SOP-Based compound fertilizer

In fertilizer commerce, “SOP” denotes sulfate of potash, crystalline K₂SO₄ containing not less than 50% K₂O and typically 45–46% SO₃. SOP-Based compound fertilizer is a homogeneous granulated NPK or NPK+MgO+TE product in which the potassium fraction is derived from K₂SO₄ rather than from potassium chloride. Commercial grade designations used for low-chloride cropping programs include 12-12-17+2MgO+0.1B, 13-5-26+2MgO+0.1B, 15-5-20+2MgO+TE, and 10-20-20+3MgO+0.1Zn. In regulatory documentation, these products fall under EU Regulation (EU) 2019/1009, Annex I Part II PFC 1(C) for inorganic macronutrient fertilizers, and the term “SOP” is distinguished from “standard operating procedure” in quality records. The central constraint is chloride: the formulation is controlled to remain below 2% by mass when a low-chloride declaration is made, and the sulfate matrix changes solubility, granulation, salt index, and handling behavior relative to muriate-of-potash-based compounds.

Because SOP contains 45–46% SO₃, the product simultaneously supplies potassium and sulfate sulfur. This dual nutrient function is relevant in cropping areas where atmospheric sulfur deposition has declined and where KCl is excluded because of chloride sensitivity. Nutrient declarations use oxide forms: 26% K₂O equals 21.6% elemental K, and 5% P₂O₅ equals 2.2% P. The chloride value is not a nominal claim; it is a controlled release parameter measured on the as-received product mass.

Granulation Behavior and Production-Line Constraints for Sulfate-Based Formulations

K₂SO₄ has an aqueous solubility of approximately 120 g L⁻¹ at 25 °C, while KCl reaches approximately 344 g L⁻¹ at 20 °C. Because granulation in a rotary drum depends on liquid-phase saturation, replacement of KCl by K₂SO₄ reduces the available liquid phase at a given moisture level. In a steam drum granulator processing a 13-5-26+2MgO formulation, granulator discharge moisture is typically held between 2.5% and 4.0%, and recycle ratio is raised to 2.5:1–4:1 to maintain seed granule survival. The moisture window is narrow: below 2.0%, granule formation collapses into fines, while above 4.5%, drum lining fouls and the mass fraction above 5.0 mm increases sharply.

Dryer exhaust gas temperature is held between 70–95 °C to avoid thermal loss of boron from borate additives; boron volatility rises above 100 °C in the presence of acidic ammonium phosphate. Dryer inlet temperature is maintained at 180–250 °C; product temperature after cooling is controlled below 45 °C because residual heat accelerates ammonium sulfate–potassium sulfate bridging. Screen oversize is milled in a chain mill and returned to the granulator at a controlled 2–5% of total recycle to limit dust load without overloading the granulator liquid phase.

Production-line records from a representative 10 t h⁻¹ drum granulation system with L/D 4:1 and peripheral speed 0.9 m s⁻¹ indicate that when K₂SO₄ exceeds 35% of total raw-material mass, the mass fraction below 2.0 mm can rise unless binder rate and recycle are adjusted. Caking tendency is evaluated under 30 °C and 75% relative humidity for 30 days. Granules with crushing strength below 25 N develop unacceptable fines during bag stacking, and 20-layer pallet loads are rejected at internal release if the 2.0–3.35 mm fraction fails the crush test.

What Specification Thresholds Govern Low-Chloride Compound Fertilizer Declarations?

The marketed grade defines the minimum declared nutrient content in the order N-P₂O₅-K₂O. A representative specification for NPK 13-5-26+2MgO+0.1B is given in Table 1. The nitrogen fraction may be supplied by ammonium sulfate, monoammonium phosphate, or urea, but the choice affects critical relative humidity and granule hardness. Phosphorus in the compound is present as water-soluble and citrate-soluble phosphate; water-insoluble phosphate is not a suitable marker for this grade because the granulation route uses acidulated phosphate rock or MAP slurry rather than direct rock addition.

The release limit for chloride is the controlling regulatory boundary. Under EU Regulation (EU) 2019/1009, a fertilizer declared as “low chloride” must not exceed 2% Cl by mass; products destined for chloride-sensitive perennial crops are often specified at 1.0–2.0% Cl, with sulfate-S substituted as the accompanying anion. The sulfur source is not inert: in the 13-5-26 grade, 15–18% SO₃ provides 6.0–7.2% elemental sulfur equivalent, which is available without biological oxidation and therefore contributes immediately to plant uptake in sulfur-deficient sandy soils.

Usage Protocols for Chloride-Sensitive Cropping Systems

SOP-based compound fertilizer is applied as basal dressing, side dressing, or split application in potato, tobacco, grape, citrus, strawberry, and greenhouse ornamentals. Application rates are calculated from soil test values, expected crop removal, and the nutrient-to-yield coefficient for the specific cultivar. A processing tomato crop yielding 80 t ha⁻¹ removes approximately 240–280 kg K₂O ha⁻¹, 45–60 kg MgO ha⁻¹, and 25–35 kg S ha⁻¹ in aboveground biomass. At a product rate of 900 kg ha⁻¹, the 13-5-26+2MgO grade supplies 117 kg N ha⁻¹, 45 kg P₂O₅ ha⁻¹, 234 kg K₂O ha⁻¹, and 162 kg SO₃ ha⁻¹. For potato, chloride-sensitive varieties may receive 600–1000 kg ha⁻¹ of 12-12-17+2MgO, split 50% at planting and 50% at tuber initiation. When banded at planting, separation from the seed piece should be at least 5 cm if nitrogen rate exceeds 100 kg ha⁻¹.

Crop-specific chloride limits determine product selection. Flue-cured tobacco programs specify fertilizer chloride below 2% and irrigation water chloride below 20 mg L⁻¹ to prevent leaf chloride exceeding 1.0% dry matter. In grapevines, petiole chloride above 0.5% dry weight at full bloom is associated with leaf scorch and reduced photosynthetic area. Citrus on salt-sensitive rootstocks may show foliar chloride toxicity above 0.25% dry weight. SOP-based compound fertilizer is used in these systems when soil or water chloride is the limiting factor; the potassium application rate remains tied to crop removal rather than to chloride avoidance alone.

Application methods include preplant broadcast, band placement, and split top-dressing. For preplant broadcast, the material is incorporated to 10–15 cm depth within 24 h to reduce ammonia volatilization if urea is present. Band placement uses GPS-guided twin-disc applicators; the fertilizer band is placed 5–10 cm to the side and 2–5 cm below the seed row. When a single-disc spreader is used, the swath uniformity test according to ASAE S341.4 is repeated after changes in granule-size distribution or spreader vane position. Coated or uncoated product is compatible with most hoppers, but dust accumulation requires cleaning of hopper slide gates at intervals not exceeding 8 h of continuous operation.

Fertigation use is limited to soluble SOP-based grades or filtered suspensions. Granular SOP-based compound fertilizer contains phosphate carriers that may not fully dissolve, and stock solutions above 10% w/v can precipitate calcium or magnesium salts in hard water. Drip irrigation systems should use 120–150 µm mesh filtration and maintain pressure below 300 kPa at the tape inlet. In soilless substrates, the product is not used as a concentrated stock because sulfate and magnesium can exceed solubility product thresholds at high concentration; pre-dissolution in a separate mixing tank with continuous agitation is required. Irrigation water with bicarbonate above 150 mg L⁻¹ is acidified to pH 5.5–6.5 before injection to limit calcium sulfate and calcium phosphate precipitation.

When Sulfate-of-Potash Replaces Muriate in the Granulation Matrix

The main difference from MOP-based compound fertilizer is the chloride payload and its agronomic consequences. In MOP-based NPK 13-5-26, the potassium source is KCl with approximately 60% K₂O and 47% Cl. To supply 26% K₂O, the formulation contains about 43% KCl by mass, resulting in a final chloride concentration near 20%. In the SOP-based equivalent, chloride is held below 2%. Table 2 summarizes the operational differences.

SOP-based homogeneous granules eliminate the segregation observed in dry bulk blends. In bulk blends, differences in particle density and size between urea, DAP, and SOP granules can produce swath coefficient of variation above 15% under broadcast application; pneumatic spreaders magnify the effect when granule size ranges differ by more than 1.0 mm. The homogeneous structure of a chemically granulated SOP-based compound ensures that each granule carries a defined N-P₂O₅-K₂O ratio within tolerance, which is verified by sieve-cut analysis rather than bulk bag sampling alone.

Compared with potassium nitrate-based formulations, where KNO₃ solubility is approximately 316 g L⁻¹ at 20 °C, SOP-based compound fertilizer supplies sulfate rather than nitrate and has lower solution-formulation flexibility. It is not intended for high-concentration liquid fertilizer programs. The lower reported salt index of K₂SO₄ (46, NaNO₃ = 100) relative to KCl (116, NaNO₃ = 100) reduces osmotic stress in the root zone at equivalent potassium rates, but the sulfate anion can contribute to leaching of exchangeable cations in acidic sandy soils. Soil acidification is governed mainly by the ammonium-N fraction; a formulation containing 13% nitrogen as ammonium or urea generates acidity during nitrification, and lime requirement should be calculated from the nitrogen source rather than from the K₂SO₄ fraction alone.

Storage conditions are bounded by moisture and temperature. Urea-containing SOP-based grades have critical relative humidity below 60% at 30 °C; warehouse relative humidity above 60% for periods longer than 48 h initiates surface dissolution and intergranular caking. The product is incompatible with calcium nitrate and concentrated ammonium nitrate in bulk blends because localized moisture films form low-melting-temperature or high-ionic-strength phases that accelerate bag set. Do not co-store with oxidizing agents, and avoid pneumatic conveying before cooling below 45 °C. In tropical coastal warehouses, pallet loads are wrapped with polyethylene film and kept off concrete floors by 10 cm battens to limit moisture migration.