POTASSIUM Potassium is the last of what might be called Potassium is also important in helping the plant the “big three” soil 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 fertilizers 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, fertilizer 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 soils 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 drought 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 soil organic matter. 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 soil test 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 Irrigation 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.