Relative Importance.

In Chapter VI. we pointed out that of the three manurial ingredients potash was the one most abundantly occurring, and that, consequently, the necessity of adding it in the form of an artificial manure existed less frequently than in the case of nitrogen or phosphoric acid. It was further pointed out that, under the ordinary conditions of agriculture, a greater restoration to the soil of the potash removed in the crops was made in the straw used in farmyard manure than was the case with regard to the other two ingredients. Despite these facts, there are many cases where the addition of potassic manures is of the highest importance in increasing plant-growth. It will be well, therefore, to devote a little space to considering our different potassic manures and their respective action.

Scottish Soils supplied with Potash.

Potassic manures are not so valuable in this country since experience has shown that most Scottish soils are abundantly supplied with this manurial ingredient. Moreover, under the conditions of most European farming, there seems to be a steady gain to the soil of potash. In America, however, the action of potash as a manure seems to be more strikingly illustrated. Indeed, wherever forage crops or straw are sold off the farm in large quantities, or where beets, cabbages, carrots, potatoes, onions, &c., are also grown in large quantities, the necessity for potash manuring generally arises.

Sources of Potassic Manures.

The value of potash as a manure first came to be recognised from the favourable action of wood-ashes. Of course their favourable action is not due solely to potash, as they contain, in addition to the other ash ingredients of the plant, phosphates; and their value as a manure may also be said to depend not a little on their indirect action. They contain a certain percentage of caustic alkali, which promotes the decomposition of the nitrogenous matter of the soil. But making due allowance for these other valuable properties, the chief value of wood-ashes is undoubtedly due to the potash they contain. Hence the use of the commercial article called potash, which is a mixture of potassium carbonate and hydrate, and which is obtained from wood-ashes, was formerly common to a considerable extent as a manure, especially for clover. Barilla, a rich potassic manure prepared by burning certain strand plants, especially the saltwort, was also in the past largely exported from Sicily and Spain. Kelp, a product got by burning sea-weed in Scotland, is also a rich potassic manure. Since, however, the discovery of the Stassfurt mines, all potassic manures have come from these.

Stassfurt Potash Salts.

Huge salt deposits exist at Stassfurt in Germany. They have been formed by the evaporation of an inland sea. Salt was first discovered in these deposits in 1839, but for long the presence of potash salts was little suspected, and it was not until 1862 that the potash salts were worked. We have already, in the Appendix to Chapter VI., given a list of the chief potash minerals occurring in the Stassfurt deposits. These minerals are found in layers, the lowest layer consisting of almost pure salt; while immediately above this we have a layer of salt mixed with the mineral polyhallite (containing potassium sulphate) of about 100 feet thick. Above this last layer there is a layer of about 90 feet, containing kieserite (magnesium sulphate) mixed with potassium and magnesium chlorides; and above this again is a layer (90 feet) of carnallite, which furnishes the chief source of the potash salts used for manurial purposes.

At first the crude salts, as obtained direct from the deposits, were sold as manures under the name of Abraum salts. Now, however, they are purified. Of potash salts in 1888 some 25,000 tons were exported from Stassfurt for manurial purposes. Of these salts there may be mentioned, viz., kainit, an impure form of the sulphate, containing on an average about 12 per cent of potash, and the muriate and the sulphate—both salts, in a more or less pure form, being used. A word or two may be added on the effect of the two forms of potash—viz., as the sulphate and as the muriate.

Relative Merits of Sulphate and Muriate of Potash.

It is a well-known fact that muriate of potash, far from having a beneficial effect on certain crops, is actually harmful. Of these, sugar-beets, potatoes, and tobacco may be mentioned. In the case of beets it seems to have an effect in lessening the percentage of crystallisable sugar, while potatoes are rendered waxy. With regard to the tobacco-plant, it seems to impair the value of the leaf from the smoker's point of view. That this deleterious action is due to the form in which the potash is present, and not to the potash itself, seems to be pretty clear, since potash in the form of sulphate has not this deleterious effect on these plants. Another objection which has been urged against muriate of potash is that, when applied as a manure, it is apt to give rise to the formation of calcium chloride,—a compound which is distinctly hurtful to many plants. A similar charge cannot be brought against sulphate of potash, since gypsum, which is the chief compound it is likely to give rise to, is of much value, as we have already pointed out, as an indirect manure. On the whole, therefore, sulphate of potash seems to be the safest form in which to add potash. Unfortunately, however, most of the commercial sulphates are very impure, and contain generally considerable quantities of muriate. In favour of the muriate, it may be said that it is the more concentrated manure, and that it diffuses better in the soil than the sulphate—a point of great importance. It has, moreover, been used without any bad effect for clover, corn, grass, and some root crops.

Application of Potash Manures.

The extreme tenacity with which the soil-particles fix potash salts, when applied as manures, is a point which ought to be borne in mind in their application. This, as we have just noticed, is greater in the case of the sulphate than in the case of muriate, and it has been observed that certain other fertilisers seem to exercise a considerable influence in hindering their fixation. Among these may be mentioned bone-meal and farmyard manure. Nitrate of soda also seems to increase the diffusibility of potash salts. Conversely, potash salts seem to help to fix ammonia.For the above reasons potash manures ought to be applied to the soil a considerable period before they are likely to be used by the crop. There is little risk of any serious loss taking place owing to rain. Autumn application is generally recommended. Even in very light soils it has been proved in the Norfolk experiments that autumn application has an immense advantage over spring application. It has been found that where potash is applied as sulphate, little sulphuric acid is absorbed by the plant.

Soils and Crops suited for Potash Manures.

Of soils best suited for potash manures, it has been found that light soils, and those largely charged with peaty organic matter (such as the moorland soils of Germany), are most benefited; while on heavy clayey soils the percentage of potash which these latter contain is already sufficiently abundant for the needs of plants. At Flitcham the value of potash on chalk soils has been strikingly demonstrated. Of crops, it is now pretty generally acknowledged that those of the leguminous order are most benefited by potash. Especially in the case of clover has potash always proved itself a manure worth applying.

Rate of Application.

Potash is best applied in small quantities. From 1 to 2 cwt. of the muriate or sulphate is a common amount, and from 6 to 8 cwt. of kainit.