Ferti-Facts:  Potassium

Written by Dr. Wes Chun Ph.D, Grower's Secret Chief Science Officer

Introduction

Potassium (K) is an alkali metal that is a member of the highly active metals Group 1 (1A) of the periodic table.  This group also includes lithium, sodium, rubidium, cesium and francium.  The letter K, used to symbolize potassium, comes from the German word for potassium, kalium.  Potassium is so active that it never appears free in nature.  It was first discovered in 1807 by the English chemist Sir Humphry Davy who produced pure potassium by passing a current through a melted sample of potash (potassium carbonate that was obtained by leaching wood ash), and named the metal potassium.

Potash = pot ash. During Colonial times, people would burn wood and other organic matter in pots.  Water that was used to rinse the ashes, was collected and dried leaving a residue of potassium salts which were called pot ash or potash. These salts were boiled with animal fat to produce soap.

Pure potassium is a soft, silvery-white metal with a density lighter than water (0.862 g/cu cm) and thus can float on water (not recommended as it is highly reactive with water, produces hydrogen gas and heat, and will eventually catch fire or explode).  It occurs in minerals such as sylvite (potassium chloride), sylvanite (sodium potassium chloride), carnallite, langbeinite, etc.

Potassium is essential for all living cells and is present in both animal and plant tissues.  It is absolutely essential in animals to maintain the proper balance of fluids in in the body and in cells, transmission of chemical messages between nerve cells and muscles, food digestion, and eye function.

A human weighing 150 pounds contains 5 ounces of potassium.  This level is maintained by a daily intake of 3.3 grams from potassium rich produce such as apricots, avocados, cantaloupes, dates, honeydews, grapefruit, greens, lima beans, molasses, mushrooms, oranges, peas, potatoes, prunes, raisins, spinach, and tomato.  It is obvious that an adequate supply of potassium fertilizer is needed to produce high quality nutritious food.  Thus, understanding of how potassium works in plants, and how plants obtain potassium are very important to the successful grower.

Potassium in Plants

Potassium is taken up in large quantities by plants.  It moves as a free ion and has many essential functions such as regulation of water pressure in plants cells, enzyme activation, protein synthesis, pH balance within plant cells, carbon dioxide fixation during photosynthesis, transport of chemical compounds, and balancing electrical charges.  Plants that have adequate K supply are better able to withstand stress, insect damage, and plant diseases.

Plants can remove large quantities of potassium and the amount removed from the soil varies with the crop.  For example, an 8 ton per acre alfalfa crop can remove up to 392 pounds of potassium. A rice crop yield of 7,000 pounds per acre can remove 25 pounds of K.  Thus, fertilization with K will depend on the type of crop being grown.

Potassium in Soils

There are four types of potassium in soils:  Unavailable Potassium; Fixed or Slowly Available Potassium; Exchangeable or Readily Available Potassium; and Soil Solution Potassium. Unavailable Potassium is found in the crystalline structure of feldspars, clay minerals, and micas that make up soil structure. Potassium from these sources are unavailable to plants except for the small amounts that are released to the soil solution with weathering.  Fixed potassium is trapped between layers of clay minerals that are fixed after repeated cycles of wetting and drying.  When wet, small amounts of potassium are released.  This type of potassium is usually not measured in soil testing. Most soil testing measure Exchangeable Potassium.  This is the potassium that is held on the surface of clay particles and organic matter in the soil and easily released to the soil solution.  Soil Solution Potassium is readily available to the plants.

Potassium can be introduced into the soil from irrigation water, fertilizer, manure, biosolids, and sediment deposition.

Potassium Deficiency Symptoms

Since potassium is a mobile nutrient, symptoms may occur first as brown scorching, curling of leaf tips, and chlorosis (yellowing) between leaf veins in older leaves.  Purple spots may also appear on the leaf undersides.  Plant growth, root development, and seed and fruit development are usually reduced in potassium-deficient plants.  Plants are generally more susceptible to frost and plant pathogens.

Potassium deficiencies are common in fruit and vegetable crops.  In addition to general potassium deficiency symptoms described above, individual crops have unique potassium deficiency responses.  Here are a few examples:

Alfalfa – First symptoms of K deficiency are small white or yellow dots around outer edges of leaves. Edges eventually turn yellow, brown, then die. 

Apples - Apple leaves may show scorching around the edges and interveinal chlorosis.   Potassium deficiencies can result in fruits with slightly acidic or woody taste.

Brassicas – K deficiencies will result in slow leaf growth.  Leaves may be blue-green in color, exhibit slight interveinal chlorosis, and may have scorching of the outside edges.

Corn – K deficiencies can result in scorching of outer edges of leaf.  Yellow striping with green midribs may show on lower leaves.  K deficient plants will have poor root development, unfilled, chaffy ears, and lodging.

Cotton – K deficiency symptoms progress from yellow or bronze mottling of leaves, to yellowish green leaves with brown specks at tip, margin, and outer veins.  Whole leaves eventually turn reddish brown and prematurely drop. 

Potato – K deficiencies can lead to low yields and reduced tuber size.  Leaves can appear dull and blue-green in color with interveinal chlorosis. Undersides of leaves may have dark brown spots and top sides of leaves may take on a bronzed appearance.

Soybeans – K deficiencies lead to leaf scorching, interveinal chlorosis, and ragged edged leaves. Maturity is delayed and bean quality is severely affected.

Tomatoes – K deficiencies result in slow growth and woody appearing stems.  Leaves are blue-green in color, and the interveinal areas often fade to a pale gray color.  Leaves may appear bronzed, and leaflets may develop yellow and orange patches.  Fruits often ripen unevenly and have green patches near the stalks.

Wheat – K deficiencies usually result in a smaller plant, lodging, and may show withering or burn of older, and then new leaf tips.

Potassium deficiencies in gooseberries, currants, and raspberries commonly lead to dieback of shoots and branches.  Plants may still produce blossoms but fruit yields and quality will be low.

Plants are more prone to frost damage and plant disease when potassium is deficient.

Fertilizing with Potassium

Organic potassium fertilizers include quick release (wood ash, kelp, muriate of potash, sulfate of potash) and slow release (Greensand, Granite dust) forms.  Larger less frequent applications work as well as smaller annual applications.  Application amounts are dependent on crop removal and crop needs.  The type of potassium depends on the farming method (organic, conventional), the current crop type and health, the chlorine level in the soil, and tolerance of the crop to chlorine.

Crop Responses to Element

Fertilization with potassium generally results in good plant growth, and profitable yields.  Potassium plays a backstage role in every facet of crop production.

Element Toxicity Symptoms

Excess potassium can cause a cation imbalance in plants.  This results in symptoms that resemble magnesium (interveinal chlorosis and reduced chlorophyll synthesis in older leaves and then newer leaves), or calcium (growing tips of leaves and roots turn brown and die, reduced nutrient and water uptake) deficiencies. 

Summary

Potassium is the third of the three major macronutrients (NPK) for plants.  To remember the major functions of NPK, remember “Top-Down-All Around.”  Thus, nitrogen deficiencies general manifest in the foliar portions of the plant.  Phosphorous deficiencies mainly affect root growth.  Potassium deficiencies manifest in the whole plant. Potassium is particularly important in stress situations since it regulates the opening and closing of stomata. Consultation with a crop advisor is advised if you suspect potassium deficiency since it can be confused with other nutrient imbalances.