What is Potassium Sulfate Fertilizer (SOP)?
Potassium sulfate fertilizer, also known as sulfate of potash (SOP), arcanite, or potash sulfur, is the most widely used chloride-free fertilizer. It contains approximately 48% to 53% potassium in the form of K₂O and 18% sulfur in the form of S, making it an excellent source for fulfilling the plant's requirements for these two essential nutrients. The use of this fertilizer is highly significant due to the vital role of potassium, often referred to as the "quality element." Potassium enhances the water content in plant cells and improves the quality, color, and yield of agricultural products.
Crop Name | Application Rate with Irrigation |
|---|---|
Fruit Trees (Pome and Stone Fruits) | 20 to 30 kilograms per hectare |
Field Crops | 10 to 20 kilograms per hectare |
Greenhouse Crops | 15 to 35 kilograms per hectare |
Advantages of Potassium Sulfate Fertilizer
- The salt index of potassium sulfate (K₂SO₄) fertilizer is lower than other common potassium fertilizers. The salinity (EC) of potassium sulfate solution is less than one-third that of potassium chloride solution at the same concentration.
- The potassium in this fertilizer enhances the plant's resistance to drought, frost, pests, and diseases, thereby improving yield and quality.
- After the application of potassium sulfate, the pH of the root zone significantly decreases, increasing the availability and uptake of nutrients like phosphorus, zinc, iron, manganese, copper, and boron.
- In saline and sodic conditions, it reduces the toxic effects of sodium, calcium, and magnesium, ultimately improving the balance of these cations and increasing crop performance.
- By reducing the impact of diseases, potassium sulfate prevents the formation of burn spots on fruits and vegetables, improving their appearance and promoting uniformity in size and shape.
- The potassium source in this fertilizer is suitable where there is a risk of soil salinity development.
- It is an excellent source of sulfur for plants. Sulfur deficiency has become increasingly common in recent years, making its supplementation essential.
- The sulfate in this fertilizer acidifies the soil, enhancing the uptake of phosphates and other micronutrients.
- Potassium sulfate is chlorine-free and is the best fertilizer source for chlorine-sensitive crops such as potatoes, tobacco, and vegetables.
- It is the best potassium source for oilseed crops like olives, sunflowers, canola, peanuts, and soybeans.
- Potassium sulfate enhances the firmness of fruits and vegetables, improving their transportability and shelf life.
- The use of potassium sulfate improves the color and taste of fruits and vegetables by increasing pigment content, sugars, acids, and water content in the produce.
Types and Forms of Potassium Sulfate (SOP)
Potassium sulfate is generally produced in three standardized forms: granular, crystalline, and powdered.Potassium sulfate can be produced by one of the following two methods:Mannheim Process: 2KCl+H2SO4→K2SO4+2HCl2KCl + H₂SO₄ → K₂SO₄ + 2HCl2KCl+H2SO4→K2SO4+2HCl This method involves the reaction of potassium chloride (KCl) with sulfuric acid (H₂SO₄) in a Mannheim furnace. It is an industrialized process and accounts for nearly all of the potassium sulfate used as agricultural fertilizer worldwide.Alternative Process: (NH4)2SO4+2KCl→K2SO4+2NH4Cl(NH₄)₂SO₄ + 2KCl → K₂SO₄ + 2NH₄Cl(NH4)2SO4+2KCl→K2SO4+2NH4Cl This method involves the reaction between ammonium sulfate ((NH₄)₂SO₄) and potassium chloride (KCl). However, it has lower efficiency and is not commonly used in industrial applications.Chemical Composition and Standards:According to agricultural fertilizer standards,
including the Iranian National Standard No. 128, potassium sulfate must meet the following criteria:A minimum of 50% water-soluble potassium (as K₂O by weight).A minimum of 17.5% water-soluble sulfate (as S by weight).Other impurities in potassium sulfate include magnesium, calcium, sodium, and chloride compounds, with the following maximum content limits:Magnesium (as MgO): 2% by weightCalcium (as CaO): 2.5% by weightSodium chloride (as NaCl): 2% by weightChloride (as Cl): 2.5% by weightThe maximum moisture content of potassium sulfate fertilizer should not exceed 1.5% by weight.Mannheim Furnace Process Details:During the production of potassium sulfate in the Mannheim furnace, a pH regulator (lime) is added to adjust the pH of the final product. Lime reacts with the free sulfuric acid present in the mixture to produce gypsum (CaSO₄). Gypsum is insoluble in water, and its presence in the product contributes to the partial insolubility of potassium sulfate in water.
Powdered potassium sulfate
also known as Soluble Potash, should have a particle size of less than 0.015 mm.
Crystalline potassium sulfate
should have a particle size of 0.2–1 mm.
Granular potassium sulfate
should have a particle size of 1–4 mm (at least 85% by weight).
Visualizing the Impact of Potassium Sulfate Fertilizer
Key Uses of Potassium Sulfate Fertilizer
Potassium sulfate fertilizer is a versatile and highly effective solution for a wide range of agricultural and horticultural applications. Its unique composition, providing potassium and sulfur, makes it ideal for improving crop quality, enhancing resistance to stress, and promoting healthy plant development. Potassium sulfate ensures plants achieve optimal productivity and superior quality, making it a reliable choice for farmers and gardeners worldwide. Below are the primary applications of potassium sulfate fertilizer:
Fruit Trees (Stone and Pome Fruits)
Enhances fruit firmness and shelf life while improving color and taste in crops such as apples, cherries, and peaches.
Greenhouse Crops
Supports balanced growth in crops like tomatoes, cucumbers, and bell peppers by providing potassium without chlorine, which is harmful to sensitive plants.
Root Vegetables
Boosts the quality and size of root crops such as potatoes, carrots, and beets. Improves starch and sugar content for better flavor and processing quality.
Oilseed Crops
Increases the oil content in crops like sunflowers, soybeans, and rapeseed, ensuring better yields and higher-quality oil production.
Flowering and Ornamental Plants
Promotes vibrant blooms and stronger stems in flowers and ornamentals such as roses and carnations.
Leafy Vegetables
Improves leaf structure, color, and marketability in crops like spinach, kale, and lettuce, ensuring better quality and yield.
Dosage and Timing of Potassium Sulfate Fertilizer Application:
Crop Name | Application Rate with Irrigation |
|---|---|
Field Crops | Start at the tillering stage and repeat every two weeks. |
Fruit Trees | After petal fall and repeat one month later. |
Vegetables and Herbs | Apply at the 3-5 leaf stage and repeat monthly. |
Greenhouse Crops | Begin at fruiting stage and repeat every 15 days. |
Citrus Trees | Apply after fruit set. |
Ornamental Plants | Start with the onset of vegetative growth and repeat monthly. |
Pistachio Trees | Apply at the pea-sized fruit stage and repeat two weeks later. |
The amount of potassium sulfate required will vary depending on the age and variety of the plant. Soil and leaf testing can also influence the amount of this fertilizer needed for each plant. Therefore, it is recommended to consult with relevant experts before application. It is worth mentioning that, despite the common belief, the potassium requirement of some plants is even higher than their nitrogen (N) requirement. For example, the potassium requirement of sunflower is twice its nitrogen requirement. In developed countries, the ratio of nitrogen to potassium consumption is nearly 1:1, but unfortunately, in our country, this ratio is often greater than 15.
Additionally, due to the presence of the sulfate anion (SO4-) and the absence of chlorine in potassium sulfate, this fertilizer is one of the best options for oilseed crops and sulfur-loving, chlorine-sensitive plants, such as canola, sesame, soybeans, olives, grapes, citrus fruits, potatoes, and tobacco.
Classification of plants based on potassium uptake:
- Potassium-loving plants: such as sunflower, potatoes, celery, canola, grapes, dates, and citrus fruits.
- Plants with moderate potassium needs: such as tomatoes, sugar beets, corn, soybeans, and cotton.
How to Apply Potassium Sulfate Fertilizer for Optimal Results
Proper application of potassium sulfate fertilizer ensures efficient nutrient delivery, improved crop quality, and higher yields. Here’s a step-by-step guide for effective use in various irrigation systems and agricultural practices:
Drip Irrigation
Description: This method delivers potassium sulfate directly to the root zone, ensuring precise nutrient uptake and minimizing waste.
- Dosage
5–10 kg/ha per application, applied weekly or as needed based on crop requirements. - How to Apply
- Dissolve potassium sulfate in water and integrate it into the drip irrigation system.
- Ensure the solution is evenly distributed to the root zone for efficient absorption.
- Regularly check and clean the system to avoid clogging due to undissolved particles.
Foliar Application
Foliar spraying provides quick access to potassium and sulfur during critical growth stages, such as flowering and fruiting.
- Dosage
2–5 grams per liter of water, applied every 10–14 days. - How to Apply
- Dissolve potassium sulfate in water and mix thoroughly in a sprayer.
- Apply evenly to the foliage during the early morning or late evening to avoid leaf burn.
- Repeat applications during peak nutrient demand periods for best results.
Broadcasting
Broadcasting is suitable for large-scale field applications, ensuring even distribution across the soil surface.
- Dosage
50–150 kg/ha, depending on the crop and soil nutrient levels. - How to Apply
- Spread potassium sulfate uniformly over the field.
- Incorporate it lightly into the topsoil using mechanical equipment to prevent nutrient leaching.
- Apply before planting or as a top-dressing during crop growth.
Fertigation
Fertigation combines fertilization with irrigation, ensuring nutrients are evenly delivered throughout the root zone.
- Dosage
10–20 kg/ha per application, depending on crop requirements. - How to Apply
- Dissolve potassium sulfate in irrigation water.
- Apply through the irrigation system, ensuring uniform distribution.
- Monitor the solution's pH and salinity levels to optimize nutrient availability.
Band Placement
Band placement is a localized application method where potassium sulfate is placed in narrow bands near the root zone to ensure efficient nutrient uptake and minimize losses due to leaching. This method is particularly effective for row crops and plants with well-defined root zones.
- Dosage
30–50 kg/ha, depending on crop and soil requirements. - How to Apply
- Apply potassium sulfate in a narrow band either below or to the side of the seed or plant rows.
- Ensure the fertilizer is placed at least 5–7 cm away from the seeds or roots to prevent salt injury.
- Cover the band with soil immediately after application to reduce nutrient volatilization and enhance availability.
Production Process of Potassium Sulfate Fertilizer by Kimia Exir Shargh
The furnace is equipped with burners, a refractory brick lining, and paddles that move the produced potassium sulfate within the furnace, eventually directing it toward the furnace outlet. The temperature, pressure, and pH within the furnace are regularly monitored by the industrial control and automation system. The Mannheim furnace process is one of the most widely used and cost-effective methods for producing potassium sulfate fertilizer. The production process at Kimia Exir Shargh is also based on the Mannheim furnace, commonly utilized both globally and in Iran. In this process, concentrated sulfuric acid (98% by weight) and potassium chloride (KCl) powder are introduced into the Mannheim furnace, where they react to produce potassium sulfate.
The product is then transported to granulation equipment, where it undergoes a compaction granulation process. After granulation, the product is cooled using a cooling system, graded by a sieving system, and finally packaged and stored.The potassium sulfate produced in the Mannheim furnace is in powder form with particle sizes under 1 mm and a temperature exceeding 500°C. To cool the product, a cooling system is used, reducing its temperature to below 40°C. If the powdered product is intended, it is directly transferred to the packaging system and packed into 1-ton jumbo bags or 10 kg and 25 kg bags.The potassium sulfate production line uses Mannheim furnace technology and includes the following sections:
Raw Material Storage Area
Potassium Chloride Refining System
Raw Material Storage, Transfer, and Charging System
Mannheim Furnace System
Product Transfer System from Furnace to Granulator/Cooling System
HCl Gas Scrubber System for Capturing and Storing By-product Hydrochloric Acid (33%)
Granulation System
Grading, Packaging, and Storage System
Important Notes
- Potassium sulfate can form insoluble compounds when mixed with calcium-based or high-phosphate fertilizers. Use it separately or ensure compatibility through small-scale testing before combining in irrigation systems.
- Potassium sulfate slightly lowers soil pH, which can benefit alkaline soils but may not suit acidic soils. Regularly monitor soil pH and, if needed, use lime or other pH adjusters to maintain an optimal range for crops.
- Uneven application of potassium sulfate can result in nutrient imbalances, leading to some plants receiving too much potassium while others suffer deficiencies. Use calibrated equipment and apply uniformly for best results.
- Potassium sulfate is less hygroscopic than some fertilizers but can still absorb moisture over time, forming clumps that make application difficult. Store in a cool, dry place and use moisture-proof containers to maintain product quality.
- Hard or saline water can reduce the solubility of potassium sulfate, affecting its delivery and effectiveness in fertigation systems. Test water quality regularly and, if needed, treat it to reduce hardness or salinity before use.
Certifications and Standards
Frequently asked questions
What are the optimal soil and climatic conditions for using potassium sulfate?
The factors affecting potassium uptake are divided into two main categories: soil factors and plant factors. The most important soil factors include:
- Type of clay minerals
- Cation exchange capacity (CEC)
- Amount of exchangeable potassium
- Potassium fixation capacity
- Potassium in the subsoil
- Root development depth
- Soil moisture, aeration, and temperature
- Soil pH
- Amount of calcium and magnesium
The most important plant factors include:
- Cation exchange capacity of roots
- Root network
- Expected yield
- Type of plant
- Plant density per unit area
- Timing of potassium application
How can potassium sulfate be applied and stored safely?
- Potassium sulfate is preferable for saline soils and water and for crops sensitive to chloride, such as tobacco and potatoes.
- It is the best fertilizer for oilseed crops like olives, cotton, rapeseed, and sunflower.
- In soils requiring high amounts of potassium sulfate, split applications are recommended.
- Foliar spraying of potassium sulfate at high concentrations can cause leaf burn.
- Potassium sulfate should not be stored alongside urea, ammonium nitrate, or calcium nitrate fertilizers.
- It must be stored in a dry place away from direct sunlight.
- Potassium sulfate is stable under humid conditions and can be stored effectively.
What distinguishes potassium sulfate from other potassium fertilizers?
The main potassium-containing fertilizers include:
- Granular potassium sulfate
- Powdered potassium sulfate (SOP)
- Potassium nitrate
- Potassium chloride
- Potassium carbonate
- Potassium thiosulfate
The global consumption of potassium fertilizers is about 34 million tons, with Iran’s share being approximately 750,000 tons. Due to the saline and calcareous nature of most soils in Iran, potassium sulfate (both granular and powdered forms) is the most commonly used potassium fertilizer.
Its advantages include:
- High potassium content (50–53% K₂O)
- Contains sulfur in the form of sulfate (SO₄⁻)
- Neutral to slightly acidic pH
- Low salt index (0.8)
- Very low chlorine content
- High solubility (120 grams per liter)
- Suitable for soil application, fertigation, and foliar spraying
How does the efficiency of potassium sulfate differ in alkaline soils compared to acidic soils?
Soil pH is one of the most critical chemical properties affecting nutrient solubility and microbial activity.
- Potassium uptake is optimal at a pH of 6 or higher and is easier in acidic soils compared to alkaline soils.
- Potassium sulfate, being neutral, and its powdered form (SOP), which is slightly acidic, perform better in alkaline soils.
- The sulfate ion (SO₄⁻) in potassium sulfate makes it one of the best fertilizers for use in alkaline soils.
What is the role of potassium and sulfur in potassium sulfate for plants?
Potassium:
- Second most abundant nutrient in plants after nitrogen.
- Participates in the activity of over 60 enzymes, photosynthesis, water use efficiency, and resistance to pests, diseases, and stresses like drought and cold.
- Regulates anion-cation balance and stomatal opening/closing, affecting plant water relations.
- Critical for nitrogen fixation in legumes and contributes to overall plant health and disease resistance.
- Known as the “quality” nutrient due to its role in improving crop quality.
Sulfur:
- Fourth most essential nutrient after nitrogen, phosphorus, and potassium.
- Plays a key role in plant defense against stresses and pests.
- Involved in the synthesis of amino acids, chlorophyll, oils, and nitrogen metabolism.
- Improves water quality, lowers soil pH in saline and alkaline conditions, and enhances the uptake of micronutrients like iron, phosphorus, zinc, and manganese.
What are the symptoms of potassium deficiency in plants?
Potassium deficiency symptoms include:
- Yellowing (chlorosis) along leaf edges, followed by browning (necrosis) at the tips of older leaves.
- Leaf curling and purple spots on the underside.
- Short internodes, stunted growth, weak stems, and increased susceptibility to lodging.
- Reduced yield, wrinkled seeds, and poor crop quality.
Potassium deficiency is more common in crops and fruits like potatoes, rice, apples, citrus, strawberries, grapes, and tomatoes. It may also increase the uptake of magnesium and calcium, leading to their toxicity symptoms.