Unit 13 - Fertilizers and Fertility

Chapter 5 - Potassium Fertilizers

Role of Potassium

Potassium affects the life and life-style of every living being. Athletes often drench themselves with special nutrient fortified drinks to restore K lost with body fluids or eat a banana

Potassium's role in plant growth is quite similar to that for humans. Potassium is not an integral part of any organic molecules in plants. K is important in many of the biochemical reactions of the plant including translocation of carbohydrates. Since it does not complex with organic compounds, it is only minimally related to the soil's organic matter.

K found in soils is from the weathering of minerals (Feldspars and Micas) that release K ions, which are than adsorbed onto CEC sites. Since potassium will weather from clay minerals, the more clay a soil has, the higher the amounts of potassium and the less fertilizer required.

K Deficiency Symptoms

Under severe deficiency, plants will often develop visible symptoms. For potassium the edges of older leaves will turn brown while yield and quality decline. Sometimes the signals are even more specific. For example, orange trees will drop their fruit; strawberries do not fully develop their sweet taste; corn stalks will break, sending ears to the ground; tomatoes will be small and contain too much white tissue. Alfalfa will show typical yellowing along the outer margins of the leaves.

K Deficiency symptoms on corn is the yellowing of margins on lower leaves

Potassium is also noted for increasing the stalk strength of many grain crops. Low K levels can lead to lodging (broken and fallen stalks) of corn. Maple leaves show K deficiency symptoms that are similar to corn--a burning of leaf edges.

Specialty crops such as turf for home lawns and golf courses also benefit from the wise use of K. It helps build in winter hardiness and tolerance to moisture stress, and increases plant regrowth vigor. Potassium also helps N produce the deep green color associated with healthy turf.

Turfgrasses may not exhibit a visual response to potassium in terms of shoot color, density and growth. However, the potassium does influence turfgrass in the following terms: a) Rooting - potassium enhances root development and branching. b) Drought tolerance - high potassium levels contribute to improved drought tolerance. Potassium regulates the absorption and retention of water by plants, influencing the heat and hardiness of turfgrass. c) Stolon and Rhizome development correlates well with the soil potassium levels. d) Reduced wilting - potassium levels are also linked to increased leaf turgor pressure that reduces the wilting tendency. e) Disease resistance - higher potassium levels reduce the incidence of numerous fungal diseases of turf including Fusarium Patch and Dollar Spot. f) Wear tolerance - is also reported to increase proportionally with the plant potassium level. The increased wear may be attributed to increased vigor and a higher cellulose content. The wear tolerance around cups on putting greens is frequently increased by the application of potassium (source: Dr. Robert Shearman, UNL).

K Use by Plants

Crop uptake of potassium will depend on the type of crop, the amount of biomass that is being produced, and the soils ability to supply potassium.

Potassium Deposits & Fertilizer

K is often referred to as "potash". Early American settlers coined that name. They produced potassium carbonate needed for making soap by evaporating water filtered through wood ashes. The ash-like residue remaining in the large iron pots was called "pot ash" and contained some potassium. This process is registered as the first US patent.

Commercial production of potash in the US began when supplies from Germany were stopped due to military conflicts. Carlsbad, New Mexico, became the hub of U.S. production. Other production from brines was developed in Utah and California. Then, in the early 1960s potash from vast, high quality Canadian reserves became available. As a result, Canada now supplies about three-fourths of the potash used in US crop production. Canadian Sylvite Deposits

International Minerals and Chemicals Corp. (IMC) began construction of a mine near Esterhazy, Canada in 1960. The mine shaft reached a depth of 1,030 meters when it was completed in 1962. Today, IMC operates two mines in the area. They have a combined production capacity of 4.2 million tons of potash a year - still the largest potash mine in the world.

Most of the known world reserves of K were developed as sea water evaporated and K salts crystallized to become the beds of potash ore being mined today. The deposits are a mixture of crystals of potassium chloride (KCl) and sodium chloride (NaCl) or table salt. Separation of the KCl from the mixture produces a high analysis natural K fertilizer (0-0-60). Other production methods result in the crystallization of potassium sulfate or potassium chloride from brines, such as those in the Great Salt Lake of Utah. Nearly 95 percent of the commercially produced K is used in agriculture. The remaining 5 percent is used for industrial purposes and for products common in the home.

The earth's crust is loaded with potassium. Most mineral soils are high in potassium with the exception of sandy soils which, due to leaching, may have lost their potassium. (Remember, even cations will eventually leach if the soil has a low CEC.) In Minnesota, the farther west you go, the greater the native K in the soil.

K Fertilizer Program

Soil K is found in minerals like feldspars and micas (90% of total soil K). K is also fixed inside of clay minerals ( 9% of soil K) and is found on the soil exchange sites ( 1% of soil K). About 0.1% of the K at any one time is in the soil solution.

Management of potassium is especially important for soils with low CEC and where irrigation is used where leaching can readily occur. Potassium can be stored in the soil from one year to the next and thus, a build up of of potassium is possible from one year to the next. Persons who continually deposit wood ashes on their garden will build up their K levels and raise the soil pH.

Potassium is not a pollutant. Even if potassium is leached from soil, K does not cause environmental problems.

See Wood Ashes , for more information on wood ashes as a K source.

Chapter 6 Fertilizer Applications

Fertilizer Chapters

Lab Units

© Terence H. Cooper & Regents of the University of Minnesota, 2007. The University of Minnesota is an equal opportunity educator and employer.