Types of Potassium Fertilizers: Functions and Application Methods Explained in Detail

Potassium exists in crop plants in a form different from nitrogen and phosphorus. It mainly exists as ions, soluble salts, or adsorbed on the surface of protoplasm, rather than in the form of organic compounds. So what are the excellent potassium fertilizers? Fertilizers that serve as excellent potassium sources include potassium nitrate, potassium sulfate, monopotassium phosphate, potassium chloride, and others. Today, the editor has collected some information about these fertilizers to share with everyone.

I. Potassium nitrate:

Potassium nitrate is a colorless crystalline substance with low hygroscopicity, relatively easy to cake, good physical properties, and convenient application. It is an excellent water-soluble potassium fertilizer. Potassium nitrate is a chlorine-free potassium-nitrogen compound fertilizer. The total content of the plant nutrients potassium and nitrogen can reach about 60%, and it has good physical and chemical properties;

It contains 13.5% nitrate nitrogen and 46% potassium. It is a chemically neutral and physiologically neutral fertilizer with good water solubility. Long-term application will not cause soil acidification. It is suitable for use from the young fruit swelling stage to the early coloring stage of fruit trees, and can promote the expansion of fruit pulp cells. Because it contains nitrate nitrogen, it is not recommended for use in the late coloring stage, as it easily causes regreening.

Potassium nitrate has the following advantages:

(1) Since it has no volatility, it can be applied directly to the soil surface without covering;

(2) For crops sensitive to chloride ions, applying nitrate nitrogen is more beneficial than ammonium nitrogen, such as citrus, grapes, vegetables, sugar beets, and other crops sensitive to chloride ions, because the nitrate nitrogen in potassium nitrate can prevent crops from absorbing chloride from the soil;

(3) Nitrate nitrogen can rapidly provide nutrients under any conditions, and is suitable for spring, summer, autumn, and winter crops;

(4) It will not cause the accumulation of salts in the soil.

(5) Potassium nitrate is a highly efficient compound fertilizer containing potassium and nitrogen. Applied to various soils and crops, it can achieve good results, and is especially suitable for chlorine-sensitive crops, such as tobacco, tomatoes, citrus, grapes, and others;

(6) The price of potassium nitrate is higher than that of potassium sulfate, but its cost-effectiveness is also high.

II. Monopotassium phosphate:

It contains 52% phosphorus (P2O5) and about 34% potassium (K2O). It is a chemically neutral and physiologically neutral fertilizer with good water solubility. Generally, before and after flowering, it can be used to promote root germination and flower bud differentiation and provide energy for flowering and fruit setting; use during the coloring stage can promote powder formation and coloring and increase fruit sweetness; use after fruit harvesting can promote branch maturation and increase the degree of fruit lignification.

Many people ask, between potassium nitrate and monopotassium phosphate, which one is better?

Actually, there is no such thing as which one is better. The key is how to use them, and the elements they contain are different. Potassium nitrate is a nitrogen-potassium binary compound fertilizer, while monopotassium phosphate is a phosphorus-potassium binary compound fertilizer. Therefore, different products should be used according to different periods. The nitrate nitrogen in potassium nitrate is easily lost by rain leaching, so it is generally not recommended for use in paddy fields.

Regarding potassium nitrate and monopotassium phosphate, it can be understood this way: when crops need a large amount of nitrogen for growth, use potassium nitrate; when crops need more phosphorus for growth, use monopotassium phosphate. Use potassium nitrate during the swelling stage, and monopotassium phosphate during the coloring stage. Use potassium nitrate during the normal growth period, and monopotassium phosphate during the flowering and fruiting period. Strictly speaking, these two products cannot be regarded as simple potassium fertilizers. Even more, monopotassium phosphate is classified under phosphorus fertilizers in many classifications.

Both fertilizers are quick-acting fertilizers, and their market prices are higher than general potassium fertilizers. Therefore, they are not recommended for use as base fertilizer, especially potassium nitrate, which is used as topdressing or foliar application. If conditions permit, the two can be used together, which will be better. Many high-end water-soluble fertilizers use these two as raw materials. Some friends say that water-soluble fertilizers made with potassium nitrate and monopotassium phosphate as raw materials should be conscience products of the industry.

The most important point is that as long as they are from legitimate manufacturers, the domestically produced quality of these two is acceptable, and there is no need to insist on imported ones.

III. Potassium sulfate:

Theoretical potassium content is 54%, generally 50%. It is a chemically neutral and physiologically acidic fertilizer with good water solubility, but long-term use will aggravate soil acidification. It is suitable for use from the late coloring stage of grapes to the fruit maturity stage, promoting powder formation and coloring of the fruit and increasing fruit sweetness.

Reactions of potassium sulfate application in different soils and matters needing attention:

(1) In acidic soil, excess sulfate ions will aggravate soil acidity, and may even intensify the toxicity of active aluminum and iron in the soil to crops. Under flooded conditions, excessive sulfate ions will be reduced to hydrogen sulfide, causing roots to be damaged and turn black. Therefore, long-term use of potassium sulfate should be combined with farmyard manure, alkaline phosphate fertilizer, and lime to reduce acidity. In practice, drainage and field drying measures should also be combined to improve aeration.

(2) In calcareous soil, sulfate ions and calcium ions in the soil form poorly soluble calcium sulfate.

(3) It should be emphasized for use on chlorine-sensitive crops, such as tobacco, tea trees, grapes, sugarcane, sugar beets, watermelons, potatoes, and others. Increasing the application of potassium sulfate not only increases yield but also improves quality. Potassium sulfate is more expensive than potassium chloride and has fewer sources, so it should be emphasized for use on chlorine-sensitive and sulfur-loving and potassium-loving cash crops, where the benefits will be better.

(4) Potassium sulfate should not be mixed with fertilizers rich in calcium.

It is said that manganese poisoning in many fruit tree regions is caused by soil acidification, and the direct cause of soil acidification is the year-round use of potassium sulfate. Therefore, fertilizers should be rotated occasionally. Do not always use one kind. If you yourself always eat the same thing, it will cause malnutrition, let alone the land.

IV. Potassium chloride

Potassium chloride [KCl, containing 60.0% K2O (K, 50.0%)] occupies a major position among potassium fertilizers because of its high potassium content, abundant resources, relatively simple processing, and low price. The industrial production of potassium chloride began in Germany in 1861. At present, the countries with higher output include Canada, the former Soviet Union, Germany, France, and the United States, and these 5 countries account for 95% of the world's total output.

Potassium chloride is currently the most used potassium fertilizer in the world, accounting for more than 90% of total potassium fertilizer consumption. Potassium chloride is not only used directly as a potassium fertilizer or as a base fertilizer for blended fertilizers, but is also the basic potassium source for producing chlorine-free potassium fertilizers such as potassium sulfate, potassium nitrate, or potassium phosphate. Pure potassium chloride is a white glossy crystal, with a solubility in water of 34.4% at 20°C, and is slightly soluble in liquid ammonia (0.102% at 18.9°C) and ethanol (0.290% at 25°C). Commercial potassium chloride is light yellow, brick red, or white in appearance, crystalline or granular, with low free water content and a certain hygroscopicity. Potassium chloride produced in Canada is light brick red because it contains about 0.05% iron and other metal oxides. Granular (0.8-4.7mm) or coarse-grained (0.6-2.3mm) potassium chloride is mainly used for bulk blended fertilizers. The hygroscopic point of potassium chloride at 20°C is 88.3% relative humidity, and it dissolves very easily after entering the soil solution, with a salt index of 116.

After potassium chloride is applied to the soil, K+ is easily held by soil colloidal particles and is also easily absorbed by crop roots, leaving relatively more Cl- in the soil. It interacts with Ca2+, Mg2+, and other cations on soil colloidal particles or H+ exchanged from the root surface to form highly mobile CaCl2, MgCl2, or HCl, which can leach calcium, magnesium, and other bases downward in the soil around the fertilization point and may acidify the soil. Therefore, KCl is also a physiologically acidic fertilizer. When used continuously and in large amounts on acidic soil, it has the side effect of acidifying the soil. Therefore, when using KCl on acidic soil, a certain amount of lime or calcium-magnesium-containing fertilizer must be applied together, and the amount of KCl used each time can also be appropriately reduced or used intermittently.

Potassium chloride contains 45%-47% chlorine. Although the Cl- entering the soil with application is also an essential nutrient element for crops, the amount required by crops is small, only 1/20-1/10 of the crop requirement. Therefore, when KCl is used continuously in large amounts, crops easily absorb too much Cl-, affecting the product quality of some cash crops, such as reducing the sugar content in grapes and melons, reducing the combustibility of tobacco, and increasing the moisture content of potatoes.

Potassium chloride also contains a certain amount of sodium chloride (1%-3%), and is generally not suitable for use on saline-alkali soil.  When potassium chloride is used on neutral and calcareous soil, it easily causes bases to leach downward. The continuous downward leaching of bases in the soil will cause some soils or local soil layers to become base-unsaturated, causing changes or even deterioration in their physical properties. Therefore, attention should be paid to applying a certain amount of base-containing fertilizer together.

Potassium chloride can be used as base fertilizer and topdressing, and should be applied deeply into the relatively moist soil layer below the surface soil. It is generally not used as seed fertilizer or foliar topdressing. For chlorine-sensitive crops such as tobacco, grapes, and potatoes, in principle it should not be applied alone or continuously, and caution should also be exercised when it must be applied together.