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23. Potash for organic growers
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Published September 2019
Principles of manuring
The principles of manuring are the same whatever the production system. Organic and non-organic farming have many common objectives and are working with the same basic resources. It is unfortunate that promotion and advocacy of the different systems emphasises differences and encourages conflict.
The objectives of organic production as stated by the Soil Association for example are:- Organic agriculture should sustain and enhance the health of soil, plant, animal and human as one and indivisible.
With regard to plant nutrition the organic aims are:
- to work within natural systems and cycles
- to maintain or increase long term fertility
- as far as possible to use renewable resources in preference to non-renewable resources
- to use other specific materials when extreme need arises
In practice this means:
- Optimising nutrient recycling by using manures to best effect
- Balancing nutrients within the rotation
- Feeding the soil rather than the plant
For phosphate and potash these also represent the aims of non-organic farmers. Nutrients from manures are taken into account when deciding fertiliser use; phosphate and potash are applied to replace the nutrients removed in the crop in order to maintain soil fertility and nutrient status.
The main difference between an organic and non-organic approach is seen with nitrogen which, although taken up by the plant principally as nitrate in both systems, has different emphasis on sourcing. Organic systems rely on soil, manure and legume N, rather than supplementing these with purchased fertiliser N.
All systems need to pay careful attention to minimising nutrient losses for both financial and environmental reasons. With organic systems, emphasis is also placed on maximising root and biological activity in the soil but the importance of such principles is also recognised by non-organic farmers – for example even the most intensive potato growers prepare deep beds of well-structured soil to maximise rooting and soil exploration.
Organic standards favour the use of natural, untreated products, which limits the available range of nutrient sources. Non-organic farmers may normally be influenced more by the cost of materials but this often reflects the degree of processing, energy inputs, transport and convenience.
Potash – a naturally available nutrient
Potash is found in plant-available form as potassium (K) salts such as potassium chloride, sulphate, nitrate etc. These natural deposits are generally the result of the drying out of seas millions of years ago. In soils and plants these salts, which are all water soluble, separate into the potassium cation K+ and the relevant anion Cl – , SO4 2- , NO3 – etc. Potash in manures is also mainly (70-90%) in water soluble form, with a small amount bound into the organic material which is released into the soil solution as the organic matter is mineralised. Potash from manures thus behaves in the potash cycle shown below in a similar manner to fertiliser potash. Potassium is not associated with any environmental or health concerns. None of the forms of these materials produce harmful effects unless they are used incorrectly. As with other nutrients, farmers should use all potash sources with care and responsibility whatever the farming system.
The potash cycle
Potassium in soil can be thought of as existing in four pools according to the availability of the K for uptake by plant roots. It is present dissolved in the soil water, adsorbed onto particles of clay and organic matter and held within the crystal structure of clay particles.
Exchangeable K, which is determined by routine soil analysis, is the K that is most readily available for uptake by roots. It is the K in the soil solution and in the ‘readily available’ pool.
Sources of potash to the plant are indicated in the diagram. They include small amounts from rain, generally less than 5 kg/ha/year. Some K is lost by drainage from soil and studies indicate for most soils this loss is approximately 1 kg K for every 100 mm of through drainage. Potassium loss is higher from dung and urine patches after grazing because of the very high concentrations in these areas. Uneven or large applications of slurry or FYM can also lead to greater loss than from correctly managed fertiliser or manure.
Manures contain useful amounts of potash depending upon the type of livestock and litter/straw (if any) from which they are derived (see PDA Leaflet ‘Nutrient Contents of Manures‘). Potassium from these sources behaves in the same way in the soil as from fertiliser. Methods of manure storage are also important as K losses can be considerable – up to 50%. Unless imported from another farm, manures only recycle nutrients within the farm and do not replace nutrient which is removed in the products sold.
It is the soil itself which supplies the crop with nutrients and additions of fertiliser or manure are made to replenish these soil reserves. Most soils contain very large quantities of potassium – up to 100 t/ha – but most of this is not available to the plant. Plants take up potassium (as K+) from the soil solution which contains only small quantities – less than 20 kg/ha K. As plants take up K+ from the soil solution it is replenished by the release of K+ held by the clay in the soil (clay minerals are negatively charged and thus attract and hold the positively charged cations – potassium, calcium, magnesium, sodium etc.). Depending upon how strongly it is bonded to the clay, K+ may be released rapidly (exchangeable K), slowly (less readily available K) or by weathering over long periods of time (very slowly available K [also known as matrix K]).
Careful cultivation, conservation of organic matter and improvement of biological activity will maximise the total availability of potassium from all these sources according to the individual nature of each specific soil. Even under ideal management however, the natural release of potassium is unlikely to be sufficient to maintain the necessary available nutrient levels in the soil and extra supplementation will be required.
Routine soil analysis measures the potassium in the soil solution plus the exchangeable K. Unfortunately, it is difficult to measure or predict the release of less readily available K.
Whilst all the ‘pools’ are shown separately in the diagram on the previous page, in the living soil there are no sharp divisions and the entire system is dynamic with potassium becoming more or less available according to many varying factors.
Soil analysis provides the best practical guide to the adequacy of reserves of available nutrient for plant growth, and to any need for nutrient supplementation (see PDA Leaflet 24: ‘Soil Analysis, Key to Nutrient Management Planning’).
Sustainable nutrient management
All systems of production should maintain an adequate supply of potassium available to the plant. Nutrient management must balance inputs with outputs and losses.
Aims common to all systems are:
- to maintain good soil structure;
- to maximise soil volume available to the plant roots;
- to conserve organic matter;
- to promote biological activity in the soil.
Additions of potash in fertilisers or manures must be given to replace that removed by cropping. Failure to do so will affect crop performance and is not sustainable management. Sandy soils with low clay content will be most rapidly affected. If soil analysis shows exchangeable K levels to be sufficiently high, fertilisers and manures should be reduced or omitted (see PDA Leaflet 8: ‘Principles of Potash Use’).
On some heavy soils the release of potassium from less readily available reserves is sufficient to provide the needs of combinable crops without other additions, but on most soils, and where high demand crops such as roots and forage crops are grown, additional potash will need to be applied in order to maintain soil K reserves and to replace removal in harvested crops.
Penalties of low potash
There is increasing evidence that an adequate potash supply will help reduce crop stress caused by drought, chilling, high light intensity, heat and deficiencies of other nutrients. These stresses can result in oxidative damage to the plant from ‘reactive oxygen species’ (ROS) free radicals, and production of these damaging ROS can be greatly reduced by a satisfactory potassium status in the plant. Potassium plays a crucial role in maintaining the general health of the plant.
If potash is limiting, response to nitrogen will be reduced, N-fixing bacteria will be less active in legumes and crop health, vigour, and resistance to stress will suffer. Such aspects are of particular importance in organic production where natural resistance through balanced nutrition is an integral aspect of overall husbandry in the absence of agro-chemical protection. Potassium is very involved with the water relations in the plant and a deficiency will be especially serious under dry conditions.
Sources of fertiliser potash
The main reserves of potash in the world are in the clay minerals of the soils and rocks, in the water of the oceans and in the rock salt deposits containing the crystallised minerals from long dried up seas. Potassium salts, principally chloride, sulphate and nitrate derived from these evaporite rocks (and from salt pans which are in current use in certain parts of the world), are the most common forms of fertiliser potash – all of these being naturally water soluble.
Perhaps the oldest form of potash fertiliser is wood ash but supplies of this material are obviously no longer practical nor sustainable.
Various process wastes containing potassium (such as lime kiln dusts) have been considered as sources of K but these vary in K availability, can contain undesirable contamination and suffer from irregularity of supply.
Finely ground primary soil minerals, for example feldspars, are offered as ‘rock potash’ fertiliser but the plant-availability of the potassium in such materials depends on the origin and the mineralogy of the parent material. It is possible, as with matrix K in soils, for a material to contain K but not to release it except over a geological timescale.
Green wastes, composts and other waste materials are increasingly becoming available as alternative potash sources. Animal feeds and bedding bought onto farm also represent a significant supply of the nutrient.
Most organic farming bodies restrict the use of natural potash (potassium chloride) due to the high chloride content which is believed by some to be harmful to soil fauna and micro-organisms, although there is little evidence of this. They usually allow the use of potassium sulphate and more recently polyhalite, which although largely sulphur, contains 14% potassium, along with valuable levels of magnesium and calcium.
Polyhalite is a layer of rock, over 1000m below the North Sea off the North Yorkshire coast in the UK. Deposited 260 million years ago, it lies 150-170m below the potash seam. It is mined, crushed, screened and bagged with no chemical separation or other industrial processes. As such Polysulphate (the trade name for polyhalite fertiliser) is licensed as a sulphur fertiliser approved for organic use by the Soil Association in the UK.
Whenever you are planning to use a restricted material you should consult your Certification Body. You may require prior permission to ensure you do not apply a material that is not allowed.
Organic standards for potash materials
The current classification of potash materials for organic production is based on consideration of a number of characteristics which relate to the objectives listed previously:
- Natural or manufactured product.
- Chloride content.
Solubility. Most potash sources – manures and fertiliser forms – are soluble and rapidly add K to soil solution. However, despite its natural solubility, potassium applied at appropriate rates is not leached from soils as some other nutrients such as nitrogen or sulphur can be.
Chloride content is included because it has been suggested to be undesirable on the basis that some plant species are more sensitive to high Cl – levels than others. It has also been asserted that excess chloride may be deleterious to micro-organisms and earthworms, but no experimental evidence has been found to support this, at agronomic rates of application.
Chloride (Cl – ), which occurs in rainfall, fertilisers and manures, is an essential plant nutrient and must not be confused with chlorine gas, hypochlorite used as a sterilant, or other forms which do not occur in soils or plants. The quantity of chloride applied in fertilisers will usually be less than that deposited by rain and salt spray in coastal areas.
Certification of sources of potash
Council Regulation (EC) No 834/2007 of 28 June 2007 on organic production and labelling of organic products sets out the rules for organic production for all countries in the EU. Growers must register with an individual certification body and comply with the requirements of that body (which may be stricter than the EU legislation). The certification bodies operating in the UK and Ireland are:
- Bio-Dynamic Agricultural Association
- Organic Farmers and Growers Ltd
- Organic Food Federation
- Organic Trust Ltd
- Quality Welsh Food Certification Ltd
- OF&G (Scotland) Ltd
- Soil Association Certification Ltd
- Irish Organic Association
- Organic Trust Limited
The correct approach for both organic and non-organic systems is to monitor soil fertility every 4-5 years by soil analysis and by drawing up a nutrient balance calculation – this will not only demonstrate good practice, but will provide evidence to justify the use of restricted materials, if necessary. Allocation of manures and use of fertiliser materials should be undertaken to maintain available soil potassium (and similarly phosphate) appropriate for the crops being grown (see PDA Leaflet 8: ‘Principles of Potash Use’). Where soil potassium (and phosphate) status is satisfactory, nutrient policy should simply be to replace nutrients removed, using estimates based on the standard offtake data (see PDA Leaflet: ‘Phosphate and Potash Removal by Crops’). It should be noted that even the most efficient storage and recycling of organic manures may not provide adequate replacement because of the export of nutrients in products sold off the farm and retention of some of the applied nutrient in non-available reserves.
Potash Development Association 23. Potash for organic growers Home » Potash Leaflets » 23. Potash for organic growers Published September 2019 Principles of manuring The principles of
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