Potassium (K) is an essential nutrient that plants need in large quantities. Potassium does not become part of the many complex organic molecules in plants. It moves as a free ion and performs its many functions, such as regulating plant water pressure, activating enzymes, balancing electrical charges, and transporting sugars and starches.
Potash is mined from a variety of geological potash deposits around the world, some of the richest reserves being in Saskatchewan, Canada. The salts are treated to remove impurities and converted into various fertilizers. The two most popular potash fertilizers extracted from these salt deposits are potassium chloride and potassium sulfate. The term "potash" is a generic term commonly used to refer to a variety of potash salts, sometimes more specifically to potassium chloride, the most widely used potash fertilizer. Potassium chloride is referred to as "muriate (meaning chloride) of potash" or MOP, while potassium sulfate is sometimes referred to as "sulfate of potassium" or SOP.
The major difference between MOP and SOP is the anion that accompanies the potassium. The advantages and disadvantages of potassium chloride and potassium sulfate depend on the chemical properties of these salts, their behavior in the soil and the nutrients they provide in addition to potassium. Both contain all the essential nutrients required by the crop. Potassium chloride provides chloride, the ionic form of elemental chloride and essential micronutrients. Both MOP and SOP contain high potassium content (60% or 62% K 2 O as MOP and 50% K 2O for SOP). The potassium in both fertilizers is present in the same potassium form and both salts are soluble in water, but the water solubility of potassium sulfate is about one third that of potassium chloride. Potassium chloride can be dissolved in water for spray or irrigation applications, whereas potassium sulfate is not suitable for these applications.
The biggest disadvantage of potassium sulfate is the cost. Potassium chloride is about 40% to 50% cheaper per pound of K2O than SOP. The biggest disadvantages of KCl are its high salt index (116 for MOP 0-0-60 and 46 for SOP 0-0-50) and chloride content. For some chloride-sensitive crops, such as potatoes, tobacco, some vegetables and fruits, and some tree crops like almonds, walnuts and citrus, potassium sulfate is usually superior to potassium chloride, although these sensitivities are highly dependent on growing conditions, soil salinity and salinity and chloride in irrigation water. Some studies suggest that chloride may need to be a concern in some situations, but most studies show tolerance in specific crops if there is no deficiency of sulfur or chloride and total chloride in soil salinity or irrigation water. For example, potato growers often use SOP as a source of potassium because they are concerned about chloride sensitivity, even though studies showing the negative effects of chloride and the advantages of SOP over MOP are not common. Comparisons and anecdotal reports of grower experience vary.
Chloride affects plants primarily by increasing the osmotic potential of soil water. In other words, chloride salts increase soil salinity, which interferes with the plant's ability to absorb water. Even in chloride-sensitive crops, studies and observations have shown that crops will respond to an overall increase in osmotic potential before chlorides reach toxic levels. To understand whether chloride in fertilizer is a specific problem, total salt, potassium content and growth conditions must be evaluated.
The infiltration effect of the two sources can be compared by calculating the soil-water salt concentration resulting from an application of MOP or SOP at a given potassium level and multiplying by the infiltration coefficient of the corresponding salt. When applied to achieve the same potassium concentration in the soil, the infiltration potential of MOP is approximately 1.3 times the infiltration potential of SOP, but the total salt contribution of both materials remains small at actual application rates. The infiltration potential of 100 lb MOP per acre (60 lb K 2O/acre) to soil water is calculated to be about 0.1 dS/m, a measure of salinity. From this perspective, the salinity threshold for unimpaired growth of almonds - a sensitive crop - is listed as 1.5 dS/m in saturated soil extracts (electrical conductivity of extracts, or ECe, the standard laboratory measure of salinity for soils). Other sensitive crops such as strawberries, carrots, onions, and common beans have a threshold ECe of 1.0 dS/m. The amount of chloride added at normal fertilizer application rates is usually well below the published harmful chloride concentrations for chloride-sensitive crops. Leaf damage in sensitive crops begins to occur at 0.1-0.3% chlorine in leaf tissue. Therefore, unless the total soluble salt and/or chloride load in the soil or irrigation water is near or above the expected threshold salinity tolerance of the crop, it is unlikely that the potassium source effect will have any practical consequences. Note that low precipitation rates and inadequate irrigation can exacerbate salt and chloride toxicity problems. If the salinity of the soil or irrigation water is near or above the tolerance threshold, the effects of chloride and salt should be managed through potassium application rates and timing, possible batch applications, maintaining appropriate leaching rates, and replacing some or all of the MOP with SOP.
✺ ECe (measurement of salt concentration) and the chemistry of the irrigation water.
✺ Total rainfall during the growing season.
✺ Timing of fertilizer applications related to the rainy season and crop planting or growth.
✺ the rate of potash application and whether it was applied in multiple applications; and
✺ use of deficit irrigation or irrigation plus less rainfall than required by the crop.
