Author: Kavita Sinha
Potassium is an essential plant nutrient and is required in large amounts for proper growth and reproduction of plants. Potassium (K+) is the predominant inorganic ion of plant cells where it can contribute up to 10% of the dry mass. K+ is recognized as a rate-limiting factor for crop yield and quality. It plays a major role as stabilizer in metabolism and as an osmotic contributing to cellular hydrostatic (turgor) pressure, growth and responses to environmental changes. A high and relatively stable potassium concentration in certain cell compartments is important for enzyme activation, stabilization of protein synthesis, neutralization of negative charges on proteins, formation of membrane potential in cooperation with the proton motive force, and maintenance of cytosolic pH homeostasis. Potassium is considered second only to nitrogen, when it comes to nutrients needed by plants, and is commonly considered as the “quality nutrient.”
Functions of potassium
Potassium plays many important regulatory roles in development. Potassium (K) increases crop yield and improves quality. It is required for numerous plant growth processes.
Enzyme Activation- Enzymes serve as catalysts for chemical reactions, being utilized but not consumed in the process. They bring together other molecules in such a way that the chemical reaction can take place. Potassium “activates” at least 60 different enzymes involved in plant growth. The K changes the physical shape of the enzyme molecule, exposing the appropriate chemical active sites for reaction. Potassium also neutralizes various organic anions and other compounds within the plant, helping to stabilize pH between 7 and 8...optimum for most enzyme reactions. The amount of K present in the cell determines how many of the enzymes can be activated and the rates at which chemical reactions can proceed. Thus, the rate of a given reaction is controlled by the rate at which K enters the cell (Van Brunt and Sultenfuss, 1998).
Stomatal Activity (Water Use)- Plants depend upon K to regulate the opening and closing of stomata. The pores through which leaves exchange carbon dioxide (CO2), water vapor, and oxygen (O2) with the atmosphere. Proper functioning of stomata is essential for photosynthesis, water and nutrient transport, and plant cooling. When K moves into the guard cells around the stomata, the cells accumulate water and swell, causing the pores to open and allowing gases to move freely in and out. When water supply is short, K is pumped out of the guard cells. The pores close tightly to prevent loss of water and minimize drought stress to the plant (Thomas and Thomas, 2009). If K supply is inadequate, the stomata become sluggish slow to respond and water vapor is lost. Closure may take hours rather than minutes and is incomplete. As a result, plants with an insufficient supply of K are much more susceptible to water stress. Accumulation of K in plant roots produces a gradient of osmotic pressure that draws water into the roots. Plants deficient in K are thus less able to absorb water and are more subject to stress when water is in short supply.
Photosynthesis The role of K in photosynthesis is complex. The activation of enzymes by K and its involvement in adenosine triphosphate (ATP) production is probably more important in regulating the rate of photosynthesis than is the role of K in stomatal activity. When the sun’s energy is used to combine CO2 and water to form sugars, the initial high-energy product is ATP. The ATP is then used as the energy source for many other chemical reactions. The electrical charge balance at the site of ATP production is maintained with K ions. When plants are K deficient, the rate of photosynthesis and the rate of ATP production are reduced, and all of the processes dependent on ATP are slowed down. Conversely, plant
respiration increases which also contributes to slower growth and development. In some plants, leaf blades re-orient toward light sources to increase light interception or away to avoid damage by excess light, in effect assisting to regulate the rate of photosynthesis. These movements of leaves are brought about by reversible changes in turgor pressure through movement of K into and out of specialized tissues similar to that described above for stomata (Van Brunt and Sultenfuss, 1998).
Transport of Sugars Sugar produced in photosynthesis must be transported through the phloem to other parts of the plant for utilization and storage. The plant’s transport system uses energy in the form of ATP. If K is inadequate, less ATP is available, and the transport system breaks down. This causes photosynthates to build up in the leaves, and the rate of photosynthesis is reduced. Normal development of energy storage organs, such as grain, is retarded as a result. An adequate supply of K helps to keep all of these processes and transportation systems functioning normally (Van Brunt and Sultenfuss, 1998).
Water and Nutrient Transport- Potassium also plays a major role in the transport of water and nutrients throughout the plant in the xylem. When K supply is reduced, translocation of nitrates, phosphates, calcium (Ca), magnesium (Mg), and amino acids is depressed (Schwartzkopf, 1972). As with phloem transport systems, the role of K in xylem transport is often in conjunction with specific enzymes and plant growth hormones. An ample supply of K is essential to efficient operation of these systems (Thomas and Thomas, 2009).
Protein Synthesis - Potassium is required for every major step of protein synthesis. The “reading” of the genetic code in plant cells to produce proteins and enzymes that regulate all growth processes would be impossible without adequate K. When plants are deficient in K, proteins are not synthesized despite an abundance of available nitrogen (N). Instead, protein “raw materials” (precursors) such as amino acids, amides and nitrate accumulate. The enzyme nitrate reductase catalyzes, the formation of proteins and K is likely responsible for its activation and synthesis (Patil, 2011).
Starch Synthesis- The enzyme responsible for synthesis of starch (starch synthetase) is activated by K. Thus, with inadequate K, the level of starch declines while soluble carbohydrates and N compounds accumulate. Photosynthetic activity also affects the rate of sugar formation for ultimate starch production. Under high K levels, starch is efficiently moved from sites of production to storage organs (Patil, 2011).
Crop Quality- Potassium plays significant roles in enhancing crop quality. High levels of available K improve the physical quality, disease resistance, and shelf-life of fruits and vegetables used for human consumption and the feeding value of grain and forage crops. Fiber quality of cotton is improved. Quality can also be affected in the field before harvesting such as when K reduces lodging of grains or enhances winter hardiness of many crops. The effects of K deficiency can cause reduced yield potential and quality long before visible symptoms appear. This “hidden hunger” robs profits from the farmer who fails to keep soil K levels in the range high enough to supply adequate K at all times during the growing season.
Potassium Uptake
Bio-availability and uptake of K by plants from the soil vary with a number of different factors. The rate of respiration by plants is largely the determining factor for proper uptake and transport of potassium by plants. Its uptake is dependent on sufficient energy (ATP). Potassium plays a vital role in the trans-location of essential nutrients, water, and other substances from the roots through the stem to the leaves. It is also made available through fertilizers in the form of K2O. Plant tissues analyze the form in these fertilizers and convert it in a more bio-available form. It is absorbed in the form of ions- K+.
Potassium uptake by plants is affected by several factors -
Soil Moisture: Higher soil moisture usually means greater availability of K. Increasing soil moisture increases movement of K to plant roots and enhances availability. Research has generally shown more responses to K fertilization in dry years.
Soil Aeration and Oxygen Level: Air is necessary for root respiration and K uptake. Root activity and subsequent K uptake decrease as soil moisture content increases to saturation. Levels of oxygen are very low in saturated soils.
Soil Temperature: Root activity, plant functions, and physiological processes all increase as soil temperature increases. This increase in physiological activity leads to increased K uptake. Optimum soil temperature for uptake is 60-80°F. Potassium uptake is reduced at low soil temperatures.
Tillage System: Availability of soil K is reduced in no-till and ridge-till planting systems. The exact cause of this reduction is not known. Results of research point to restrictions in root growth combined with a restricted distribution of roots in the soil.
Potassium deficiency in plants
Potassium deficiency might cause abnormalities in plants, usually the symptoms are growth related.
Potassium deficiency symptoms
Cholrosis – scorching of plant leaves, with yellowing of the margins of the leaf. This is one of the first symptoms of Potassium deficiency. Symptoms appear on middle and lower leaves.
Slow or Stunted growth – as potassium is an important growth catalyst in plants, potassium deficient plants will have slower or stunted growth.
Poor resistance to temperature changes and to drought – Poor potassium uptake will result in less water circulation in the plant. This will make the plant more susceptible to drought and temperature changes.
Defoliation - left unattended, potassium deficiency in plants results in plants losing their leaves sooner than they should. This process might become even faster if the plant is exposed to drought or high temperatures. Leaves turn yellow, then brown and eventually fall off one by one.
Poor resistance to pests- K-deficient plants tend to be more susceptible to infection than those with an adequate supply of K. Use of K significantly decreased the incidence of fungal diseases by 70%, bacteria by 69%, insects and mites by 63%, viruses by 41% and nematodes by 33%.
Predicting the needs for potash
The K status of soils can be monitored with either plant analysis or routine soil testing procedures. Plant analysis can be used to either confirm a suspected deficiency indicated by visual symptoms or routinely monitor the effects of a chosen fertilizer program. An interpretation for K levels in plant tissue is provided in Table 1.
If amounts of K in the root zone are more than enough to meet crop needs, K will be absorbed by plants in amounts higher than required for optimum yield. This can lead to higher than normal concentrations of K in plant tissue and is referred to as "luxury consumption." Luxury consumption has no known negative effect on plant growth and yield. Plant analysis is a management tool that can be used to look back at nutrient supplies during the growing season. This tool cannot be used to predict the amount of potash needed for any crop in the next growing season.
Table 1. Sufficiency levels of potassium for major agronomic crops, vegetables, and fruit
| Crop | Plant part | Time | Sufficiency range (% K) | |
| Alfalfa | Tops (6" new growth) | Prior to flowering | 2.0-3.5 | |
| Apple | Leaf from middle of current terminal shoot | July 15 - August 15 | 1.2-1.8 | |
| Blueberry | Young mature leaf | First week of harvest | 0.4-0.7 | |
| Broccoli | Young mature leaf | Heading | 2.0-4.0 | |
| Cabbage | Half-grown young wrapper leaf | Heading | 3.0-5.0 | |
| Carrot | Young mature leaf | Mid-growth | 2.8-4 | |
| Cauliflower | Young mature leaf | Buttoning | 2.6-4.2 | |
| Corn | Whole tops | Less than 12" tall | 2.5-3.5 | |
| Edible bean | Most recently matured trifoliate | Bloom stage | 1.5-3.3 | |
| Grape | Petiole from young mature leaf | Flowering | 1.5-2.0 | |
| Soybean | Trifoliate leaves | Early flowering | 1.7-2.5 | |
| Spring wheat | Whole tops | As head emerges from boot | 1.5-3.0 | |
| Strawberry | Young mature leaf | Mid-August | 1.1-2.5 | |
| Sweet corn | Ear leaf | Tasseling to silk | 1.8-3.0 | |
| Sugar beet | Recently matured leaves | 50-80 days after planting | 2.0-6.0 | |
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The relative level classifications represent an estimation of the soil’s ability to supply all the needed K for a crop. An increase in production can be expected if potash fertilizer is added to the fertilizer program when soil test values are in the low and very low ranges. Added yield may or may not be observed if potash is added when the soil test values are in the medium range or greater. A response to potash fertilization should not be expected if soil test values for K are in the high or very high range.
Table 2 . Rating chart of soil test values for k for Indian soil for air-dried soil samples.
| Soil test potassium (kg/ha) | Rating |
| < 108 | low |
| 108-280 | medium |
| >280 | high |
Conclusion
Potassium is extremely important in many ways to the productivity of plant. The need for potash in a fertilizer program can be determined from plant analysis and soil testing. Soil testing is the most reliable predictor of this need.
References:
Bryson. (2014), Plant Analysis Handbook III; Rosen and Eliason (2002), Nutrient Management for Commercial Fruit and Vegetable Crops in Minnesota.
Patil RB (2011 ). Role of potassium humate on growth and yield of soybean and black gram.International Journal of Pharma and Bio sciences 2(1) 242-246.
Schwartzkopf C (1972) . Potassium, calcium, magnesium- how they relate to plant growth mid-continent agronomist, us green section role of potassium in crop establishment from agronomists of the potash & phosphate institute.
Thomas TC and Thomas AC (2009) . Vital role of potassium in the osmotic mechanism of stomata aperture modulation and its link with potassium deficiency. Plant Signal Behaviour 4(3) 240–243.
Van Brunt JM and Sultenfuss JH(1998). Better crops with plant food. In Potassium: Functions of Potassium 82(3) 4-5.
About Author / Additional Info:
I am Deputy Project Director (ATMA) In Kanker