at considerable length in a chapter specially devoted to the subject; so that we need do no more than mention them here.

Phosphoric acid is also found in the form of phosphate of lime in certain rocks as "layers" and "pockets."

_Universal Occurrence in Common Rocks._

But while it is thus found in considerable quantities in various parts of the world, and while no anxiety need thus be felt as to its abundance for artificial manurial purposes, its occurrence in the common rocks, which, as we have already pointed out, is practically universal, is in many cases very minute.

Fownes first identified it in the felspathic rocks in 1844; and since then its percentage in granite, lava, trachyte, basalt, porphyry, dolomite, gneiss, syenite, dolerite, diorite, and a number of other rocks, has been determined by numerous investigators. For analyses of these rocks the reader is referred to the Appendix.[124]

_Occurrence in the Soil._

That no soil is actually without phosphoric acid is highly probable, but in many soils it is present in the merest traces, and even in fertile soils it is rarely present in quantities over two-tenths of a per cent; while half that amount may be taken as an average for most fairly fertile soils. This would be about 3500 lb. per acre, calculating the soil to a depth of 9 inches. In exceptional cases it has been found to the extent of .3 per cent; and in the famous Russian _black earth_ it has been found to amount to .6 per cent.[125] Like nitrogen, it is found in greatest amount in the surface portion of the soil, but its amount at different depths does not vary to the same extent as we have found to be the case with nitrogen.

_Condition in which Phosphoric Acid is present in the Soil._

Unlike nitrogen, phosphoric acid occurs in the soil almost entirely in an _insoluble_ form; and when applied to the soil in a soluble form, is speedily converted into an insoluble condition. Its most commonly occurring forms are as phosphates of lime, iron, and alumina. These facts are of importance to remember, as they explain why phosphoric acid is not found in drainage-water in any quantity. It also shows how little the risk of loss from drainage is in the application of artificial phosphatic manure to the soil.

_Occurrence in Plants._

The percentage of phosphoric acid in plants, like other ash-constituents, is subject to considerable variation, and depends on a variety of conditions, such as the state of the plant's development, nature of soil, climate, season, treatment with manures, &c. All these conditions have a certain influence. The different parts of the plant have been found to contain it in different quantities. The tendency of phosphoric acid is to travel up to the higher portions of the plant with the progress of growth, and to finally accumulate in the seed. As illustrating this, it may be mentioned that the inner portion of the stalk of a ripe oat-plant has been found to contain only a seventeenth of the amount of phosphoric acid found in the same portion of the stalk of a young oat-plant. Similarly it may be mentioned that, while the ash of the grain of rye and wheat contains nearly half their weight of phosphoric acid, the percentage present in the ash of other parts of the plant amounts only to from 5 to 16 per cent. The percentage of phosphorus is greater in young plants than in mature plants; it is greater also in quickly developed plants than in slowly developed plants.

In the plant, phosphorus is present chiefly in the albuminoids; and its absorption from the soil takes place in greatest quantity during the period of maximum growth. In beans and peas an oil containing phosphorus has been found.

_Occurrence in Animals._

That phosphorus in different forms exists in animal tissue is well known. It is found both in the brain and in the nerves, as well as in nearly all the fluids of the animal body. It is, however, in the bones that it is most abundant, the mineral portion of which is almost entirely made of phosphate of lime,--a fact which renders bones such a valuable artificial manure. Altogether, phosphoric acid occurs in the animal body to the extent of 2.3 per cent. There is a point which we shall have occasion to draw the student's attention to further on in discussing the nature of farmyard manure--and that is, that the urine of the common farm animals is practically devoid of phosphoric acid.

_Sources of Loss of Phosphoric Acid in Agriculture._

As we have already done in the case of nitrogen, we may now attempt to form some conception of the sources of loss and gain of phosphoric acid in the soil. The sources of loss may be divided into natural and artificial. Of natural sources of loss we have only one, and that is loss by drainage.

_Loss of Phosphoric Acid by Drainage._

We have already seen that the condition in which phosphoric acid is present in the soil is as insoluble phosphate. In drainage-water it occurs in mere traces. Minute though the amount seems when stated as percentage, and small as it appears beside the loss (from the same source) of nitrogen, it is yet, if considered for large areas, sufficiently striking. Thus it has been estimated that in the river Elbe there is carried off by drainage from the fields of Bohemia 2-3/4 million pounds (1200 tons) of phosphoric acid annually. This, it is true, is a very trifling amount compared with the annual loss of nitrogen from an equal area; but then it must be remembered, on the other hand, the sources of gain to the soil of this ingredient are not so numerous as are those of nitrogen, the only sources of phosphoric acid being in the manure applied to the soil, and that coming from the gradual disintegration of phosphatic minerals.

_Artificial Sources of Loss._

The other sources of loss may be classed under the term artificial, and are connected with agricultural practice. Just as we have seen that in the case of nitrogen enormous quantities of that substance are constantly being removed from the soil in those crops which are consumed off the farm, so, too, enormous quantities of phosphoric acid are being removed in the same way. As illustrating this fact, it may be mentioned that Professor Grandeau has recently estimated that in the entire crops grown in France in one year there are about 298,200 tons of phosphoric acid; while the amount returned in the dung of farm animals is only 157,200, or only about one-half of what is removed in the crops, leaving a deficit of 147,000 tons to be made good by the addition of artificial phosphatic manures, if the fertility of the soil is to be maintained. The same authority has calculated that in the bones of the entire farm animals in France there is no less a quantity than 76,820 tons of phosphoric acid.

As an example of how, in many cases, the amount of phosphoric acid removed from the farm is very often much greater than that restored, a case quoted by Crusius may be cited. This was a farm of 670 acres (Saxon) which had received only farmyard manure, and from which, during sixteen years, 985.67 cwt. of phosphoric acid had been sold off in the crops; while only 408.33 cwt. had been restored in the manure, leaving a loss of 577.34 cwt.

_Phosphoric Acid removed in Milk._

A further source of loss is the phosphoric acid removed in milk. In the total annual yield of milk from one cow there may be from 11 to 12 lb. of phosphoric acid.

_Loss in Treatment of Farmyard Manure._

The risks of loss of phosphoric acid in the treatment of farmyard manure are not so great as in the case of nitrogen. There is, however, a considerable risk, through want of proper precautions, of the soluble phosphates being washed away by rain.

_Loss in Sewage._

The loss of phosphoric acid incurred by the present method of sewage disposal is not so large as the loss of nitrogen, inasmuch as the quantity of phosphoric acid contained in human excreta is very much less. Roughly speaking, it may be said to amount to a little less than one-third of the nitrogen lost in this way.

_Sources of Artificial Gain of Phosphoric Acid._

To balance these losses, we have a practically unlimited supply of mineral phosphates for application as artificial manure, as well as large quantities of other manures, many of them already mentioned in connection with nitrogen, such as bones and guanos of all kinds. Quite recently, also, a large source of phosphoric acid has been opened up in the basic slag, a rich phosphatic bye-product obtained in considerable quantity in steel-works from the basic process of steel manufacture. We have also large quantities of phosphoric acid in the imported feeding-stuffs, for statistics regarding which we would refer our readers to a previous chapter. The question of the actual amount contained in these sources is not of the same interest as in the case of nitrogen, and need not therefore detain us. We have sufficiently indicated the importance of phosphoric acid in agriculture by the statements above given. All further consideration of phosphoric acid must therefore be deferred to future chapters.

FOOTNOTES:

[123] See Appendix, Note I., p. 210.

[124] See Appendix, Note II., p. 211.

[125] These results, as indeed all soil percentages, are calculated on the soil in a dry condition.

APPENDIX TO CHAPTER V

NOTE I. (p. 201).

COMPOSITION OF APATITE (Voelcker).

(_Krageroee, Norway._)

Lime 52.16 Phosphoric acid 41.25 Chlorine 4.10 Fluorine 1.23 Oxide of iron 0.29 Alumina 0.38 Potash and soda 0.17 Water 0.42 ------ 100.00 ======

Apatite is found in considerable quantities in America, Germany, France, Spain, Hungary, Norway, and Great Britain. According to Rose, apatite is made up of three molecules of tribasic calcium phosphate (Ca(PO_4)_2), combined with one molecule of calcium fluoride (Ca F_2) or one molecule of calcium chloride (CaCl_2) respectively.

The composition of the pure mineral should be--

_Chlorapatite._ Per cent. Calcium phosphate 89.38 Calcium chloride 10.62

_Fluorapatite._

Calcium phosphate 92.31 Calcium fluoride 7.69

NOTE II. (p. 203).

The following is a list of the commoner rocks in which the percentage of phosphoric acid has been determined. The results are taken from analyses by Nesbit, Schramm, Bergemann, Rose, Deherain, Handtke, Petersen, Nessler, Muth, Fleischmann, Storer, and others:--

Per cent. Felspar 1.7 Granite 0.09 0.25 0.58 0.68 Lava 1.21 1.8 Trachyte 0.30 0.66 Basalt 0.50 1.11 Porphyry 0.26 Marl 1.45 2.31 3.8 Calcareous stones 0.064 0.176 Dolomite 1.24 Lias chalk 1.39 Gneiss 0.18 0.78 1.51 Syenite 0.10 Dolerite 0.3 1.1 1.2 Diorite 0.5 0.69