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About potash


Autumn is the time to tackle potassium deficiency in silage fields

Spring is usually the time of year when soil sample results appear on advisors' desks across the country and farmers start looking for advice on the appropriate fertiliser to spread.

Approximately 60pc of our soils nationally are at index 1 and 2 for potassium.

This means that six out of 10 silage fields are deficient in potassium, and this poses a challenge. Autumn time is the ideal time of year to rectify potassium deficiencies.

So why is potassium such an important fertiliser? Potassium is the nutrient taken up in the greatest quantity by grassland swards and has a wide-ranging role in the plant, affecting nutrient uptake, photosynthesis, rate of growth and feed value.

It is particularly important for increasing stem strength, improving drought resistance and cold tolerance, and importantly for increasing yield.

Potassium fertilisation is vital, especially in autumn and on older grass. If adequate amounts of potassium are not available, the rate of growth and yield will be restricted.

There is also a relationship between nitrogen and potassium, as the response of grass to nitrogen is dependent on an available supply of potassium to allow N uptake as nitrate and conversion into proteins.

In silage fields in particular, stem strength is of huge importance. Silage crops low in potassium are more prone to lodging, as the stem cannot hold up the seed head and grass plant.

Padraig O'Kiely's work in Teagasc, Grange on the factors that impact most on silage quality show that lodging is the factor that can lead to the greatest deterioration in the quality of a silage crop and estimates that up to nine units of DMD percentage could be lost by your crop lodging. This means your 70pc DMD silage pit might now be only 61pc DMD.


The table below shows the amount of potassium (K) required for first-cut silage crops. Fields that are index 1 and 2 for potassium require 120-140 units of potassium/acre.

However, as the recommended amount of K to be applied in a single application is 90 units (three bags 0-7-30/acre), the advice is generally to spread the 90 units and wait until the autumn to spread the balance. In practice, this balance is rarely spread.

The reason why it is advised to only spread 90 units in a single application is because luxury uptake of potassium occurs in rapidly growing spring grass, which has the ability to take up a lot more potassium than it needs for normal plant functioning.


High levels of potassium in the plant interfere with the uptake of magnesium, and low magnesium in the animal's diet is one of the factors causing grass tetany.

Many of us depend on cattle slurry to rectify our potassium deficiencies; as 1,000 gallons of thick cattle slurry will contain approximately 30 units of K, it will go some distance to replacing some of the K off-take from cutting or grazing.

However, in a silage situation, 3,000 gallons/acre provides only 90 units of K at best, meaning we are still short 30-50 units of K on index 1 and 2 soils. Now is the best time to apply this amount.

This autumn there is increased risk of stripping the land of potassium because of the volume of late cuts of silage that are going to be taken in September/October.

It is vital to replenish this K to ensure next year's silage crops will grow to their potential and help fill the empty silage pits around the country. A bag of Muriate of Potash (50 units K) should be adequate in most situations.

As there is no legal deadline or maximum rates in relation to potassium, every farmer should look at getting a pallet or two of potassium spread over the next month.

Joe Kelleher is a Teagasc consultant based in Newcastle West, Co Limerick


Autumn is the time to tackle potassium deficiency in silage fields


High potassium content will increase yield in paddy

Research have proved that paddy production can be increased several folds by increasing the potassium content in fertilizers. Belgium and Ireland have more production of paddy as compared to other South Asian Countries

Potassium is often the most limiting nutrient after nitrogen (N) in high yielding rice systems. K fertilizer needs to be applied in adequate amounts in most irrigated rice fields. Other nutrients need to be applied in balanced amounts to ensure a good crop response to K fertilizer application and to achieve a healthy and productive crop.

  • Potassium deficiency symptoms. Stunted dark green plants with yellowish brown leaf margins and/or older leaves with necrotic tips and margins; leaf symptoms of K deficiency can be confused with that of Tungro disease, but Tungro occurs in patches in a field (not in the whole field) and usually has more pronounced yellow and orange leaves and plant stunting; leaf symptoms often appear in late growth stages; unhealthy or black roots; greater lodging; higher level of unfilled grains; lower grain weight.

  • Occurrence of K deficiency. K deficiency occurs in intensively cropped areas with high levels of N and P application. K is often deficient in coarse-textured/sandy soils; acid upland soils; degraded lowland soils; acid sulfate soils; and, organic soils.
  • Note: additions of K from irrigation water can be significant in some areas (e.g. South Vietnam).
  • How much K to apply? At optimum plant nutrition, the rice crop (straw plus in grain) takes up around 19 kg K2O (16 kg K) per ton of grain yield (2.2 kg K2O in grain and 16.8 kg K2O in straw). Recommendations for K are based on yield target and soil K status (see Table on opposite page) as determined by grain yield in K-omission plots (see also Fact Sheet on Nutrient Omission Plot Technique for P and K).
  • When to apply K fertilizer? If fertilizer K rates are small, incorporate all fertilizer K before the last soil puddling before transplanting or topdress all K within 10−15 days after direct seeding. At rates > 30 kg K2O/ha, apply 50% basal and 50% at early panicle initiation. Split K in at least two doses if soil is sandy with leaching. Use of K at flowering increases resistance to lodging and diseases in dense canopies with high yield target, but may not increase yields.


High potassium content will increase yield in paddy


Watch: Alternative Materials for Rechargeable Batteries

Lithium, a metal with a name that has become synonymous with rechargeable batteries, has been instrumental in pushing technology into producing batteries with better life and better performance for a wide range of applications.

But lithium won’t last forever.

That’s why researchers are looking at alternatives. New evidence from the Georgia Institute of Technology suggests that sodium and potassium could potentially fill lithium’s big shoes. The two metals have been known to hold less energy than other alternatives, and also to decay and degrade quickly.

"But,” explained Matthew McDowell, an assistant professor in materials science and engineering, “we've found that's not always the case.”

As batteries charge and discharge, the ions on which they are based (whether lithium, sodium or potassium) are constantly reacting with and penetrating the battery’s electrodes. This causes large volume changes, often breaking the electrode particles into small pieces and causing degradation. Sodium and potassium are believed to be worse offenders because their ions are larger than lithium.

In experiments using iron sulfide — also known as pyrite or “fool’s gold” — in the role of a battery electrode, researchers were able to use an electron microscope to observe what happens during internal battery reactions. They found that iron sulfide was more stable during reaction with sodium and potassium — meaning that batteries based on either could have a much longer life than expected. In fact, the iron sulfide expanded like a balloon when exposed to sodium and potassium — but when exposed to lithium, it appeared to almost explode.

The study also casts doubt on the idea that volume changes occurring during electrochemical reactions are always a precursor to particle fracture.

It is possible, the researchers said, that different ions reacted in different ways because lithium is more likely to concentrate its reaction along the particle's sharp cube-like edges. By contrast, the reaction with sodium and potassium is more diffuse along all of iron sulfide’s surface.

The research findings could help scientists design battery systems that use alternate materials.

"Lithium batteries are still the most attractive right now because they have the most energy density; you can pack a lot of energy in that space," McDowell said. "Sodium and potassium batteries at this point don't have more density, but they are based on elements a thousand times more abundant in the earth's crust than lithium. So they could be much cheaper in the future, which is important for large-scale energy storage."

The research first appeared today in the journal Joule.

Watch: Alternative Materials for Rechargeable Batteries


Nitrogen & Potassium Interactions

Justus von Liebig’s Law of the Minimum states that yield is proportional to the amount of the most limiting nutrient, whichever nutrient it may be (Figure 1).

Figure 1: Liebig’s barrel

Nitrogen is the nutrient that most frequently provides the largest response, suggesting that this is usually most limiting. However, the plant available potassium status of a soil has a considerable influence on the uptake of nitrogen by crops, as shown through field experiments. Yield response to applied fertiliser nitrogen is decreased when the exchangeable K content of a soil is below a critical target level, index 2-.

Potassium in the Plant

Potassium is very important in the relationship between water and crop growth through helping to regulate the amount of water within a crop. The importance of this for plants is to keep them turgid and upright, which occurs as a result of water flowing into cells, causing them to swell. Simplistically, individual cells consist of an expandable cell wall enclosing a central space, the vacuole (Figure 2).

Figure 2. Simplified representation of a plant mesophyll cell, showing the vacuole, from Mengel and Kirkby ‘Principles of Plant Nutrition’, IPI, 1987.

The vacuole contains an aqueous solution (plant sap), which is largely water. It also acts as a ‘general storage compartment’ for nutrient ions, like potassium, phosphate and magnesium, and other solutes like sugars. The movement of water into cells relies on the process of osmosis, where a higher solute concentration is required within cells to allow the flow of water in that direction (Figure 3). The solute most plants tend to use is potassium, which explains why it is vital for maintaining the turgidity (rigidity) of plant cells and tissues.

Figure 3. Representation of water regulation
within cells creating turgidity

The major determinant of growth and a prerequisite for large yields in most crops is the rapid expansion of the leaf canopy in spring. This allows improved capture of sunlight energy, required to convert carbon dioxide to sugars and then into dry matter. The increase in the size of the leaf canopy is driven by cell division and expansion, for which nitrogen is a major driver. As cells expand, the volume of water contained within them increases considerably, which has implications for the K requirement of the plant, with crops often containing in excess of 300kg K20/ha (Figure 4).

Typical potash uptake for an 8t/ha wheat cropFigure 4. Typical potash uptake for an 8t/ha wheat crop

The uptake of this potassium is needed to maintain the osmotic concentrations of leaf tissues (which otherwise will become diluted) at an effective level to maintain turgor. Much of the total N and K required by crops is therefore taken up to sustain development and expansion of the leaf canopy during the early months of growth.

Trial Work

Figure 5: Response of spring barley to N fertiliser on soils with different levels of exchangeable soil K (Kex), Hoosfield, Rothamsted.

These interactions have been shown through field experiments, a lot of which have been conducted by Rothamsted Research over the years. One example comes from the Hoosfield Barley experiment started in 1852, where large differences in soil exchangeable K have developed over the years between plots which either received fertiliser K or did not (Figure 5). When four application rates of nitrogen were tested on both high and low K soils there was a considerable difference in the response of spring barley to the applied N. From this the principle of N x K interactions is clearly demonstrated. The average yields between 2000 and 2005 show that there was no justification for applying more than 50 kg N/ha to the barley grown on the soil with only 55 mg/kg exchangeable K, but yield was further increased by an application of N of up to 96 kg/ha when the soils contained adequate amounts of exchangeable soil K.

High yielding crops need large supplies of both nitrogen and potash and it is no accident that the addition of N stimulates the need for, and uptake of, K. Nitrogen is mostly taken up as the negatively charged anion nitrate (NO3) and to maintain neutrality the plant needs to take up equal amounts of a positively charged cation. Potassium as K+ is the preferred cation.

Without sufficient potassium, crops fail to use water efficiently and consequently become more seriously affected by water stress in periods of drought. Plants also use N less efficiently and are less able to handle stress caused by frost, heat, water-logging and wind.

Role of Potassium in Disease Resistance

Disease Rating, Yield and Potassium contentFigure 6. Representation of plant potassium content on disease severity

Potassium has an essential role in plant disease resistance, probably the most effective of all the nutrients. It is a regulator of enzyme activity, and therefore involved in nearly all cellular functions that influence disease severity (Figure 6). From over 200 literature reports on the role of potassium on plant diseases 70% improved plant health. Conversely, the uptake of nitrogen has the potential to increase a plants susceptibility to disease and pest attack, due to the rapid growth response that can occur initially, the changes in plant tissue colour and increase in production of sugars.

Potassium is required for the synthesis of proteins, starch and cellulose, cellulose being a component of cell walls. The function of cellulose on cell wall thickness not only acts on the plants standing power, but also as a mechanical barrier to invasion and infection by pathogens by reducing the movement of sugars out of cells. Potassium deficiency reduces cellulose production, leading to thinner cell walls and higher sugar levels in the space between the cells that stimulates fungal attack and germination of spores.

Potassium Applications

Where potassium may be in short supply from the soil there may be a benefit from top dressing with nitrogen plus potash in the spring, to ensure the shortfall from the soil is filled. If potash is short and not corrected it will becoming limiting and full response to nitrogen will therefore not be obtained, resulting in lower yields and poorer crop performance.

When there is insufficient potassium for this purpose, leaf expansion and stem elongation become too slow during the early stages of growth for the leaf canopy to expand and rapidly cover the ground. This results in inefficient interception of sunlight and photosynthetic production of assimilates required for the crop to grow rapidly and achieve maximum yield.


When used carefully, nitrogen is usually the greatest agronomic stimulant to crop growth. But for it to be used efficiently in crop production and play its full role in increasing yield, a crop must also have access to, and take up, an adequate amount of potassium from the soil. This is because there is a strong interaction between these two nutrients in crop growth which decreases the crop’s response to applied fertiliser N when the exchangeable K content of a soil is below the critical target level. Because of this interaction, there is little point in applying large amounts of N when there is too little exchangeable K in the soil as the N is used inefficiently. There is also the risk that any excess unused fertiliser N lost from the soil will have undesired adverse effects on the environment.

Nitrogen & Potassium Interactions


University of Cambridge scientists use potassium to boost efficiency of perovskite solar cells

An international team of scientists at the University of Cambridge have found that the addition of potassium iodide ‘healed’ the defects and immobilised ion movement, which have been the limiting factor for efficiency in cheap perovskite solar cells.

The next generation solar cells could be used as an energy efficiency-boosting layer on top of existing silicon-based solar cells.

The solar cells in the study were based on metal halide perovskites, which are cheap and easy to produce at low temperatures. These features make perovskites an attractive for next-generation solar cells and lighting.

But, even with their advantages, there have been certain limiting factors that have hampered their efficiency and consistency. Tiny defects in the crystalline structure of perovskites called traps can cause electrons to get ‘stuck’ before they can release their energy.

Cambridge’s Cavendish Laboratory's Dr Sam Stranks, who led the research, said: "So far, we haven’t been able to make these materials stable with the bandgap we need, so we’ve been trying to immobilise the ion movement by tweaking the chemical composition of the perovskite layers.

“This would enable perovskites to be used as versatile solar cells or as coloured LEDs, which are essentially solar cells run in reverse.”

In the study, scientists changed the chemical composition of the perovskite layers by adding potassium iodide to perovskite inks.  

The potassium iodide formed a layer on the top of the perovskite layer on top of the perovskite which had the effect of ‘healing’ the traps so that the electrons could move more freely, as well as immobilising the ion movement.

Stranks said: “Potassium stabilises the perovskite bandgaps we want for tandem solar cells and makes them more luminescent, which means more efficient solar cells.

“It almost entirely manages the ions and defects in perovskites.”

Image: Researchers use potassium to ‘heal’ perovskite solar cells. Photo: Courtesy of University of Cambridge.

University of Cambridge scientists use potassium to boost efficiency of perovskite solar cells


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