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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).
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 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).
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.
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).
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.
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.
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.
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.
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.
High-quality fruit production is the cornerstone of marketability. The optimum performance of soil-less plants depends on the well-balanced and prompt availability of minerals in nutrient solutions. In addition to concentration, nutrient ratio also plays an essential role in growth, productivity, quality and nutrient absorption.
Dec. 26, 2017 - by Jeff Gelski
KANSAS CITY — Potassium chloride remains a popular ingredient in order to reduce sodium and add potassium in food products, but that’s not the only way potassium may be used in sodium reduction strategies. Tripotassium citrate (potassium citrate) is one example as it is an option for several applications, including beverages.
Foods rich in potassium are important in managing high blood pressure because potassium lessens the effects of sodium, according to the American Heart Association, Dallas. The more potassium a person eats, the more sodium is lost through urine. Potassium also helps to ease tension in blood vessel walls, which helps further lower blood pressure.
Potassium citrate has a mild, pleasant and salty taste with a potassium content of 36%, said Caitlin Jamison, market development manager of health and nutrition for Jungbunzlauer, Inc. and based in Newton Centre, Mass. Additionally, it is water-soluble.
“This makes it easy to use for beverages,” she said. “Beyond fortification, citrates play a functional role to improve emulsification and reduce heat-related fouling deposits in protein-based beverages.”
The dairy industry and the dairy alternative industry have used sodium citrate for this purpose.
“However, potassium citrate offers similar functionality with the added benefit of providing trace amounts of potassium and a cleaner label,” Ms. Jamison said.
Potassium chloride remains in demand, too. Minneapolis-based Cargill this year opened a new potassium chloride production facility in Watkins Glen, N.Y., that has become the company’s primary production facility for potassium chloride, said Mike Beaverson, senior marketing manager for Cargill Salt.
Potassium chloride has been shown to reduce sodium by up to 50% in a range of applications, including baked foods, soup, ready-to-eat meals, snacks and sauces, according to Cargill. The company offers a variety of potassium chloride products.
“Matching the correct sodium reduction solution will vary depending on the food application,” said Janice Johnson, Ph.D., research development manager for Cargill Salt. “For brine solutions or beverages, Potassium Pro is ideal. FlakeSelect products have a unique shape that promotes faster dissolution, which can be beneficial for protein extraction in meat applications. The FlakeSelect process combines salt and potassium chloride to help prevent segregation, which may have taste benefits in some food applications such as bakery and snacks.”
An NHANES report has shown that 98% of the U.S. population does not consume enough potassium, said Alice Wilkinson, vice-president of nutritional innovation for Watson, Inc., West Haven, Conn.
“Potassium is difficult to formulate with as it has a flavor issue, and the dose is so large that even if it were flavor-neutral, it simply takes up more space than most formulas have,” she said.
Watson has developed a line of encapsulated potassium sources that help reduce flavor concerns.
“We work with potassium phosphates and chlorides in either hot melt lipid encapsulation (or) cellulose encapsulation,” Ms. Wilkinson said. “Some are designed as thin layer to keep use rates down, and our customers are having success with these products in a variety of food types, including bars and beverages.”
Potassium bicarbonates may be used as direct replacements for sodium bicarbonates in leavening applications. Church & Dwight, Inc., Ewing, N.J., offers Flow-K potassium bicarbonate, a food grade potassium bicarbonate product composed of a proprietary flow aid system that assures excellent storage and handling properties, according to the company. It allows for reduced sodium levels while maintaining overall quality and flavor. This product commonly is used in the leavening system for cakes, muffins, and cookies. It also may be used in effervescent drink mixes.
Jungbunzlauer’s sub4salt line of products uses blends of salt with potassium-based salts to provide 35% to 50% sodium reduction when used as a 1:1 replacement for salt, Ms. Jamison said. The products include combinations of potassium chloride, potassium gluconate and potassium citrate.
Potassium chloride has been linked to a bitter and metallic taste that may require flavor adjustments in formulations.
“Potassium chloride does have this typical profile, but other salts, including potassium citrate, potassium gluconate and potassium lactate, have a very clean, pleasant taste at intended use levels,” Ms. Jamison said. “In addition, Jungbunzlauer has also found that they can actually help with masking the off-taste in many food products. Some of these may be a bit surprising. For example, potassium citrate may be a logical replacement for sodium citrate when buffering a beverage. However, for certain fruity flavor profiles, such as berries, lactates work very well for enhancing the flavor character.
“Potassium-based salts can also be helpful for masking the linger and off-notes associated with high-intensity sweeteners in low-calorie beverages. Jungbunzlauer has also done work with using potassium citrate as part of a masking system for infant formulas made with protein hydrolysates.”
From the different nutrients that are needed by plants we have known for more than 4 decades that potassium is of critical importance to flowering/fruiting plants. Potassium is one of the most highly limited nutrients in soil due to its high mobility and great increases in yields have been achieved with both potassium fertilization in soil and the use of properly balanced nutrient solutions containing enough potassium in hydroponics. But how important is potassium and what is its ideal concentration in hydroponic nutrient solutions when growing flowering plants? Today we are going to take a look at the scientific literature about potassium and what the optimum levels of potassium for different flowering plants might be in order to maximize yields.
There are many studies in the scientific literature dealing with the effect of potassium on various flowering plants. Earlier evidence from the 1980s pointed to optimum concentrations of potassium being close to the 160-200 ppm range. The book "mineral nutrition" by P.Adams (here) summarizes a lot of the knowledge that was available at the time and shows that for the growing techniques available at the time using greater concentrations of K was probably not going to give a lot of additional benefit.
However newer evidence from experiments carried out within the past 10 years shows that optimum potassium concentration might depend on a significant variety of factors, from which media, other nutrient concentrations and growing system type might play critical roles. For example study on strawberries in 2012 (here) showed optimum concentrations of K to be around 300 ppm for strawberries and the optimum media to be a mixture of peat+sand+perlite (image from this article included above).
Evidence from experiments on tomatoes (link here and image from this article above) also shows that for tomatoes the actual optimum concentration of K might actually be larger under some condition with the optimum in this study in terms of fruit quality and yields being 300 ppm. In this last case the tomatoes were grown using a nutrient film technique (NFT) setup. However there have also been studies under other growing conditions - like this one on reused pumice - which shows that increasing K concentrations to 300ppm can actually have detrimental consequences. In this case tomatoes fed at 200, 290 and 340ppm of K had very similar results when using new substrate but the old substrate heavily underperformed when high K concentrations were used.
Papers published on the effect of different K concentrations in melons (here) and cucumbers (here) also point to optimal concentrations in the 200-300 ppm range and for the optimum N:K ratio to be between 1:2 and 1:3 for these plants. This is probably the reason why you will often find suggested nutritional guidelines for flowering plants - like those below taken from here - mostly suggesting K concentrations in the 250-350ppm range. However you will often find that they directly contradict research papers, like this guideline suggesting K of 150 ppm for strawberries while we saw in a recent paper that 300ppm might be better. This is most probably due to differences in the sources used which might have used different growing systems or plant varieties which responded to other conditions better.
All in all the subject of K concentration in hydroponics is no simple one. Using low K will limit your yields tremendously but increasing your K very high can also harm your plants, especially depending on the type of media you are using. In general aiming for a K concentration between 200-250 ppm is safest but in many cases increases to the 300-400ppm range can bring significant increases in plant yields. A careful study of the available literature and the actual growing conditions that the plants will be subjected to will be key in determining what the best K concentration to use will be. Alternatively carrying out adequately designed experiments under your precise growing environment will help you carry out an evidence-based decision about what K concentration to use.
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