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¶ Introduction
Potassium (K) fertilizers are commonly used to overcome plant deficiencies. Where soils cannot supply the amount of potassium required by crops, it is necessary to supplement this essential plant nutrient. “Potash” is a general term used to describe a variety of potassium-containing fertilizers used in agriculture. The word "potash" comes from the term "pot ashes", which in colonial days were the primary potassium source for fertilizers, soaps, gun powder, and other industrial uses.
The potassium contents of potash fertilizers are expressed as "percent dipotassium oxide" or"% K2O". To convert expressions:
% K2O x 0.86 = %K
%K x 1.16 = %K
Potassium chloride (KCl), the most commonly used source, is also frequently referred to as “muriate of potash (MOP)” (muriate is the old name for any chloride-containing salt). Potassium is always present in minerals as a single-charged cation (K+).
¶ Table 1. |
|
| Analysis | 0-0-60, 0-0-62 |
| Formula | KCl |
| Composition | 50% - 52% K |
| 60% - 63% Cl | |
| Water solubility @ 68°F | 344 g/L |
| Solution pH | ≈ 7 |
| Density, lb/Ft | |
| red granular | 62 – 75 |
| red standard | 65 – 82 |
| white standard | 74 – 83 |
| crystal granular | 66 – 67 |
¶ Production
Deeply buried potash deposits are found throughout the world. The dominant mineral in these deposits is sylvite (potassium chloride) mixed with halite (sodium chloride). This forms a mixed mineral called sylvinite.
During World War II, potash was produced by the U.S. and exported to Canada. Today three-fourths of U.S. potash comes from Canada, mainly Saskatchewan. Primary U.S. potash deposits are found near Carlsbad, New Mexico.
Most potassium minerals are harvested from ancient salt beds found beneath the earth's surface or formed by the brines of dying lakes. They are transported to a processing facility where the ore is crushed and the potassium salts are separated from the sodium salts.
¶ Figure 1. Forms of Potassium Chloride
The color of potassium chloride can vary from red to white, depending on the source of the sylvinite ore. Most “standard” potash sources are reddish in color and contain impurities, like trace amounts of iron oxide and the flotation agents used in manufacturing. If standard potash sources are used in making suspension grade fertilizers, these impurities may form iron gels and surface scum which plug nozzles, lines, and other parts of liquid fertilizer application equipment.
Some potassium chloride is produced by injecting hot water deep into the ground to dissolve the soluble sylvinite mineral. The brine is pumped back to the surface. Solar evaporation is used to recover the potash salts from brine water. Potash is produced this way in the Dead Sea and the Great Salt Lake (Utah).
Potassium chloride is sold in several particle sizes ranging from 8 to 100 mesh. "Granular" size potash ranges from 8 through 20 Tyler mesh and is made by compacting finer materials. "Coarse" size potash ranges from 8 through 28 mesh and "standard" from 8 through 65 mesh. These materials are commonly used in making dry bulk blend fertilizers.
"Soluble fine" potash has high purity, is white, and has a typical analysis of 0-0-62. Size ranges from 35 to 100 mesh. Soluble potash is often mixed with UAN or ammonium polyphosphate solutions to make suspension grade fertilizers.
¶ Agricultural Use
Potassium chloride is the most widely used potassium fertilizer due to its relatively low cost and because it includes more potassium than most other sources - 50 to 52% K and 45 to 47% Cl.
Over 90% of global potash production is used for plant nutrition. Potassium chloride is often spread onto the soil surface prior to tillage and planting. It may also be applied in a concentrated band near the seed. Since dissolving fertilizer will increase the soluble salt concentration, banded potassium chloride is placed to the side of the seed to avoid damaging the germinating plant.
Potassium chloride rapidly dissolves in soil water, but is considered a slightly mobile nutrient when soil applied. The K+ cation is retained on the negatively charged cation exchange sites of clay and organic matter. The Cl- anion moves readily through the soil along with water.
An especially pure grade of potassium chloride (“white potash”, 0-0-62) can be dissolved for fluid fertilizers or applied through irrigation systems. There are no agronomic differences between the red and white forms of potassium chloride.
¶ Management Practices
Potassium chloride is primarily used as a source of potassium nutrition. However, there are regions where plants respond favorably to application of chloride. Potassium chloride is usually the preferred material to meet this need.
There are no significant environmental impacts on water or air associated with normal application rates of potassium chloride. The effect of elevated salt concentrations surrounding the dissolving fertilizer on adjacent plant seedlings may be the most important factor to consider.
¶ Non-agricultural Use
Potassium is essential for human and animal health. It must be regularly ingested because the body does not store it. Potassium chloride can be used as a salt substitute for individuals on a restricted salt (sodium chloride) diet.
It is used as a deicing agent and has a fertilizing value after the ice melts. It is also used in water softeners to replace calcium in water.
¶ References
IPNI. Nutrient Source Specifics No. 3: Potassium Chloride. International Plant Nutrition Institute, Norcross Georgia. 1 pg.