POTASSIUM

Potassium is the last of what might be called Potassium is also important in helping the plant the “big three” nutrients – nitrogen, fight diseases. When potassium is deficient, phosphorus, and potassium. These three fertilization can reduce the severity of several primary nutrients are, by far, the nutrients most crop diseases, including; commonly limiting for crop production. Potassium has long been referred to as • Corn and Sorghum stalk rot. K strengthens “potash.” A few hundred years ago, potassium stalks against invading organisms and were commonly obtained by reduces lodging. However, stalk rot can extracting wood ashes – hence the name result from several factors other than K “potash.” In fact, much of the forest in eastern deficiency. United States was cut, burned, and the potash was sold to England. Today, potash usually • Corn Leaf blight. Symptoms often occur in refers to the oxide form K2O.By law, K deficient fields. analyses guarantees are still in terms of the K2O equivalent. • Soybean Molds and Mildews. K reduces shrunken, moldy seeds.

Functions of Potassium in Plants • Potato Black Spot and Stem End Rot.

A unique aspect of K is that it is not a part of • Grass Leaf spot and Dollar spot. K thickens any structural component of plants and as a leaf cuticles. K can improve the quality of soluble ion in the plant sap, it is required to lawns, even on with high K soil tests. activate at least 60 plant enzymes. As a result, potassium in new crop residues is very soluble • Root disease in small grains may be due in and is readily washed from the residue and part to chloride in KCl. returned to the soil

Potassium plays an essential role in Potassium Deficiency Symptoms photosynthesis and metabolism of plants. It is important in carbohydrate breakdown which Potassium deficiencies are not easy to detect furnishes energy for plant growth. Potassium visually. Moderate deficiencies produce only a also increases resistance in plants and reduction in growth. Only with severe deficiencies aid in reducing plant water loss. do classical potassium deficiencies appear.

Potassium aids in the conversion of N into Typical deficiency symptoms includes spotting, proteins in the plant. Grasses need K to streaking or curling of leaves, starting with the balance high rates of N fertilizer. If K is lacking, lower portion of plant. Lower leaves appear some N will remain as non-protein nitrogen. scorched or burned at margins and tips. In corn, Crops with high protein levels, such as alfalfa grains, and grasses, burning starts at tip of leaf and soybeans, tend to have higher potassium and proceeds down the edge, usually leaving the requirements. midrib green. Potassium moves readily in the plant; thus leaf symptoms appear in older leaves Other roles of potassium in plants include first. Dead areas fall out leaving ragged edges. assisting plant enzyme systems, reducing respiration, translocation of food within the plant Alfalfa: Slow growth, white or yellowish spots and increasing the starch and oil content of around tips of older leaflets. crops. Corn: Slow growth, lower leaf tips and margins 65 turn brown and fire, chaffy ears; plants die Removal of K can be very high in forage or prematurely from stalk rot and leaf diseases. silage crops where the entire plant is harvested. With these crops, relatively high rates of Small Grains: Slow potassium fertilizer are required to replace the growth, delayed potassium removed - providing soil tests maturity, shriveled indicate a need. If only grain is harvested, grain, tendency to much less K is removed by the crop. lodge. Alfalfa Potassium Removal By Crops Soybeans: Leaves Crop Unit P2O5K2O turn yellow and later Alfalfa lbs/ton 12.0060.00 brown on the tips and Red clover lbs/ton 12.0050.00 edges. The base of Corn Bermudagrass lbs/ton 12.0040.00 the leaf usually Firing remains green. Seeds Bromegrass lbs/ton 12.0040.00 are shriveled. Fescue, tall lbs/ton 12.0040.00 Corn lbs/bu 0.330.26 Cotton: Yellow spots Corn silage lbs/ton 3.208.70 between veins, Grain sorghum lbs/bu 0.400.26 browning of leaf Soybeans Sorghum silage lbs/ton 3.208.70 margins, premature Wheat lbs/bu 0.500.30 dropping of leaves. Sunflowers lbs/cwt 1.500.60 Oats lbs/bu 0.250.20 Potassium Uptake By Crops Soybeans lbs/bu 0.801.40 Native grass lbs/ton 5.4030.00 Plants utilize potassium as the positive charged K+ cation. Potassium is required in large amounts by crops and is taken up much faster Forms of Soil Potassium than dry matter is accumulated by crops. For corn, K uptake is completed soon after silking. Soils commonly contain many thousands of Similar potassium accumulation relationships pounds of total potassium. However, most are true for other crops as well. potassium exists in structural components of soil minerals and is not available to plants. Only Potassium is moved from the leaves and stalks a few hundred pounds of potassium per acre into the grain as the plant matures. A larger exist in forms potentially available to plants. percentage of K is moved to the grain by soybeans than by cereal crops, such as wheat, The characteristic behavior of plant nutrients in sorghum, or corn. Thus, more K is removed the soil are similar in some respects, yet vary per bushel of soybeans harvested than with greatly depending on the kind of soil minerals cereal crops. involved, the solubility of these soil minerals and the mobility of nutrient forms in soils Potassium is unlike phosphorus in that most mineral soils contain large amounts of potassium, but like phosphorus, much of the total potassium is in soil minerals that are only very slowly become potentially available to plants.

Unlike nitrogen and phosphorus, only a very small amount of potassium is associated with . Potassium does not enter into organic plant combinations like nitrogen, 66 phosphorus and sulfur, but remains active in Slowly Available K is plant tissues. In fact, even washing dry plant trapped between the leaves removes significant amounts of layers of “plates” of potassium. certain kinds of clay particles. This is Various forms of potassium that occur in soils sometimes called “fixed” potassium. Plants can be classified on the basis of potential cannot use much of the slowly available availability to plants in three broad general potassium during a single growing season. groups: 1. unavailable, 2. slowly available, and However, the soil’s ability to supply potassium 3. readily available. Most potassium is in the over a longer period of time is related closely to unavailable form, therefore, the others are more its supply of fixed potassium. Fortunately, some significant from the standpoint of crop of the trapped ions are in equilibrium with production. These forms interchange as shown available forms of K and slowly escape. Thus, in the diagram below: even though this "fixed" potassium is not immediately available, it does provide an important reservoir of potassium that continually replenishes the ‘readily available’ pool.

Readily Available K is held on the surface of clay and other soil colloids and dissolved in soil solution. Plants easily absorb potassium in this form although there is relatively little potassium dissolved in soil water at any one time. Because of this, the potassium is an index value related to the soils ability to resupply solution K+ from the CEC after crop uptake. Soil tests for available potassium are intended to extract only K+ ions in soil solution and on exchange sites.

Unavailable K is contained in micas, feldspars Potassium is a nutrient that plants absorb in and clay minerals. Plants cannot use potassium amounts far in excess of their requirements if in these crystalline, insoluble forms. Over long readily if available. While this is also true for time periods these minerals weather and their some other nutrients, potassium is outstanding potassium is released as the available in this respect. Therefore, excess amounts + potassium (K ) ion. This process is far too slow applied infrequently result in ‘luxury to take care of the potassium needs of field consumption’ of applied potassium fertilizers. crops. However, trees and long-term perennials obtain a substantial portion of the potassium In soils where the amount of slowly available K they require from the weathering of minerals is low, it is possible to draw down the level of containing potassium. readily available K after a few years of cropping. However, in soils with a large It should be noted that most of the soils in the reservoir of slowly available K, such as for Great Plains and western U.S. have been many Kansas soils, it takes much longer. subjected to much less precipitation and weathering than in the more humid eastern water can be an important source of K states. As a result, soil minerals in these western in some areas. In the Great Plains, 10 to 60 lb of soils are rich in K compared to states with annual K2O per acre foot of water are common. precipitation of greater than about 30" annually. 67 Since K+ ions are held in an exchangeable form with ridge till is that roots follow old root by negatively charged clay particles, potassium channels that are K depleted from previous does not move readily in most soils. When crops. Compaction can also reduce K uptake potassium fertilizers are applied to medium- or by restricting aeration and root growth. fine-textured soils, it takes many years for K+ ions to move over an inch or two by natural Soil Moisture. Potassium deficiency processes. In these soils, potassium is symptoms in crops are frequently associated considered an immobile nutrient. In very sandy with dry soil conditions. Subsequent rain or soils, K+ is somewhat mobile and can move irrigation often reduces or eliminates downward in the soil. However, little applied development of potassium deficiency potassium ever leaches out of the rooting zone. symptoms. Extremely wet soils can also Although organic matter particles have a high reduce root growth, aeration, and K uptake cation exchange capacity for most cations, by plants. attraction for K+ is weak. As a result, of potassium from organic or peat soils may result Soil Temperature. Cool temperatures in low exchangeable potassium levels. decrease the metabolic processes of the plant which affects the uptake of potassium As potassium is removed from exchangeable or by plants. Also, cold soils slow the release of solution phases, a shift of potassium from slowly potassium to the soil solution. available forms to readily available forms occurs. On the other hand, if the K+ ion concentration of Crop Varieties/Hybrids. Some corn varieties the soil solution and exchange complex is in particular are more prone to potassium increased by adding potassium fertilizer, the deficiency. Apparently the root systems of reverse is true. Some of the applied potassium these corn hybrids are less efficient in may become trapped (fixed) between the "plates" absorbing potassium, particular in ridge till or of the clay particles. Some of the fixed potassium no-till fields. is then slowly available over time. This process may explain why additions of potassium fertilizer do not result in rapid increases in potassium soil Fertilizer Sources of Potassium test values in some soils. Potassium is found combined with other elements in salt beds beneath the earth's surface and in brines of lakes. The most important potash deposits occur in Canada, the U.S.S.R., New Mexico, and Utah in the U.S. The Saskatchewan deposit is 3,000 feet deep on the northern edge and 7,000 feet deep at the North Dakota border. While more than 80% of the potash used in the Factors Affecting K Uptake by Plants U.S. comes from Canadian deposits, much of the potash fertilizer used in Kansas is comes from Since K is relatively immobile in soils, any mines in New Mexico. condition that restricts root growth can decrease root contact and absorption of potassium. New Mexico potash ore is mined directly with Factors restricting K uptake include: mining equipment while Canadian potash (potassium chloride) utilizes solution mining Poor Soil Aeration. Poor soil aeration affects where a hot brine solution is pumped into the plant uptake of potassium more than uptake potash bed. The potash salt is dissolved and of most other nutrients. Minimum brought to the surface as a brine and dried. As a systems, such as ridge till or no till, often result, most New Mexico potash is reddish accentuate potassium deficiency problems, colored and contains 60% K2O equivalent while presumably due to less aeration in these Canadian potash is white and contains 62% K O. untilled soils. Another possible explanation 2

68 White soluble material is used for fluid fertilizers and contains 62% K2O equivalent Potassium fertilizers can be broadcast and incorporated, preplant deep banded or applied Common potassium fertilizers include: near the seed in a band. Broadcasting is preferred when relatively large amounts of Potassium Chloride (KCl) accounts for potassium are required. Banding near the row approximately 95% of all potassium fertilizers concentrates the fertilizer where the young plant used. Potassium chloride (also referred to as can obtain it for early growth. Banding potassium muriate of potash) is the least expensive source is often recommended for ridge till or no-till crops of potassium used today. Analysis is normally in soils where subsoil potassium levels are low. 60% or 62% K2O equivalent and about 45% Care must be taken not to apply potassium chloride. The color ranges from deep red to pink where it will be in contact with the seed or to white, but color has no effect on its value to emergence problems may crops. The small amount of iron (~0.05%) in red potash does not make it any less desirable as a fertilizer material. Potassium Summary

Potassium-Magnesium Sulfate (K SO ·2MgSO ) 2 4 4 In eastern Kansas, especially the southeast is commonly referred to as K-Mag, and contains counties, good responses have been obtained 22% K O, 11% Mg and 22% S. It is an excellent 2 from applications, both broadcast and banded, source when magnesium or sulfur in addition to of potassium on alfalfa, soybeans, wheat, corn, potassium is required. and grain sorghums. Grasses have been less

responsive. In the western half of Kansas soil Potassium chloride is marketed in several particle tests generally show high levels of potassium sizes but granular material is best suited for and responses to applied potash fertilizers have direct application and dry blending because it been infrequent except on sandy soils. Where most closely matches size grades of other dry heavy cropping is taking place under irrigation fertilizers and contains a uniform particle size. potassium should be tried occasionally to see if

soil reserves have been depleted. This is Potassium Sulfate (K SO ), or sulfate of potash, 2 4 especially true in the sandy soil areas. contains 50% K O equivalent and 18% sulfur. It 2 is preferred for chloride-sensitive crops such as Potassium soil test values over the past decade tobacco which will not burn properly if excess or so have decreased in the northeast part of chloride is present. Potassium sulfate is not very the state as well. As a result, potassium common in because of it’s relatively deficiencies for row crops has become more high cost. common. This is especially true for reduced

tillage systems and is more severe in dry

growing seasons. Potassium Nitrate (KNO ), or saltpeter, contains 3 44% K2O and 13% N. While an excellent source of potassium and nitrogen, it is relatively costly to produce and not commonly used

Application of K Fertilizers

As an immobile nutrient, potassium needs to be placed in the root zone by incorporation or banding for most crops. However, topdressing is effective on perennial crops such as alfalfa and pastures since these crops develop a massive root system near the soil surface. 69

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