POTASSIUM SALTS The need for potassium salts in plant nutrition was recognized long ago by agricultural chemists. As early as 1840 Liebig had insisted on the fertilizing value of "salts of the alkalis," without, however, specifying precisely which was the most important. Lawes and Gilbert from the outset included these salts in their fertilizer trials and by 1852 had laid out field plots for comparing potassium, sodium and magnesium sulphates : these experiments soon showed that potassium salts had most value. Farmers did not, however, begin to use them as fertilizers for many years ; the need on the farm was met by farmyard manure and by wood ashes, on light soils by salt, and on farms near the seashore by seaweed. It was about 186o that the great deposits of potassium salts in the neighbourhood of Stassfurt in Saxony were first worked to supply agriculturists with definite potassic fertilizers, but little use was made of them in Britain till after 189o. The industry has developed and is now under the charge of the German Potash Syndicate.
Four potassium salts are available on the market : potassium sulphate ; potassium chloride ; a mixture of potassium and sodium, and, if from Stassfurt, magnesium chlorides known as kainit ; and a mixture of the chlorides and sulphates of potassium, sodium and magnesium known as potash manure salts. Prior to sale all these are graded and standardized to contain definite amounts of potassium. The sulphate and muriate are sold on a basis of potassium equivalent to 48.5% muriate (i.e., impure chloride), also on a basis of 45% kainit 12.5%. Potash manure salts are of several grades, from 20 to 3o%. The world consumption was in 1926 equivalent to 1.4 million tons annually, of which Great Britain took 3% (40,000 tons .
Up till the time of the World War the Stassfurt mines supplied all the potassium salts used in the world, but during the war other sources were exploited in the various countries, notably flue dusts and other industrial wastes and residues in Great Britain and certain natural deposits in the Western United States. Most of these proved unsuitable for peace conditions and have not survived. Two only remain of importance : the Alsace potash mines, which differ from those of Stassfurt in that the salts are free from sulphates and from magnesium, consisting of potassium and sodium chlorides only, and the Searle's Lake and other deposits in California, which can now supply a potassic fertilizer free from boron, which caused difficulty when this particular source was first exploited. Attempts are also being made to exploit the potassium salts in the Dead Sea, but so far this is done only on an experimental scale.
The four potassic fertilizers above mentioned all have certain properties in common due to potassium ; their differences arise from the salts and radicals associated with them. Four distinct effects are produced on the growing plant by potassium salts: (i.) on the general health and vigour of the plant ; (ii.) on the efficiency of the leaves for synthesising and translocating carbo hydrates; (iii.) on certain processes occurring in leguminous plants; and (iv.) on the formation of grain.
The effect on the growth and vigour of the plant makes potassic fertilizers of considerable value in adverse weather conditions, especially in sunless seasons. If but little potash is supplied the potato crop becomes very dependent on the amount of sunshine during its growing period : at Rothamsted the lowest yield in the five years 1922-26 was 2.47 tons per acre in 1922, when there was also the lowest number (519) of hours of sunshine during the four months July to October inclusive, and the highest yield was 9.72 tons in 1923, when the hours of sunshine were also highest, numbering 708. But where potassic fertilizers were given, the loss of crop following the lack of sunshine was much less: the potash in some way making up for deficient sunshine and enabling the plant to produce a good crop in spite of the dis advantage of a bad season.
Another result of the increased vigour conferred by potassic fertilizers is that they increase the resistance of plants to fungus disease. At Rothamsted the wheat receiving much nitrogen but no potassium is, in certain years, very liable to rust, while the adjoining plot receiving the same amount of nitrogen and equally exposed to infection, but receiving potassic fertilizers, is much freer from rust. The mangolds receiving much nitrogen but no potassium are badly attacked by Uromyces betae, while those receiving potassium largely escape. The permanent grass receiv ing nitrogen but no potassium is much more liable to attacks of Epicliloe than the neighbouring plot receiving potassium. The results suggest—and this is emphasized by the experiments of W. F. Bewley on the bacterial stripe disease at Cheshunt—that the important factor is the ratio of nitrogen to potassium: if this is too high the plant tends to become susceptible to fungus attack. It does not necessarily follow that potassium salts alter the physiological susceptibilities of the plant to the disease; their effect may be indirect, conditioned by the change in habit of growth or rate of maturation they bring about ; more work is needed, however, to clear up this subject. (See E. C. Stakman and 0. S. Aamodt, Journal Agricultural Research, 380, for a recent discussion of this aspect of the problem as applied to stem rust of wheat in the United States.) Flax growers in the north of Ireland have found that potassic fertilizers decrease the liability of the plant to the attacks of the wilt organism. In T. Wallace's sand cultures of fruit trees (Long Ashton), omission of potassium caused a characteristic leaf scorch and premature defoliation : the shoot growth of apples was much restricted. W. W. Garner (Washington, D.C.) found that potassium deficiency caused a "chlorosis" of tobacco leaf characterized by leaf spotting and distinct from that following magnesium deficiency, while H. S. Reed and A. R. C. Haas (Cali fornia) observed a bronzed appearance on the leaves of young orange trees suffering from potassium starvation.
The effect of potassic fertilizers in increasing the efficiency of the leaf as an agent for assimilating carbon dioxide and syn thesising and translocating carbohydrates gives these fertilizers special importance for crops like potatoes, mangolds and sugar beet which depend for their value largely on the amount of carbo hydrate per acre they can produce. On the mangold plots at Rothamsted a ton of leaf on plants receiving potassic fertilizers produces a much greater weight of root and of sugar than an equal weight of leaf receiving no potassium, especially when sufficient nitrogen is given to ensure adequate leaf area. In like manner the weight of potatoes and starch per acre are both increased by additions of potassic fertilizers. Potassic fertilizers are of great importance to sugar beet : indeed the possession of the world's supply of these fertilizers was an important factor in the development of the sugar beet industry in Germany.
Associated with these marked differences in crop there are great differences in the colours of the leaves especially towards the end of the season. The leaves of mangolds well supplied with potash are healthy green in colour, well spread out and gradually die and become yellow from the bottom row upwards as the season advances. Without potash, however, the mangold leaves are of an unhealthy dark green colour, crinkled and, instead of spreading out, they tend to bunch together; they do not ripen normalcy but tend to die late : the stems are often orange colour. The facts are clear, but the physiological explanation has not yet been given. The harmful effect of potash starvation on carbo hydrate production does not seem to be the result of a pathological condition of the chloroplastids. H. S. Reed found that they remained normal for two months, and even increased in numbers in potash-starved algae.
For some reason not fully understood, the turnip crop, although a great producer of sugar, responds less to potassic fertilizers than do mangolds or potatoes. In general, cereals also respond but little : deficiency of potash is shown first by falling off in yield, and a slight rise in the percentage of nitrogen in the grain; i later, however, when it becomes severe there is a shrinkage in size of the grain. These effects are most pronounced on light sandy, chalky and peaty soils : on these the need for potash seems to be considerable.
A further effect of potassic fertilizers is to bring about a longer continuance of the growing period, and a longer function ing of the leaves; on some of the light soils fruit trees receiving potassic fertilizers retain their leaves in a green healthy state for some time of ter other trees have shed them. This extension of the vegetative processes seems to explain the retardation in maturation seen on lighter soils. An example of this is furnished by the North Carolina experiments on cotton, where phosphates markedly hasten maturation and potash equally delayed it. Cot ton does not ripen all at once, but requires several pickings. The percentage of the cotton open at the first picking was nearly twice as great under phosphatic manuring as under potassic manuring.
The effects of potassic manures on leguminous crops are dif ficult to explain but very important in practice. All leguminous crops need potash, and especially clover where, as often in Britain, it is grown not by itself but in association with grasses. Lawes and Gilbert showed many years ago that in a mixed grass field, clovers have less capacity than the grasses for absorbing potassium from the soil, and in absence of potassic fertilizers they suffer from the competition ; the potassium starved grass plots at Rot hamsted contain notably less clover than those fully manured, the actual depression fluctuating according to the season.
In conclusion, potassic fertilizers are of special importance for: Crops, sugar beet, potatoes, mangolds, leguminous crops, especially clover, and therefore meadow hay.
Soils, chalk, sands, pests.
Seasons, cold, sunless, dry.
Other conditions, to promote resistance to fungous diseases and to extend the period of vegetative growth.