Efficient Fertilizer Use — Potassium : Dr. Bob Thompson 1 SECTION CONTENTS: • Introduction • K in Plants • K Uptake by Plants • K Removal by Crops • K Deficiency Symptoms • K in Nature • K Interactions • Placement • Potassium Fertilizers Introduction Potassium (K) is one of sixteen essential nutrients required for plant growth and reproduction. It is classified as a macronutrient, as are nitrogen (N) and phosphorus (P). The chemical symbol for potassium is "K." It is taken up by plants in its ionic form (K + ). The word potassium translates from the Latin or German word, Kalium. The term "potash" comes from the colonial practice of burning wood in large pots and using the ashes as fertilizer and making soap, gunpowder and glass. "Potash" is defined as K 2 O and is used to express the content of various fertilizer materials containing potassium, such as muriate of potash (KCl), sulfate of potash (K 2 SO 4 ), double sulfate of potash and magnesium (K 2 SO 4 · 2MgSO 4 ), and nitrate of potash (KNO 3 ). Frequently, the expressions "K" and "K 2 O" are used interchangeably, although technically incorrectly. Potassium In Plants While potassium is not a constituent of any plant structures or compounds, it plays a part in many important regulatory roles in the plant. It is essential in nearly all processes needed to sustain plant growth and reproduction. Potassium plays a vital role in: • Photosynthesis • Translocation of photosynthates • Protein synthesis • Control of ionic balance • Regulation of plant stomata and water use • Activation of plant enzymes • And, many other processes It is known to activate at least sixty enzymes involved in plant growth. And, this may be its most important function in the plant. Plants deficient in potassium are less resistant to drought, excess water, and high and low temperatures. They are also less resistant to pests, diseases and nematode attacks. Potassium is also known as the quality nutrient because of its important effects on quality factors such as size, shape, color, taste, shelf life, fiber quality and other quality measurements.
Transcript
Efficient Fertilizer Use — Potassium: Dr. Bob Thompson
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SECTION CONTENTS: • Introduction • K in Plants • K Uptake by Plants • K Removal by Crops • K Deficiency Symptoms • K in Nature • K Interactions • Placement • Potassium Fertilizers
Introduction
Potassium (K) is one of sixteen essential nutrients required for plant growth and reproduction. It is classified as a macronutrient, as are nitrogen (N) and phosphorus (P). The chemical symbol for potassium is "K." It is taken up by plants in its ionic form (K+). The word potassium translates from the Latin or German word, Kalium. The term "potash" comes from the colonial practice of burning wood in large pots and using the ashes as fertilizer and making soap, gunpowder and glass. "Potash" is defined as K2O and is used to express the content of various fertilizer materials containing potassium, such as muriate of potash (KCl), sulfate of potash (K2SO4), double sulfate of potash and magnesium (K2SO4 · 2MgSO4), and nitrate of potash (KNO3
). Frequently, the expressions "K" and "K2O" are used interchangeably, although technically incorrectly.
Potassium In Plants
While potassium is not a constituent of any plant structures or compounds, it plays a part in many important regulatory roles in the plant. It is essential in nearly all processes needed to sustain plant growth and reproduction. Potassium plays a vital role in:
• Photosynthesis • Translocation of photosynthates • Protein synthesis • Control of ionic balance • Regulation of plant stomata and water use • Activation of plant enzymes • And, many other processes
It is known to activate at least sixty enzymes involved in plant growth. And, this may be its most important function in the plant. Plants deficient in potassium are less resistant to drought, excess water, and high and low temperatures. They are also less resistant to pests, diseases and nematode attacks. Potassium is also known as the quality nutrient because of its important effects on quality factors such as size, shape, color, taste, shelf life, fiber quality and other quality measurements.
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Potassium increases crop yields because it:
• increases root growth and improves drought tolerance • builds cellulose and reduces lodging • enhances many enzyme actions • aids in photosynthesis and food formation • helps translocate sugars and starches • produces grains rich in starch • increases protein content of plants • maintains turgor, reduces water loss and wilting • helps retard crop diseases and nematodes
Potassium Uptake By Plants
Table 7.1: Potash Uptake by Crops
Time of potassium uptake varies with different plants. However, plants generally absorb the majority of their potassium at an earlier growth stage than they do nitrogen and phosphorus. Experiments on potassium uptake by corn showed that 70-80 percent was absorbed by silking time, and 100 percent was absorbed three to four weeks after silking. Translocation of potassium from the leaves and stems to the grain was much less than for phosphorus and nitrogen. The period during grain formation is apparently not a
critical one for supply of potassium. Cotton takes up about 30 percent of its potassium during the first twelve to fourteen days of blooming. At this peak period of potassium
Crop Yield Uptake (K2O)
Alfalfa 10 ton/acre 600 lb/acre
Banana 31 ton/acre 1286 lb/acre
Clover-grass Mixture 6 ton/acre 360 lb/acre
Coastal Bermudagrass
10 ton/acre 480 lb/acre
Coffee 2233 lb/acre 160 lb/acre
Corn 200 bu/acre 266 lb/acre
Corn Silage 32 ton/acre 266 lb/acre
Cotton 1500 lb/acre lint 210 lb/acre
Grain Sorghum 8000 lb/acre 240 lb/acre
Oil Palm 11 ton/acre 268 lb/acre
Peanuts 4000 lb/acre 185 lb/acre
Soybeans 60 bu/acre 205 lb/acre
Wheat 80 bu/acre 162 lb/acre Source: PPI
Note: Potassium content of fertilizers is expressed as K2O, although there is no such compound in fertilizers, nor is it absorbed by or found in the plant in that form. Soil and plant tissue analyses values are usually expressed in terms of percent potassium (K) but fertilizer recommendations are expressed as K2O. To convert from K to K2O, multiply K2O by 0.83. To convert from K2O to K, multiply K2O by a factor of 1.20.
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uptake, 3-4 lb/acre are taken up daily. Sixty-six percent of the total potassium is rapidly translocated from the leaves and stems to the bur of the boll during boll fill. Nitrogen and phosphorus are translocated to the seed.
Plant requirements for potassium differ widely. Amounts of potassium utilized by several agronomically important crops are given in Table 7.1. More detailed nutrient utilization data is presented in the Appendix.
Potassium Removal By Crops
Table 7.2: Potash Removed by Crops
Nutrient uptake or utilization is an important consideration but crops take up far more potassium than they remove with the harvested portion. For example, a 200 bu/acre corn crop takes up or utilizes about 266 lb/acre of potash (K2O). But when the corn is harvested as grain, only 0.29 lb/bu is removed, or 58 lb/ton K2O is harvested and removed from the field. However, if the crop were harvested as silage, then 8.3 lb/ton K2O are vested and removed from the field. Therefore, a 32 ton/acre silage crop would remove 266 lb/acre K2O. Harvest management is the major consideration in developing a potash fertilization program. Crops harvested where the whole plant is removed from the field, like alfalfa hay, must have more
potash applied than crops where only grain, lint or fruit are removed. Often with hay and silage crops, removal is an excellent guide for planning the potash fertilization program. With other crops, such as grain, soil tests offer the best guide.
Crop Removal (K2O)
Alfalfa 60.0 lb/ton
Coastal Bermudagrass 50.0 lb/ton
Corn 0.29 lb/ton
Corn Silage 8.30 lb/ton
Cotton 20.0 lb/bale
Grain Sorghum 0.38 lb/bu
Peanuts 17.0 lb/bu
Rice 0.18 lb/bu
Soybeans 1.4 lb/bu
Sugarcane 3.50 lb/ton
Tall Fescue 52.0 lb/ton
Tobacco (Burley) 4.70 lb/cwt
Tobacco (Flue-cured) 5.20 lb/cwt
Wheat 0.34 lb/cwt Source: PPI
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Potassium Deficiency Symptoms
Plants absorb potassium as the potassium ion (K+). Potassium is a highly mobile element in the plant and is translocated from the older to younger tissue. Consequently, potassium deficiency symptoms usually occur first on the lower leaves of the plant and progress toward the top as the severity of the deficiency increases. One of the most common signs of potassium deficiency is the yellow scorching or firing (chlorosis) along the leaf margin. In severe cases of potassium deficiency the fired margin of the leaf may fall out. However, with broadleaf crops, such as soybeans and cotton, the entire leaf may shed resulting in premature defoliation of the crop.
Potassium deficient crops grow slowly and have poorly developed root systems. Stalks are weak and lodging of cereal crops such as corn and small grain is common. Legumes are not strong competitors for soil potassium and are often crowded out by grasses in a grass-legume pasture. When potassium is not sufficient, winter-killing of perennial crops such as alfalfa and grasses can occur.
Seeds from potassium deficient plants are small, shriveled, and are more susceptible to diseases. Fruit is often lacking in normal coloration and is low in sugar content. Vegetables and fruits deteriorate rapidly when shipped and have a short shelf life in the market.
Figure 7.1 Potassium deficiency symptoms in corn and soybeans.
Corn: Firing or scorching appears on outer edge of leaf, while midrib remains green. May be some yellow striping on lower leaves. (Sorghum and most grasses also react this way.) Poor root development, defective nodal tissues, unfilled, chaffy ears, and stalk lodging are other symptoms in corn.
Soybeans: Firing or scorching begins on outer edge of leaf. When leaf tissue dies, leaf edges become broken and ragged…delayed maturity and slow defoliation…shriveled and less uniform beans, many worthless.
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Figure 7.2 Potassium deficiency symptoms in alfalfa.
Alfalfa: With classical symptoms (shown at top right), first signs of K deficiency are small white or yellowish dots around outer edges of leaves…then edges turn yellow and tissue dies and becomes brown and dry. However, for alfalfa grown on soils high in sodium (Na), the K deficiency symptoms has a different appearance, as indicated in the photo at left above.
Figure 7.3 Potassium deficiency symptoms in cotton.
Cotton: Cotton “rust” …first a yellowish or bronze mottling in the leaf. leaf turns yellowish green, brown specks at tip around margin and between veins. As breakdown progresses, whole leaf becomes reddish brown, dies, sheds prematurely. Short plants with fewer, smaller bolls or short, weak fibers. In the past, K deficiency symptoms have been described as occurring on older, mature leaves at the bottom of the plant. In recent years, symptoms have been observed at the top on young leaves of some heavily fruited cotton varieties.
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Figure 7.4 Potassium deficiency symptoms in wheat.
Wheat: Frequently, no outstanding hunger signs on leaf itself (no discoloration, scorching, or mottling), but sharp difference in plant size and number, length, and condition of roots. Lodging tendency. Smaller kernels. In advanced stages, withering or burn of leaf tips and margins, beginning with older leaves.
Figure 7.5 Potassium deficiency symptoms in potatoes, apples, rice and sugarcane
Potatoes: Upper leaves, usually smaller, crinkled and darker green than normal with small necrotic patches…middle to lower leaves show marginal scorch and yellowing. Early indicator: dark green, crinkled leaves, though varieties differ in normal leaf color and texture.
Apples: Yellowish green leaves curl upward along entire leaf…scorched areas develop along edges that become ragged. Undersized and poorly colored fruit may drop prematurely. Poor storage, shipping and canning qualities in fruit.
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Rice: Rice deficient in K may show symptoms as stunted plants, a slight reduction in tillering, and short, droopy, dark green upper leaves. Yellowing may appear in interveinal areas of lower leaves, starting from the top and eventually drying to a light brown. Long thin panicles and black, deteriorated roots may be related to K deficiency.
Sugarbeets: The first sign of K deficiency appears as tanning and leathering of the edges of recently matured leaves. When the soil solution is very low in Na, a severe interveinal leaf scorch and crinkling proceeds to the midrib. Under high Na conditions, tanning and leaf scorch lead to a smooth leaf surface.
Figure 7.6 Potassium deficiency symptoms in canola, peanuts, coastal bermudgrass and grapes.
Canola: Potassium deficiency reduces growth, resulting in smaller leaves and thinner stems. Plants are more easily lodged and may wilt. Under severe deficiency, the edges of older leaves become yellow, or scorched and may die completely, but remain attached to the stem.
Peanuts: Because K is easily redistributed from mature to younger organs, deficiency symptoms are first observable in the older, lower leaves. Deficiency is expressed by chlorosis of the leaves, beginning at the leaf margin. Potassium deficiency occurs frequently in acidic soils, and symptoms usually appear within five weeks of planting.
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Coastal Bermudagrass: Potassium plays an important role in heat, drought and cold tolerance of forage grasses. Leafspot diseases may be the first symptom of K deficiency recognized in Coastal and other hybrid bermudagrasses. Yellowing of older leaves, followed by leaf tip and leaf margin chlorosis, can occur with severe deficiency. Reddish-brown to purple spots, caused by fungal infection, may also be scattered over younger leaf blades. Thinning stands and reduced growth, followed by death of older leaves, are frequent symptoms.
Grapes: Potassium deficiency symptoms typically appear in early summer on leaves on the middle portion on the shoots. The leaves fade, becoming chlorotic beginning at the leaf margin, while the center portion of the leaf and veins remain green. The leaves tend to cup downward. In white wine varieties (such as Chardonnay, shown in photo) the leaves become mostly yellow or yellow bronze.
Occurrence Of Potassium In Nature
Potassium is abundant in nature, comprising about 2.4 percent of the earth’s crust. The potassium content of soils varies widely, ranging from only a few hundred pounds per acre (furrow-slice 6" depth) to over 50,000 pounds per acre or more in fine-textured soils formed from rocks that are high in potassium-bearing minerals. All naturally occurring potassium contained in the soil originated from the disintegration and decomposition of potash-feldspars (orthoclase and microcline) and micas (muscovite and biotite). Much of the natural potassium occurring in soils is not available to plants and crops; therefore, soils containing relatively large amounts of total potassium usually respond to potassium fertilization.
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Relatively Unavailable Potassium
From 90-98 percent of the total potassium present in soils is found in insoluble primary minerals such as feldspars and micas. These minerals consist of potassium-aluminum silicates which are resistant to chemical breakdown. They release potassium slowly, but in small quantities compared to total needs of growing crops.
Slowly Available Potassium
This form comprises 1-10 percent of the total potassium supply and may originate from dissolved primary minerals or from potassium fertilizers. This potassium is attracted to the surface of clay minerals where it may be firmly bound or fixed between the clay layers in a form slowly available to plants. The actual amount available depends on the type and amount of clay present.
Readily Available Potassium
Readily available forms of potassium comprise only 0.1 to 2 percent of the total potassium in the soil and consist of potassium dissolved in the soil solution and held on the exchange positions of the clay and organic matter. This potassium is referred to as "exchangeable" because it can be replaced by other positively-charged ions (cations) such as
hydrogen, calcium, and magnesium. This exchange happens rapidly and frequently. The potassium in the soil solution may be taken up by the plant or lost from the soil by leaching, especially on sandy coarse-textured soils in regions of high rainfall.
Potassium Interactions With Other Nutrients
Adequate supplies of other plant nutrients are required to obtain maximum responses to potassium fertilization; however, there are several unique relations between potassium and other nutrients, due to the complementary ion effect (other cations held on the cation exchange positions of the clay) that are important in plant nutrition.
Figure 7.7 The Potassium Cycle
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High potassium fertilization can decrease the availability of magnesium to the plant and may result in magnesium deficiency of crops grown on soils that are already low in magnesium. This problem is often encountered with crops grown on sandy soils, particularly in the Coastal Plain soils of the southern United States. Conversely, crops grown on soils high in magnesium can suffer potassium deficiency, especially if the soils are high in phosphorus and low in potassium. This problem is especially severe in the soils of the Mississippi River flood plain.
Correct these deficiency problems by adding the deficient nutrient through a well-planned soil fertility program. High levels of potassium fertilization along with ammoniacal nitrogen (NH4
+) also depress the magnesium content of forage grasses and may result in grass tetany (hypomagnesemia) of cattle consuming the forage.
Sodium is an element similar to potassium in its chemical properties. Sodium has been shown to substitute partially for potassium in some crops.
Leaching of potassium on acid, sandy soils may be reduced by liming the soil to a pH of 6.2 to 6.5; however, applications of high rates of limestone to a soil low in potassium may induce potassium deficiency of crops growing on those soils. This problem occurs more on soils with predominantly 2:1 type clays (such as montmorillonite clays) rather than the 1:1 type (such as kaolinitic clays).
Placement of Potassium Fertilizers
The common potassium fertilizers are completely water-soluble and, in some cases, have a high salt index. Consequently, when placed too close to seed or transplants, they can decrease seed germination and plant survival. This fertilizer injury is most severe on sandy soils, under dry conditions, and with high rates of fertilization—especially nitrogen and potassium. Some crops such as soybeans, cotton, and peanuts are much more sensitive to fertilizer injury than corn. Placement of the fertilizer in a band approximately three inches to the side and two inches below the seed is an effective method of preventing fertilizer injury. Row placement of potassium fertilizer is generally more efficient than broadcast application when the rate of application is low or soil levels of potassium are low.
Broadcasting and mixing with the soil before planting is usually a convenient and effective method of applying potassium fertilizers. Fertilizer injury is minimized by this method but on deep sandy soils some potassium may be lost by leaching, especially if considerable time elapses between application and planting and heavy rainfall occurs. In some soils that contain clay minerals (2:1 type) that fix potassium, some fertilizer may become unavailable.
Split application of potassium fertilizers on long season crops such as alfalfa or grass crops that are harvested several times during the growing season is often recommended. This practice prevents the crop from absorbing more potassium than is
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needed for maximum growth during the early growing season (luxury consumption) and provides adequate available potassium during the latter part of the growing season.
Broadcast application of potassium under minimum tillage results in much of the applied potassium remaining in the top 1 to 2 inches of the soil; whereas with conventional tillage it is distributed throughout the plow layer. Corn usually absorbs sufficient potassium under no-till due to its extensive root system in the surface layer of the soil. Leaf analysis of corn shows a lower potassium content under minimum tillage than with conventional tillage due to either the location of the applied potassium or to poorer aeration. Sufficient potassium can be supplied by using a higher rate of potassium fertilization with no-till systems.
Potassium Fertilizers
Elemental potassium (K) is not found in pure state in nature because of its high reactivity. It can be purified, but must be kept in oil to retain its purity and prevent violent reactivity. Potash deposits occur as beds of solid salts beneath the earth’s surface and brines in dying lakes and seas.
Potassium is mined from a number of minerals. Sylvinite, sylvite, and langbeinite are the most important mineral sources.
Sylvinite
Sylvinite is composed primarily of potassium chloride (KCl) and sodium chloride (NaCl) and the unrefined ore contains 20-30% K2O.
Sylvite
The mineral sylvite is composed mainly of muriate of potash (KCl) and the refined ore contains about 60-62% K2O.
Langbeinite
The langbeinite mineral is composed largely of potassium sulfate (K2SO4) and magnesium sulfate (MgSO4). The chemical formula is K2SO4 · 2MgSO4. It contains about 22% K2O, 11% Mg and 22% S. In addition to the mineral name, it is called potassium magnesium sulfate, double sulfate of potassium and magnesium, and either K-Mag® or Sul-Po-Mag®. K-Mag is marketed within the United States, whereas K-Mag or Sul-Po-Mag may be used internationally. The product is 100% water-soluble and essentially chloride-free.
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Mining Potash
Potash is mined three major ways:
1. Conventional shaft mining method similar to coal mining. This technique undercuts the face, drills, and blasts.
2. Continuous mining method. This shaft mining technique uses specially developed machines that take the ore directly from the vein.
3. Solution mining. This process pumps hot water down to the potash ore bed, dissolves the salts and returns the potash brine to the surface for refining.
Potassium Chloride
Muriate of potash, potassium chloride, or KCl accounts for more than 90 percent of the potassium used in the United States. It is water-soluble and contains 60-62% K2O.
Most muriate of potash is produced from sylvinite, but some comes from brines. The raw impure ore is refined to fertilizer by crystallization or flotation processes. Most agricultural KCl is produced by the flotation processes.
Fertilizer grade KCl is available in five particle sizes: white soluble, special standard, standard, coarse, and granular. Granular is very well suited to bulk blending. The white soluble grade is ideal for the manufacturing of clear liquid fertilizers.
Potassium Sulfate
Potassium sulfate, sulfate of potash or K2SO4 contains about 50% K2O and 18% sulfur (S). Because the chloride content is below 2.5%, it is used for chloride-sensitive crops (such as tobacco, fruits, and some vegetables) to supply sulfur as a crop nutrient. It accounts for about four to six percent of total agricultural potassium sales. Potassium sulfate can be used where chloride buildup becomes a problem.
Double Sulfate of Potash and Magnesium
Double sulfate of potash and magnesium, potassium magnesium sulfate, (Sul-Po-Mag®, K-Mag®) K2SO4· 2MgSO4 are names used to describe the mined and processed mineral langbeinite. It contains about 22% K2O, 11% magnesium (Mg), and 22% sulfur (S). The typical chloride content is 2.5%.
K-Mag/Sul-Po-Mag is a naturally occurring mineral that is a good source of water-soluble magnesium, potassium, and sulfur...all in the sulfate form.
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Potassium Nitrate
Potassium nitrate or KNO3 contains little or no chloride or sulfur. It can supply both nitrogen and potassium nutrients to chloride sensitive crops. It contains about 44% K2O and 13% nitrogen (N).
Table 7.3: Various Potassium Fertilizer Materials and Their Percent Nutrient Content
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