APPENDIX TO CHAPTER V.
NOTE I. (p. 201).
Composition of Apatite (Voelcker).
(Krageröe, 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 |
| 100.0 |
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(PO4)2), combined with one molecule of calcium fluoride (Ca F2) or one molecule of calcium chloride (CaCl2) 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 |
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, Dehérain, 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 | ||
CHAPTER VI.
THE POSITION OF POTASH IN AGRICULTURE.
We may, lastly, consider the position of potash in agriculture, the only ash ingredient of the plant, in addition to phosphoric acid, which it is as a rule necessary to add as a manure.
Potash of less Importance than Phosphoric Acid.
It is of far less importance than phosphoric acid, from the fact of its much more abundant occurrence in the soil, as well as from the fact that under the ordinary conditions of agriculture, although removed from the soil in considerable quantities by crops, it finds its way back again in the farmyard manure; for it has not the same tendency to accumulate in large quantities in the grain or seed as we have seen to be the case with phosphoric acid. On this account straw contains a much greater proportion of potash than phosphoric acid, and hence farmyard manure may be regarded as fairly rich in potash.
Of all sources of potash the ocean must be regarded as the chief. Millions and millions of tons are present in a state of solution in the salt water of the ocean.[126] Like phosphoric acid, its occurrence in the rocks forming the earth's crust may be said to be practically universal. Many of the commonly occurring rocks and minerals are extremely rich in it, and by their disintegration furnish large quantities to the soil. Some of these rocks contain it in such abundance that they have been tried as potash manures; and were other more valuable sources less available than they actually are, such a practice might well be recommended. A volcanic rock known as palagonite, and that most commonly occurring of all potash minerals—viz., felspar—have both been experimented with in this way with considerable success.
Felspar and other Potash Minerals.
That felspar should prove, when finally ground, a valuable source of potash, is not to be wondered at when we remember that some varieties of it contain over 16 per cent. It has been calculated that a single cubic foot of this mineral is sufficient to supply an oak-wood, covering a surface of 26,910 square feet, with potash for a period of no less than five years.[127] Some idea of the enormous potential fertility of a soil containing felspar, so far as potash is concerned, may be obtained from this statement. It must be remembered, however, that it is only the orthoclase or potash felspars which contain large quantities of potash—other felspathic rocks, such as oligoclase and labradorite, being comparatively poor in it. Another commonly occurring mineral which is rich in potash is mica, which has been found to contain from 5 to 13 per cent. From this it follows that rocks which have large amounts of these minerals in their composition—such as granite, for example, which often contains 5 or 6 per cent of potash—form by their disintegration soils rich in this ingredient.
Stassfurt Salts.
But in addition to the sources of potash already mentioned, it exists in other forms in the earth's surface. Till within recent years it was obtained for commercial purposes from the ashes of plants, which, as we shall immediately see, are extremely rich in this ingredient; from salt water—this source giving rise to the so-called "salt gardens" on the coast of France; and from nitre soils in different parts of India, referred to already at considerable length. Large mineral deposits, however, have been recently discovered in the neighbourhood of Stassfurt in Germany, and have since their discovery supplied all the potash required for manurial and other purposes. In these deposits (similar ones have also been found at Kalusz in the Carpathian Mountains) there are no less than five different minerals which contain potash. The form in which it is present is as sulphate or chloride, so that it is readily available for plants, and is of altogether very much greater value than the form in which it occurs in the minerals already mentioned—viz., as an insoluble silicate. Of the Stassfurt potash salts, the best known as a manure is kainit, which contains about 32 per cent of sulphate of potash. A list of the other potash minerals, with the particulars of their composition and the percentage of potash they contain, will be found in the Appendix.[128]
Occurrence of Saltpetre.
We have already had occasion, in Chapter IV., when discussing the question of nitrification, to refer to the occurrence of nitrate of potash in certain soils in India, which have formed a large source of saltpetre used in commerce in the past.
Occurrence of Potash in the Soil.
From what has been said regarding the richness in potash of certain commonly occurring minerals, such as felspar, it is only natural to infer that most soils must contain large quantities of this substance; and this is so. The wonder is that potash, when applied as an artificial manure, should have such a marked effect in increasing the fertility of the soil, as is often the case. We must remember, however, that although a soil may contain large quantities of potash, there may be a very small percentage of the whole in an available form for the plant's needs.
Potash chiefly in insoluble Condition in Soils.
Potash occurs almost entirely in soils in a very insoluble form—viz., combined with silica as a silicate of potash. It is only by the slow disintegration of potash rocks that the potash they contain is set free for plant uses. When it is applied as an artificial manure, on the other hand, it is in a soluble form. In most soils the amount soluble in water probably lies between .001 and .009 per cent; that soluble in dilute acid solutions from .1 to .5 per cent; and that insoluble from .2 to 3.5 per cent of the soil. It is highly probable that a certain quantity of potash in the soil may exist in combination with humic and ulmic acids, forming insoluble potassium humates and ulmates.
Potash in Plants.
Of all the ash ingredients of plants, potash is the most abundant, as it forms on an average about 50 per cent of the total ash of plants—about 90 per cent of the alkalies. The ash of plants, indeed, was for long the chief source of potash. Certain plants remove very large quantities from the soil. Of these roots, potatoes, the vine, the tobacco-plant, and hops may be mentioned as examples. It is present in large quantities in the grain of cereals, although, as we have already pointed out, not to the same proportional extent as phosphoric acid. It is found in the plant's extremities, such as twigs and new leaves, in greatest abundance.[129]
Potash in the Animal Tissue.
It is also found in all parts of the animal body. Especially rich in potash salts are the blood corpuscles, which contain about ten times the amount contained in the serum. It is found in especial abundance in the fleece of sheep, which may contain more potash than that in the whole body of the sheep. Animal urine also contains potash in considerable quantities.
Sources of Loss of Potash.
The capacity of the soil to retain soluble potash compounds, while not equal to its capacity for retaining phosphoric acid, is yet very much in excess of its capacity for retaining nitrates. The result is, that potash is only found in comparatively minute traces in drainage water.[130] Taking the same example as we already cited in illustration of the loss of phosphoric acid, we find that the amount carried away in the course of a year in the waters of the Elbe from Bohemia is 97,000,000 lb. (43,300 tons).
Potash removed in Crops.
The amount of potash removed by the different crops from the soil will be considered in a subsequent chapter. We need only say here that the class of crops which remove the largest quantity are the root crops, especially mangels. The loss is least in the case of the cereals. The amount of potash contained in the straw of cereals is about three times the amount of that removed in the grain.
Potash removed in Milk.
Lastly, we may refer to the potash removed in milk, which, on an average, may be taken at 10 lb. per annum for each cow.
Potash Manures.
Of potash manures the chief are the sulphate and the chloride, or, as it is commercially known, the "muriate." The chief source of potash manures are the Stassfurt deposits already referred to. Wood-ashes have also been used in large quantities in the past (chiefly as a potash manure), and in some parts of the world are still used. A considerable source of artificial potassic manures is the refuse manufacture of sugar-beet, such a large industry in Germany. Potash occurs as a constituent of certain other manures, more valuable for nitrogen and phosphoric acid, such as guano and dried blood.