Fertilizer potassium is sometimes called “potash”, a term that comes from an early production technique where potassium was leached from wood ashes and concentrated by evaporating the leachate in large iron pots (“pot-ash”). Clearly, this practice is no longer practical and is not environmentally sustainable. In food production, potassium is removed from the soil in harvested crops and must be replaced in order to maintain future crop growth.
Over 350 million years ago, the huge Devonian Sea was slowly drying up in the area of Central Canada and northern U.S., leaving behind concentrated salts and minerals. This process continues today in places such as the Great Salt Lake and the Dead Sea.
These ancient marine salts are now recovered and used in a variety of useful ways, with the majority being used as potassium fertilizer. Potassium is a natural plant food because fertilizers such as potassium chloride and potassium sulfate are widely found in nature. Fortunately, there are huge reserves of potash in the earth that can meet our need for this nutrient for many centuries to come. This fertilizer is clearly not an artificial or manufactured chemical, since it comes directly from the earth and is simply recycled through very long geological processes.
Potassium is an important mineral required for human health. Since potassium is not stored in the body, it is necessary to continually replace this nutrient on a regular basis with potassium-rich foods. Diets high in potassium and low in sodium have been shown to be beneficial for avoiding high blood pressure.
Potassium is essential for plant health and there must be an adequate supply in the soil to maintain good growth. When the potassium supply is limited, plants have reduced yields, poor quality, utilize water less efficiently, and are more susceptible to pest and disease damage.
In many parts of the world, agricultural soils are gradually becoming depleted of potash. Some soils were high in potassium when they were first cultivated long ago. However, after many years of intensive cropping and repeated nutrient removal during harvest, many fields now require regular inputs of potash to maintain their productivity.
High yielding crops remove large amounts of potassium in the harvested portion of the crop. For example, harvesting 9 ton alfalfa/A will remove over 450 lb K2O. Similarly, a potato yield of 450 cwt/A removes 500 lb K2O, and harvesting 40 ton/A of tomatoes will take off over 450 lb K2O/A. But these high rates of nutrient removal are not usually being matched with fertilization. For example, In Idaho an average of four pounds of potash are removed in crops for every pound that is added back. In the Pacific coast states, over two pounds of potash are removed on average for every pound returned to the field as fertilizer. It’s little wonder that K deficiency is becoming a more common occurrence in agricultural fields.
There are many excellent sources of potassium that can be used to sustain a productive and healthy ecosystem and replenish the soil’s nutrient reserve. So which one should you use? Some of the most popular include:
Potassium chloride (Muriate of potash) (KCl; 0-0-60)
Potassium sulfate (Sulfate of potash) (K2SO4; 0-0-50-18S)
Potassium-magnesium sulfate (K2SO4-2MgSO4; 0-0-22-22S-11Mg)
Potassium thiosulfate (K2S2O3; 0-0-25-17S)
Potassium nitrate (KNO3; 13-0-44)
The potassium in all these fertilizers is identical and this nutrient will be rapidly available to the plant regardless of the source. The primary difference is in the companion nutrients that come along with the potassium.
Potassium Chloride: The importance of chloride is frequently overlooked, but it is an essential nutrient for plant growth. Recent research has demonstrated that many crops respond favorably to chloride applications with greater yield and quality. Like any soluble fertilizer, salt-induced damage can result if large amounts are placed in close proximity to seeds or seedlings. Potassium chloride is usually the least expensive source of potash.
Potassium Sulfate: All crops require an adequate supply of sulfur to develop proteins and enzymes. Sulfur-deficient plants appear light green and have reduced yields and quality. Sulfate that is present in potash fertilizers is immediately available for plant uptake.
Potassium-Magnesium Sulfate: Because its vital role in chlorophyll, magnesium deficiency is first exhibited by yellow leaves in the lower part of the plant. Magnesium requirements vary considerably, with legumes generally containing more of this element than grasses.
Potassium Nitrate: An abundant supply of nitrogen is essential for all high-yielding crops. For crops that prefer a nitrate source to an ammonium source of nitrogen, this potash source can be a good option.
There are many excellent potash sources available for meeting the nutrient requirements of crops. When making a decision on which source to use, choose the one that meets your needs and provides the accompanying anion that will help keep your high-yielding crops in top shape.
About the Author
Dr. Rob Mikkelsen is the Vice President of Communications and North American Program Director for the International Plant Nutrition Institute (IPNI). Prior to his work with IPNI, he served as the Professor of Soil Science at North Carolina State University in Raleigh and Soil Chemist at the National Fertilizer Development Center in Muscle Shoals, Alabama.
Dr. Mikkelsen is well known for his research and expertise in nutrient management, and has authored and co-authored numerous publications. His research has focused on basic agronomic and fertilizer technology, as well as nutrient interactions with the environment, animal waste management, and nutrient budgets.
Dr. Mikkelsen received his B.S. degree in Agronomy/Soils at Brigham Young University, and his Ph.D. in Soil Science at the University of California, Riverside. He currently resides in Merced, California.