Potash Salts.

A High value is justly put on the manurial salts of potassium. These rank on a slightly higher level, in this respect, than even soluble phosphate of lime, and are exceeded only by the nitrates and the salts of ammonia. Potash is contained in all soils, ranging from two parts in 1,000 to two parts in 100. The rocks of our mountain ranges contain it in still more abundance. Some of the felspars contain as much as 16 per cent, of this compound, the micas from 1 to 3 or 4 per cent., and the basalts and trachytes of Dunedin, Otago Peninsula, Banks Peninsula, and of Lyttelton about 3 per cent. It is, of course, contained in the ashes of all the cultivated plants, and is therefore required for their food.

The proportion of potash in the ash indicates the kinds of plants that need it most, and will guide the farmer in its application.

At the same time it should not be overlooked that the amount of it already in the soil varies widely, so that it may often happen that money is spent on potash (kainit, &c.) which is put on land that already contains an ample supply of that ingredient. Only an analysis of the soil will show in the case of potash, as well as everything else, what ingredients are abundantly there already, and what require to be supplied to make up for its deficiencies and strengthen the weakest link in the chain.

For milk it would work out this way: Suppose one cow yields 800gal. of milk per annum, weighing, say, a little over 8,000lb. This would give 68lb. of ash, including 16lb. of potash. These results show at a glance that root-crops—turnips, mangels, and potatoes (including, though erroneously, potatoes among roots for the present)—require a great deal of potash, and would rapidly remove that constituent from the soil. Potash should therefore figure well in special manures for these crops. Next to root-crops we have the leguminous crops-peas, clovers, beans, lucerne, lentils, &c.; and after them the cereals-wheat, oats, barley, maize, and other grain-crops, all of which still require potash, and remove it from the soil. In dairying also we send away potash as well as phosphorus and lime in the milk and the cheese. The very highest English authorities, Lawes and Gilbert, gave potash as the specialty for leguminous plants, phosphates for turnips, and active nitrogen (nitrates and ammonia salts) for grain.

The sources of potash for manures may be summarised as follow: (1) Wood ashes, (2) common saltpetre or nitrate of potash, (3) muriate of potash, (4) sulphate of potash, (5) kainit, carnallite, and sylvin, all derived from the Stassfurt deposits in Prussia, and (6) carbonate of potash, derived from various sources.

Concerning each of these a few words of explanation will be required.

Wood Ashes.—Not only do plants vary widely as to the quantity of ash they yield, but even in the same plant there is quite as great a difference in the amount as well as quality of the ash got from various parts of it. The leaves, roots, and branches, for example, yield, weight for weight, at least ten times as much ash as the heartwood. The ash of young plants, also of shrubs (and, indeed, of all small plants), and of the leaves, roots, bark, and branches of trees, is richer in potash than that of hard, large timber.

The amount of ash, taking plants in the average, may be put as probably 1½ per cent.: that is, 100 tons of mixed timber, of all sizes, will yield one ton and a half of ashes. Wood ashes contain from 5 to page 31 15 per cent., or an average of, say, about per cent., of pure potash. The ashes of 100 tons of timber, therefore, may yield about 2½cwt. of that fertiliser. From sawdust, looking at its origin, we would not expect quite so much potash as this. Of course, when bush country is burned, or a considerable proportion of the branches and crops of the trees are allowed to decay on the ground, there is a very valuable contribution of potash, and sometimes of phosphates and lime, made to the surface soil, brought from below by the far-reaching roots.

It is in the form of carbonate of potash (K₂CO₃) that this valuable nutrient is found in the ashes. It is quite soluble in water, and therefore instantly available for plant-food. This high degree of solubility, however, is often the cause of its speedy removal from the hard sloping surface of the land by rain-water washing it away into the creeks and off to the sea. It would be very desirable, therefore, to grub-hoe or break up somehow the burnt land at once and lay it down in grass, so as to get the first benefit of the fertiliser, and get it into the soil. Besides supplying directly potash food for plants, wood ashes have a valuable chemical action on the soil itself, by which they liberate lime and magnesia, and even potash, from clay, and thus render these constituents available to the growing plants. The carbonate of potash in the ashes also neutralises the very objectionable low acids in damp soils (humic, geic, ulmic acid), and thus warms and sweetens cold, sour lands after a wet winter and a late spring.

There is a whole caste of native Indians whose sole occupation it is to search out patches of rich soil impregnated with these white crystals of saltpetre. They dig out the fat earth so charged, rub it up with hot water in wooden tubs to dissolve this salt, using the same water over and over again till it has as much saltpetre as it can hold in solution. The sediment is then allowed to settle; the clear liquid is decanted off and boiled down, or evaporated in shallow vessels by the heat of the sun, till, page 32 on cooling, the saltpetre in it crystallizes out. If intended for manure, it is now ready for the market; but for making gunpowder and nitric acid it has to be purified by resolution and recrystallization repeated a number of times.

In France, Belgium, and Sweden, and in some parts of Germany, since the time of the first Napoleon, when England cut him off from getting supplies of that ingredient of gunpowder from India, saltpetre has been made artificially in what are called the nitre plantations of these countries. These are simply mounds of rich loam mixed with offal and other dead nitrogenous animal matter, wood ashes, and lime. These mounds are protected from rain (which would dissolve out the nitrates) by a loose, open roof or shed, which, however allows the air free access. They are watered from time to time with stable runnings and similar putrefying nitrogenous liquids, and turned over occasionally to expose all parts to the action of the air and the liquids.

These mounds, it is now known, become the homes of countless millions of the nitric-acid microbe (which is the real nitre-maker) working in the dark, feeding on the nitrogen, and converting it into nitric acid, which then unites with the lime and the potash present, converting them into soluble nitrates. These heaps are then treated for the recovery of the saltpetre just as the saltpetre Indian treats his white and grey patches of nitre soil as already described.