Chapter 12. Potassium
Potassium’s unique function is as a regulator of metabolic activities. It is the only nutrient which remains in the plant fluids in a soluble state. In some plants, more is required than any other soil nutrient.
Potassium is highly mobile in the soil, but leaching is minimized by cation exchange and by trapping within clay crystals.
Table 21. Comparison Of Potassium Fertilizers compares potassium fertilizers. Constant use of plant residues and animal manure, which contain significant potassium, will assure a satisfactory supply, sometimes an excess.
Potassium In The Plant
Potassium is the Great Regulator. It is active in numerous enzyme systems which control metabolic reactions, particularly in the synthesis of proteins and starches. Micronutrients, which have similar functions, are required only in minute amounts. In contrast, potassium must be present in large quantities, although it seems to be completely unsuited for its role 1 . As tables 3. Estimated Fertilizer Requirements – Field Crops , 4. Estimated Fertilizer Requirements – Vegetables And Fruits and 5. Average Nutrient Requirements For Vegetables show, some plants require more potassium than any other soil nutrient, even nitrogen.
Since potassium functions as a regulator, it is not a constituent of the plant tissue, but rather of the fluids which flood the tissue. Consequently it affects the balance in water pressure inside and outside the plant cells. When potassium is deficient, water fills the plant cells and they become flabby. A potassium deficiency also causes plants to be more sensitive to drought, frost and a high salt content. Sometimes winter hardiness can be increased by adding potassium in the fall.
The connection with both protein and starch formation puts potassium in a central role. Potassium is involved in photosynthesis and protein synthesis in leaves, cellular structure of the stalks, and starch synthesis in the roots. A potassium deficiency will lead to an excessive accumulation of simple sugars and free amino acids, photosynthesis will be retarded, and cereal plants will be weak and subject to lodging. In addition, a deficient plant is susceptible to attack by pests and disease organisms .
Biennials and perennials especially require a sufficient supply of potassium in order to synthesize the starches necessary to carry the plants through winter.
The complementary effects between nitrogen and potassium are analogous to those between nitrogen and phosphorus. The disturbances brought about by a potassium deficiency will also occur with a nitrogen excess. In either case the high priority in the metabolism of nitrogen uses the available supply of potassium, and not enough remains for other essential functions.
Unfortunately, the importance of potassium does not immunize the plant against the effects of an excess; a plant will absorb as much as is available. The loser is usually magnesium – but sometimes calcium in an acid soil. Magnesium is necessary for proper utilization of phosphorus, and a magnesium deficiency can produce effects similar to a phosphorus deficiency.
Potassium In The Soil
Both nitrogen and phosphorus are constituents of the soil organic matter, but potassium is not. Soil organisms have a much lower requirement for potassium than plants do. Consequently, as organic residues decompose, most of the potassium is quickly released. The behavior of potassium in the soil is determined more by physical than by chemical or biological processes.
Two mechanisms limit the leaching of potassium from the soil. One is that the potassium ion is small and may be trapped inside crevices within clay particles, where it is held by crystalline forces. This happens also to ammonium ions. Both are trapped and become unavailable, although they are released slowly if the amount in solution drops. Potassium so held is sometimes called fixed or non-exchangeable potassium.
The second soil mechanism for conserving potassium is cation exchange, which comes about because small clay and humus particles develop a negative electrical charge. The negatively charged particles attract positively charged ions, or cations, which include potassium. Cation exchange is discussed in chapter 14. Calcium And Soil Ph – Soil pH And Cation Exchange in relating soil pH to the calcium content. It is sufficient now only to state that exchangeable potassium associated with cation exchange usually is much greater than the quantity dissolved in the soil water – the only exceptions are those soils low in both clay and organic content.
Soluble and exchangeable and non-exchangeable potassium make up the pool of available potassium. Unfortunately, commonly available soil tests do not evaluate the non-exchangeable component. Plants grown in clay soils may be receiving enough potassium even when soil tests indicate a deficiency.
Some plants, either with the help of soil bacteria or where roots create a local acid environment, are able to extract potassium directly from rock powders. According to a survey of the literature , tobacco, oats, rye, alfalfa, clover, sweetclover and tomatoes are good at doing so, while soybeans, cow peas, corn and buckwheat are not.
The potassium content of several common materials is shown in table 21. Comparison Of Potassium Fertilizers . In summary, all animal manures and most plant residues are good potassium fertilizers. Hay and straw are representative of such plant residues, but other materials would do as well. Cocoa shells, commonly available commercially for use as a mulch, supply a significant amount of potassium.
In practice, the liberal use of organic residues of almost any kind supplies enough potassium with no need for an additional inorganic fertilizer. Indeed, with heavy applications of residues, the potential for an excess of potassium exists, especially in many soils of the eastern U.S., where magnesium is often low.
Where inorganic potassium is necessary, wood ashes are popular, and they also contain lime and a small but highly available amount of phosphorus.
Rock powders which contain significant amounts of potassium are granite dust and greensand. They are popular among organic enthusiasts because, like rock phosphate, nutrients become slowly available via the soil’s biological activity. Basalt is not available commercially, but it can sometimes be obtained locally. Its potassium content is highly variable, but basalt weathers more quickly than granite dust or greensand, and its potassium is more readily available .
Sulfate of potash magnesia (often sold under the trade names of sul-po-mag and K-mag) is a naturally occurring crystalline material known as langbeinite. Potassium chloride is also found as a natural crystal, sylvite, although chemical means are usually used to purify it. Potassium sulfate is currently produced by a number of methods, most of which involve the use of potassium chloride.
With a steady program of recycling organic residues, potassium is unlikely to be deficient, except when the residues are predominantly nitrogenous with a poor balance in potassium, as in poultry manure, blood meal and cottonseed meal. Usually if the C/N ratioC/N ratio is high, the potassium/nitrogen ratio will also be high.
Wood ashes are a good source of potassium and are probably the only fertilizer necessary for growing clover. Three limitations are:
- they are caustic
- they may cause the soil pH to rise excessively
- it is difficult to obtain enough to add significant amounts of potassium to moderate or large areas.
The usual practice with rock powders such as granite dust and greensand is to spread quantities of the order of 3-5 tons per acre, which should suffice for about 3-4 years, probably more if other sources of potassium are used.
Granite dust has an approximately neutral pH, but greensand is acidic, with pH levels of 1.0 to 3.5 possible. However, this low pH figure is misleading, and the amount of lime required to neutralize the acidity is low. The soil may be temporarily disturbed locally by the acidity of greensand, but the long-term effect should be negligible with normal applications.
Three advantages of potassium rock powders over soluble fertilizers are:
- In mimicking the natural tendency of the soil minerals to release their potassium slowly, rock powders eliminate luxury consumption by the plants if no other significant source of potassium is present;
- Potassium rock powders contain trace elements, to varying degrees;
- Potassium rock powders require less energy to produce, but this saving is partially offset by the greater amount of energy required for transportation.
Whether the above features warrant the high price of potassium rock powders is a question being considered by an increasing number of farmers and gardeners. The traditional justification for the use of potassium rock powders is their slow release of potassium. In this respect, rock powders certainly do mimic the soil minerals; but they do not mimic organic residues, the potassium of which is soluble and released rapidly.
Three options among the soluble commercial fertilizers are potassium chloride, Potassium sulfate, and sulfate of potash magnesia. The first, also known as muriate of potash, is the most common, accounting for 95% of all potassium fertilizers used in the world. Following is a brief summary of the virtues of chlorides vs. sulfates:
- Some crops have a low tolerance to chlorides, mainly tobacco, fruit trees, potatoes and some beans; and others cannot tolerate a high amount of chlorides, (strawberries, alfalfa, some beans, grapes, tomatoes, cucumbers and onions). On the other hand, chlorides have no discernible effect on many plants, such as most field crops, and they seem to be beneficial to some, for example asparagus, beets and buckwheat.
- Chlorides have little nutrient value, and the small amount that is required is easily met by the normal chloride concentrations in the soil. The sulfur in potassium sulfate, however, is an important plant nutrient.
- Potassium chloride acidifies the soil, because chlorides leach out calcium and magnesium. Potassium sulfate also has an acifying effect, but not so strongly; this is because calcium sulfate is less soluble than calcium chloride.
- Chlorides appear to inhibit nitrifying organisms (those which convert soil ammonium to nitrates). This is desireable with the use of ammonium fertilizers, because it slows down nitrification of the ammonium and thus minimizes the chances of denitrification. If one is depending on the natural soil processes, however, then potassium sulfate is preferable.
- Potassium chloride is the least expensive of the three options, and potassium sulfate the most expensive.
- sulfate of potash magnesia also supplies magnesium and is the best balanced of the three.
The potassium in organic fertilizers is highly available, because potassium is not organically bound; when the plant dies and decomposes, potassium is released immediately. Among the inorganic fertilizers, granite dust and greensand (and basalt to a lesser extent) are the only slow-release fertilizers. The others are soluble.
Unless the history of the soil is known well enough to be able to predict that potassium is deficient, additions of soluble inorganic potassium fertilizer are not wise without a soil test, particularly if organic residues are recycled. The only possible exception might be if a large quantity of nitrogen is about to be spread or if the soil is already known to be high in magnesium. One of the most common examples of an imbalance is an overlimed soil, heavily fertilized, with no regard paid to magnesium. Little can be done in such a situation until the excesses are either leached or used up.
Wood ashes add lime as well as potassium, but they contain little magnesium. The major problem with wood ashes is the danger of overliming, and without a soil test, application rates should not exceed about 1-1/2 lb/100 sq ft. This would only add the equivalent of about 20 lbs of potash per acre, but a higher rate of application could result eventually in an excessive pH.
If the soil is known to be low in potassium, then a rate of 50-100 lbs of commercial potash/acre may be reasonable. If nitrogen is also to be supplied, then the amount of applied potash should be about the same as the amount of nitrogen or slightly more.
If a high potassium application is planned for soils naturally low in magnesium, fertilizer is better spread frequently at low rates.
Table 21. Comparison Of Potassium Fertilizers also indicates the amount of fertilizer necessary to add a given amount of potash.
1 Many metals and enzymes are co-regulators, and they function by means of chelation, wherein the metal attaches itself to a specific site on the enzyme. Chelation normally requires a multivalent metal. All of the trace elements are multivalent, but potassium is monovalent, and a mechanism had to evolve in which a monovalent ion could also function as a co-regulator. The reason for such an inefficient adaptation, contrary to the usual tendency for frugality in nature, is not understood; perhaps it is simply that a lot of potassium is needed anyway to balance sodium in establishing the osmotic pressure across cell membranes. [return to text]
Potassium in plants, soil, fertilizers