Potash: IMSBC Code Schedule and Carriage

Potash is one of the highest-volume dry-bulk fertilizer cargoes on the oceans, with global seaborne trade running at approximately 50 to 60 million tonnes per year. The IMSBC Code classifies it Group C: no liquefaction risk, no IMDG chemical hazard class. The practical controls that distinguish every potash voyage from an inert mineral cargo are the strong chloride-driven corrosivity to steel (which demands thorough post-discharge hold washing), the hygroscopic tendency to cake and harden during transit (which complicates discharge if holds aren’t kept weather-tight), and the contamination sensitivity that can downgrade an entire cargo at the receiver if foreign material enters the holds.

The potash traded globally is potassium chloride, sold under the abbreviation MOP (muriate of potash). A smaller volume of potassium sulphate (SOP, sulphate of potash) also moves in bulk, with different chemistry and a narrower market. Both appear in the IMSBC Code schedule under the heading POTASH. Understanding the distinction between MOP and SOP, the agricultural trade that drives the cargo flows, and the corrosivity and caking controls that govern every shipment is the foundation of competent potash carriage.

Potassium fertilizer chemistry: MOP and SOP

Potash is potassium oxide (K2O) expressed in percent, the standard agronomic unit for comparing potassium sources. The two commercially dominant potassium fertilizers, MOP and SOP, both supply K2O but differ in their anion, their production routes, their price, and their agronomic profile.

Muriate of potash is potassium chloride, chemical formula KCl. It analyzes at approximately 60 to 62% K2O equivalent, making it the highest-potassium single-nutrient fertilizer produced at global scale. The name “muriate” is an archaic chemical term for chloride, retained in the fertilizer trade because it distinguishes MOP from sulphate alternatives. KCl forms in evaporitic deposits as sylvite, the mineral name for the geological potassium chloride that makes up the productive ore in the world’s major potash basins.

Sulphate of potash is potassium sulphate, chemical formula K2SO4. It analyzes at approximately 50 to 52% K2O. SOP is chloride-free, which makes it agronomically preferred for chloride-sensitive crops (tobacco, some fruits, some vegetables, and certain specialty crops). The premium SOP commands over MOP is substantial: SOP typically trades at 1.4 to 1.8 times the MOP price. SOP production is more complex than MOP, involving either the Mannheim process (reacting KCl with sulphuric acid) or processing of naturally occurring sulphate-bearing brines.

From a shipping standpoint, MOP and SOP share Group C classification and similar physical handling requirements. The main practical difference is that SOP is chloride-free, reducing (though not eliminating) its corrosive impact on steel. MOP’s chloride content is approximately 47% by mass, which is the dominant driver of its corrosivity to hold structure.

Potash grades and their role in cargo planning

Standard MOP (sometimes called “red MOP” because of the pink-to-red coloration from iron oxide impurities) is a granular material with particle sizes of approximately 0.2 to 1.0 mm. It’s the most commonly shipped grade and is used for direct application and bulk blending with nitrogen and phosphate fertilizers. Bulk density runs from approximately 1.0 to 1.2 tonnes per cubic metre, with stowage factor from 0.85 to 1.0 cubic metres per tonne.

Granular MOP (also called “coarse MOP”) has a particle size of approximately 1.0 to 4.0 mm. The larger granule size is preferred for mechanical bulk blending because it matches the granule size of companion fertilizers (urea at 2 to 4 mm, DAP at 2 to 4 mm) and improves blend uniformity. Granular MOP generates less dust during loading and discharge than standard MOP. Its bulk density and stowage factor are similar to standard MOP.

Soluble MOP (used in fertigation and foliar spraying systems) is a high-purity, often powder-form product that is usually bagged or palletized rather than shipped as a loose bulk cargo. Where it does move in bulk, its fine particle size increases dust hazard and caking tendency relative to granular grades.

White MOP (also called “de-colored MOP”) is standard MOP that has been processed to remove the iron oxide and mineral inclusions that give standard MOP its characteristic red-pink color. It analyzes at the same 60 to 62% K2O but is off-white to white. White MOP is used in industrial applications and in markets where buyers prefer the visual appearance of a purer product. It commands a modest premium over standard MOP and is more sensitive to contamination from colored materials.

The IMSBC Code schedule for potash

The IMSBC Code’s individual schedule for POTASH (also referenced as MURIATE OF POTASH in the Code’s index) specifies the physical data, hazard classification, and handling requirements that govern every bulk potash shipment under the SOLAS framework. The Code made the schedule mandatory under SOLAS Chapter VI Regulation 1-1 when the Code entered into force on 1 January 2011. The 2023 edition incorporates amendments adopted at MSC 105 (MSC.500(105)) and reflects MSC 107 updates (MSC.539(107)).

The key schedule parameters are:

Group C classification is the central regulatory fact: potash doesn’t liquefy (no Group A TML or Flow Moisture Point determination is required) and carries no IMDG chemical hazard class (no Group B MHB designation). The schedule does, however, record two characteristics that drive the practical handling regime for every potash voyage: corrosivity to steel, and the tendency to cake when exposed to moisture.

The schedule’s note on corrosion is more operationally significant than it appears at first reading. Chloride at the concentration present in MOP (47% Cl by mass) is aggressive to mild steel in the presence of even small amounts of moisture. The schedule requirement to clean and wash holds after every potash discharge is not a housekeeping preference; it’s the measure that prevents cumulative chloride-driven corrosion from destroying hold coating and structural steel over successive cargoes.

IMSBC Group C cargoes: the potash position

IMSBC Group C is the lowest-hazard classification for solid bulk cargoes, but “lowest hazard” doesn’t mean “no requirements.” Group C means the cargo neither liquefies nor carries a designated chemical hazard under the IMDG Code. For potash, the Group C position means:

No TML testing is required. The master doesn’t need a Flow Moisture Point certificate or a Transportable Moisture Limit figure from the shipper. The shipper’s declaration under the Code still must be provided, but it doesn’t include moisture-limit certification.

No IMDG-class labels are applied to the ship or cargo documentation. Potash isn’t an oxidizer, isn’t flammable, and doesn’t evolve toxic gas. The Bill of Lading, cargo declaration, and manifest carry no IMDG class or UN number for standard commercial MOP or SOP.

Group C classification does not eliminate cargo-specific hazards. Potash’s chloride corrosivity and hygroscopic caking are Group C special requirements, not Group B chemical hazards, but they still require active management on every voyage. The Code’s individual schedule specifies these requirements explicitly. A master who treats a potash cargo as an entirely routine inert mineral cargo and neglects the hold-washing and moisture-control requirements is non-compliant with the individual schedule, even though no chemical hazard class applies.

The comparison with other fertilizer cargoes clarifies the Group C position. Diammonium phosphate (DAP) is also Group C, but its schedule imposes ammonia evolution monitoring and enclosed-space entry precautions that potash doesn’t require. Ammonium nitrate fertilizer is Group B (MHB, subsidiary oxidizer), with strict temperature monitoring, segregation requirements, and, in high-purity grades, detonation risk. Potash sits at the lower end of the fertilizer hazard spectrum, but its chloride corrosivity makes it meaningfully more demanding than a genuinely inert cargo like pig iron or limestone.

The global potash trade

Potash geology concentrates production in a small number of basins. The Saskatchewan basin in Canada is the world’s largest potash reserve, formed during the Devonian period when an inland sea evaporated over what is now prairie farmland at depths of 1,000 to 3,000 metres. Mining is done by conventional shaft mining (Nutrien’s Lanigan, Vanscoy, and other operations) and by solution mining (Mosaic’s Belle Plaine and Nutrien’s solution operations). The Saskatchewan basin alone holds an estimated 40 to 50% of the world’s economically recoverable potash reserves.

The Verkhnekamskoye deposit in Russia’s Perm region is the second-largest basin, and the adjacent Starobinskoye deposit in Belarus is the third. These three basins, Saskatchewan, Perm, and Starobinskoye, account for the majority of global potash production and the bulk of seaborne trade.

Major export origins and their principal shipping points:

Canada. Exports run to approximately 14 to 16 million tonnes per year in recent years. The majority flows to Pacific markets through Vancouver (Pacific Coast Terminals at Port Moody and Canpotex’s Ridley Island terminal at Prince Rupert), with Atlantic exports leaving from Saint John, New Brunswick. Canpotex, the Canadian potash export consortium representing Nutrien and Mosaic, coordinates export logistics and vessel nominations.

Russia. Uralkali operates mines at Solikamsk and Berezniki in Perm Krai. Exports leave through the Baltic port of Saint Petersburg (Ust-Luga) and, for smaller volumes, the Black Sea. Sanctions applied after 2022 have disrupted some Uralkali trade routes and redirected volumes to non-sanctioning markets.

Belarus. Belaruskali, the state-owned producer, historically exported through Klaipeda in Lithuania under a transit arrangement with Belarusian Railways. That arrangement collapsed in 2021 following a political dispute, and subsequent Western sanctions on Belarus further complicated export logistics. Belaruskali now exports through Russian Baltic and Black Sea ports on a more limited scale. Pre-2021, Belarus exported approximately 12 to 13 million tonnes per year; the current figure is substantially lower.

Germany. K+S AG operates the Werra and Neuhof-Ellers mines in central Germany. German potash exports are smaller in volume than the three major producers, running at approximately 2 to 3 million tonnes per year, and are exported primarily through the port of Hamburg. K+S is a significant supplier to European markets.

Israel. ICL Group extracts potash from Dead Sea brine at Sodom and exports from Ashdod. Seaborne volumes run at approximately 1.5 to 2 million tonnes per year.

Jordan. Arab Potash Company mines at Ghor Al-Safi on the Dead Sea. Exports leave through Aqaba. Jordanian production runs at approximately 2 to 2.5 million tonnes per year.

The major potash import destinations are the world’s large agricultural economies: Brazil (the largest single import market at approximately 10 to 12 million tonnes per year, entering through Santos, Paranagua, and Vitoria), China (approximately 8 to 10 million tonnes per year through Qingdao, Yingkou, and other ports), India (approximately 4 to 6 million tonnes per year through Mundra, Kandla, and Krishnapatnam), and the United States (approximately 4 to 5 million tonnes per year through New Orleans and other Gulf ports). Southeast Asian markets (Indonesia, Malaysia, Vietnam, Thailand) collectively import approximately 4 to 5 million tonnes per year.

Vessel types and trade-lane economics

Potash moves on all standard bulk carrier sizes. Handysize vessels (25,000 to 40,000 DWT) serve regional trades and smaller port terminals. Supramax (50,000 to 60,000 DWT) and Panamax (65,000 to 80,000 DWT) are the dominant vessel sizes for the major trade lanes from Canada and the Perm basin to Brazil, India, and Southeast Asia. The economics of Canadian potash export to Brazil favor Panamax-equivalent vessel sizes: the Panama Canal transit connects the Vancouver loading range to Atlantic Brazil discharges in approximately 25 to 30 transit days.

Major export terminals can accommodate Panamax or larger vessels. Pacific Coast Terminals at Port Moody, British Columbia, operates shiploaders capable of 4,000 to 6,000 tonnes per hour and berths that accommodate Panamax vessels. The Canpotex terminal at Ridley Island (Prince Rupert) is similarly equipped. At these loading rates, a 70,000-tonne Panamax loads in approximately 12 to 18 hours of effective loading time.

Capesize vessels (above 100,000 DWT) are used on specific trades where deep-water berths at both ends allow it. The major Brazilian agribulk terminal at Paranagua has depth constraints that limit conventional Capesize access, but some terminals in the Santos complex can receive larger vessels.

Chloride corrosivity: the primary ship-structural concern

Potash (MOP) contains approximately 47% chloride by mass. In the presence of moisture, this chloride concentration creates an electrolyte that drives rapid electrochemical corrosion of mild steel, including hold plating, frames, web frames, and bilge components. The corrosion mechanism is the same as that of sodium chloride (salt), but the larger cargo volumes, higher cargo density, and the concentrated nature of the ionic solution at contact surfaces make potash a more aggressive corrosive agent per tonne of cargo than many shipmasters expect.

The corrosion pathway works as follows. Dry MOP in a dry hold is essentially inert to steel: there’s no electrolyte available to complete the electrochemical circuit. When moisture is introduced, whether from atmospheric absorption, condensation, rain ingress, or bilge leakage, the potassium chloride dissolves into a concentrated KCl solution. This solution is an excellent electrolyte. At the steel surface, it drives anodic dissolution of iron (Fe to Fe2+) and cathodic oxygen reduction, producing iron oxide corrosion products. Chloride ions at high concentration also break down the passive oxide film that ordinarily slows steel corrosion, producing the characteristic pitting and under-coating corrosion that is the dominant failure mode in potash holds.

The practical consequence is that potash holds accumulate structural damage faster than holds used for inert minerals, even when hatch covers are watertight and cargo moisture is nominally low. Cargo residues left in hold crevices after discharge dissolve during the next wash cycle (or in rain) and produce a localized concentrated solution at frame-to-plate junctions and bilge zones. This is the zone where corrosion pitting initiates.

Post-discharge hold washing: the mandatory control

The IMSBC Code schedule for POTASH requires thorough washing of holds after every potash discharge. This is not a recommendation: it’s a schedule requirement driven by the corrosivity notation. The purpose is to remove all KCl from contact with steel surfaces before the next cargo, before the next port stay, and before conditions allow the accumulated residue to create an aggressive corrosive environment.

The washing sequence used in industry practice:

Sweep out all residues. After grab discharge, crew sweep hold floors, frames, and bilge covers to remove all potash fines and residue visible to inspection. Potash is dense and the granules don’t disperse as easily as fine dust, but fines accumulate in frame crevices and at the edges of tank tops where the sweeping action can’t reach.

First water wash. Flood the hold with fresh water or seawater from hoses or pressure washers, working from the top down. This dissolves and flushes potash residues from all surfaces. Drain to bilge wells and pump out. The first wash water will have a measurable chloride content from dissolved KCl residue.

Second water wash. Repeat the wash. For heavily contaminated holds (particularly those that carried many successive potash cargoes with incomplete washing between them), a third wash may be needed to bring the hold surface to a standard where no salt residue remains.

Inspection after washing. Inspect hold coating for new damage: areas of delamination, bubbling, or pitting since the last inspection indicate corrosion has initiated under the coating. These must be spot-blasted and repainted at the next opportunity, ideally before the next potash loading.

Bilge pump test. After washing, test bilge pumps while water is still available to be pumped. A pump that’s partially blocked by potash fines drawn through a damaged bilge well cover will fail at the worst time: during a voyage when bilge water accumulates under a fresh cargo.

The comparison with salt carriage is instructive. Both salt and MOP require post-discharge washing for the same reason: chloride corrosion. The difference is that potash cargoes are far more sensitive to hold contamination (residual salt in a potash hold can downgrade the cargo at the receiver) while also being more corrosive per unit of contamination because of the higher ionic concentration at the KCl composition. A vessel alternating between salt and potash cargoes needs to be particularly thorough because each cargo adds to the cumulative chloride burden on the hold structure.

For context on the hold preparation regime that governs this kind of sequential cargo planning, the cargo hold preparation standards article covers the full procedure.

Hygroscopic caking: cause, mechanism, and prevention

Potassium chloride is hygroscopic above its equilibrium relative humidity (ERH), which is approximately 84% at 20 degrees Celsius and 80% at 30 degrees Celsius. Above those thresholds, KCl absorbs moisture from the surrounding air, and granule surfaces dissolve into a concentrated surface brine. When conditions then shift (temperature drops, surface evaporation accelerates), the dissolved KCl recrystallizes and bonds granules together. The result is a hard mass that resists grab crane penetration at discharge.

The ERH of KCl is higher than that of most other hygroscopic fertilizers. Urea’s ERH is approximately 75% at 25 degrees Celsius; DAP’s ERH is approximately 74 to 76%; NaCl (salt) is approximately 75% at 25 degrees Celsius. This means KCl (MOP) is somewhat less susceptible to initial moisture absorption than those materials under typical tropical conditions, but the advantage is marginal: tropical holds during loading at ports like Santos, Vitoria, or Qingdao in summer routinely experience ambient relative humidity above 80%, which is above the KCl ERH.

Once caking initiates, it’s cumulative. A thin surface crust after a humid loading port crossing into a humid voyage can deepen over a 20 to 30 day transit. Crust depths of 150 to 400 mm have been documented in survey reports for potash voyages where hatch cover seals were worn but not obviously leaking, allowing humid air ingress without rain penetration. That crust volume can represent several hundred tonnes of material that the grab crane can’t directly penetrate.

Moisture sources during a potash voyage

Three primary moisture sources create the conditions for caking in a sealed potash hold:

Atmospheric humidity during loading. When the hatch is open during loading, the ambient air in the hold is whatever the port’s air happens to be. At Paranagua in January (peak Brazilian summer) or at Qingdao in July, that air can be 28 degrees Celsius and 85% relative humidity. Loading into a hot, humid hold fills the hold with moist air that then gets sealed in when covers close. As the vessel proceeds to sea and the hold cools, the relative humidity of the trapped air rises further (since the dew point is fixed but the temperature drops), and condensation on hold steelwork can drip onto the cargo surface.

Hatch cover leakage. A hatch cover seal that allows water ingress during heavy rain or head seas creates a localized wet zone at the cargo surface near the hatch coaming perimeter. This zone is where caking initiates most aggressively, and it’s where survey evidence of hatch cover leakage is found: a ring-shaped crust pattern on the cargo surface under and around the hatch coaming.

Condensation from temperature cycling. A vessel loaded in a cold climate and proceeding to a tropical discharge port (or vice versa) undergoes temperature cycling that causes condensation on the cool steel surfaces of the hatch covers and hold structure. This condensation drips onto cargo surfaces. The drip rate is small per square metre, but over a 20-day voyage across temperature zones, the cumulative moisture input from condensation can meaningfully raise moisture content in the top layer of cargo.

Mitigating caking: the practical measures

Hatch cover condition is the single most effective control. A cover that seals properly eliminates the two most aggressive moisture sources: direct rain ingress and the ongoing replacement of dry hold air with humid outside air through leaks. Pressure-test (hose test) all hatch covers before loading potash. The test standard is IMO MSC/Circ.1071. A cover that fails the hose test must be repaired before loading.

Ventilation management during the voyage. General ventilation principles for hygroscopic cargoes: ventilate the hold only when the dewpoint of outside air is lower than the dewpoint of hold air. This requires a calibrated dewpoint meter, not just a temperature/humidity reading. A hold ventilated into tropical humid air when outside dewpoint is higher than hold dewpoint simply imports moisture. The default practice for potash holds in humid conditions is to keep covers closed.

Bilge well covers. Fit the bilge wells with filter fabric (burlap or coarse woven polypropylene) to prevent potash granules from entering the bilge sump. Potash granules, particularly standard MOP at 0.2 to 1.0 mm, can pass through standard 2 to 3 mm bilge well perforations and enter the bilge pump. Once inside, KCl solution attacks the pump impeller and casing. A blocked or failed bilge pump on a potash voyage creates exactly the condition that turns a minor moisture problem into a major structural one: water builds up in the bilge zone under the cargo, creating a concentrated KCl solution at the tank top.

Dust generation and control

Potash loading and discharge generate dust that is a practical nuisance and a minor environmental concern at port. Standard MOP, with its 0.2 to 1.0 mm particle size, produces the most dust. Granular MOP (1.0 to 4.0 mm) produces substantially less. The dust is not toxic but can irritate eyes and respiratory passages at sustained exposure.

Modern export terminals handle the dust issue through enclosed conveyor systems with integrated dust collection, and through water mist sprays at the shiploader head where cargo falls into the hold. The water spray adds a very small amount of moisture to the cargo surface (typically 0.05 to 0.1% by mass) that is acceptable within normal commercial moisture limits and that actually reduces dust by wetting the fine fraction. Shore-side dust scrubbers capture fine particles before they reach the port boundary.

On the vessel side, dust accumulates on deck during loading at terminals without fully enclosed shiploaders. After loading and before closing hatch covers, the crew should wash down decks to remove potash dust. KCl dust on deck, in scuppers, and around hatch coamings creates a salt-water solution when rain falls, accelerating deck corrosion at those points. Deck wash-down immediately after hatch closing and before proceeding to sea removes the source.

At discharge, grab cranes operating on open-air quaysides generate visible dust when cargo falls from the grab. Receiving port workers near the discharge point are exposed to this dust. Personal protective equipment (eye protection, dust masks) is standard for workers handling potash discharge. Some receiving ports require a brief water spray on the cargo pile as it grows, reducing airborne dust at the quayside.

Contamination sensitivity and cargo quality

Potash is sold at a specification defined by K2O content, moisture content, and limits on impurities. Standard MOP analyzes at 60 to 62% K2O, and the commercial tolerance for quality shortfall is narrow: a cargo that analyzes at 59% K2O on arrival may be rejected or subject to a price discount. More critically, contamination from foreign cargo residues can render an entire shipment unsuitable for agricultural use.

The contamination risk is asymmetric. From the vessel’s perspective, it’s relatively easy to carry a potash cargo and then load grain or another food-grade product: thorough washing is required, but the outcome is a clean hold. From the potash quality perspective, even a small residue of a previous cargo can be a problem. Coal fines in a supposedly clean hold will contaminate white MOP and downgrade it to a non-premium grade. Grain residue in a hold can create biological contamination of the potash that triggers phytosanitary rejection at an agricultural importing country. Lime or cement dust left from a structural repair activates in contact with humid potash and can cause localized decomposition of cargo.

The hold survey before loading potash is therefore not a formality. Cargo surveyors representing the shipper’s insurer and the receiver inspect holds before loading commences. In the major potash trade lanes, pre-loading hold surveys by recognized inspection companies (SGS, Bureau Veritas, Intertek) are standard. The surveyor records the state of hold coatings, the absence of foreign residue, the condition of bilge covers and hatch seals, and the results of a cleanliness test. The loading certificate that results is part of the cargo documentation.

Interaction with previous and subsequent cargoes

A potash cargo survey will reject holds that previously carried any of the following without thorough washing:

Any fertilizer containing nitrogen compounds (ammonium nitrate, urea, DAP, MAP): residues from these can contaminate the K2O analysis of the potash and create non-uniform nutrient distribution in the finished cargo.

Coal or mineral concentrates: dark-colored residues visibly contaminate light-colored MOP grades and create an obvious contamination claim at the receiver.

Lime, cement, or alkaline mineral cargo: alkalinity reacts with moisture in the KCl to create a localized corrosive alkaline brine at the cargo surface.

Conversely, vessels loading potash after another fertilizer cargo in the same vessel (a common pattern on the Canada-to-Brazil run, where DAP and potash may be loaded at adjacent terminals or on sequential voyages) require thorough documented cleaning between the two cargoes. DAP residue in a potash hold is a nitrogen contamination risk. Potash residue in a DAP hold is a chloride contamination risk that accelerates DAP decomposition and can increase ammonia evolution rates.

Hold preparation for potash loading

Hold preparation for a potash cargo follows the standard requirements of cargo hold preparation standards, with the specific adaptations required by the IMSBC Code schedule for POTASH. The sequence:

Step 1: Sweep and remove all previous cargo residues. The standard is a clean-swept hold with no visible residue of any kind. For potash cargoes that demand high white-grade cleanliness, “no visible residue” means bare metal and paint surfaces with no discoloration or contamination traceable to a previous cargo. Use brushes, brooms, and vacuums as appropriate for the residue type; avoid water at this stage if the previous cargo was hygroscopic or water-soluble.

Step 2: Inspect and repair hold paint coatings. Check all hold surfaces for corrosion damage, paint delamination, and exposed steel. Touch up damaged areas with approved hold paint and allow full cure before proceeding. Areas with active corrosion (rust pustules under the coating, or bare steel showing red-brown oxidation) require spot-blasting before repainting. A hold with bare corroded steel surfaces loaded with MOP will accumulate corrosion damage at those spots at a rate that is approximately 3 to 5 times the rate on a sound-coated surface.

Step 3: Clean bilge wells. Bilge wells must be fully clear of cargo residue, checked for corrosion, and tested for pump function. Fit new filter fabric (burlap or polypropylene geotextile, cut to fit) over each bilge cover before closing. The fabric must be secured at the edges so it can’t be displaced when potash flows in over it during loading. The bilge suction strainer (if the vessel has one in the bilge well) must also be clear.

Step 4: Hatch cover test. Pressure-test each hatch cover using the hose test method per IMO MSC/Circ.1071. Walk the full perimeter of each cover with water pressure applied and check for leakage at panel joints, coaming seals, and drain plug areas. Any leak path is a caking risk: repair before loading. Record the test result in the hold preparation checklist and the deck officer’s log.

Step 5: Pre-loading atmosphere check. This is required before any hold entry, even for a routine inspection before loading a Group C cargo. Verify oxygen content (at least 20.9% by volume) and check for any residual toxic or flammable gas from the previous cargo. Potash holds don’t generate gas during transit, but the hold may carry residual gas from a previous cargo (for example, if the vessel previously loaded a gas-evolving cargo such as DAP or certain mineral concentrates).

Step 6: Hold survey with inspector. Before authorizing loading to begin, the chief officer or master must confirm the hold survey is complete and the inspector has issued the loading certificate (or confirmed hold acceptance). Do not load potash against a rejected hold survey: correction at this stage costs hours; a cargo contamination claim costs far more.

Loading operations

Potash loads via shore conveyor and shiploader at major export terminals. The largest potash loading facilities in the world (Vancouver’s Pacific Coast Terminals and Ridley Island at Prince Rupert) operate closed conveyor systems with dust collection and shiploaders capable of 4,000 to 6,000 tonnes per hour. At that rate, a 70,000-tonne Panamax loads in 12 to 18 hours of effective loading, often spread across two calendar days with draft surveys and trim corrections.

Draft survey at loading. The draft survey is conducted by the ship’s cargo officer and a recognized independent surveyor, measuring drafts at six standard points and correcting for trim, hull deformation, and water density. For a 70,000-tonne cargo at a spot MOP price of, say, $350 per tonne, a 0.5$122,500. The survey discipline for potash is identical to the standards applicable to any high-value fertilizer cargo.

Trimming the cargo. The IMSBC Code schedule specifies trimming for potash. The typical approach is for the shiploader to extend its retractable conveyor forward and aft from the hatch opening to spread cargo across the hold length, rather than concentrating cargo in a central pile under the hatch. Untrimmed cargo peaked under the hatch imposes concentrated local loads on the tank top that can exceed the structural design stress for the central tank top section. After shiploader trimming, manual trim corrections with a bulldozer lowered into the hold may be needed in some holds; this requires an enclosed-space atmosphere check before entry even for a Group C cargo.

Cargo temperature check. Record cargo temperature at the hold surface at the start of loading and again 24 hours after loading. Potash doesn’t self-heat (unlike certain Group B mineral concentrates or some problematic cargo combinations), so a cargo temperature well above ambient at the start of voyage warrants investigation. In some cases, cargo conveyed through sun-heated exterior conveyors at a major terminal arrives at the hatch at 35 to 40 degrees Celsius, which is normal operational variation, not a hazard signal. But a sustained temperature rise on voyage is not expected.

Shipper’s declaration. The shipper must provide a cargo declaration per the IMSBC Code requirements, including the cargo name as it appears in the schedule (POTASH or MURIATE OF POTASH), the Group classification (C), the physical properties (bulk density, moisture content), and the statement that the cargo is appropriate for bulk shipment. The absence of a TML figure or FMP certificate is normal for Group C: this is one way the document differs from a Group A fertilizer shipment.

Voyage management and monitoring

A potash voyage doesn’t require the intensive atmosphere monitoring of a DAP or Group B cargo, but it does require consistent attention to hatch cover condition and bilge status.

Hatch cover inspection after weather. After any heavy rain event or significant sea state that drives water across the deck, the chief officer should visually inspect the hatch cover seals and coaming perimeters from deck level. If any water penetration is suspected (a small trickle at a coaming corner, or a wet patch on the cargo surface spotted from the hatch access ladders), the hold should be inspected after atmosphere clearance. Moisture staining on the cargo surface near hatch coaming edges is the primary visual indicator of leakage.

Daily bilge checks. Bilge well levels should be checked daily. Water in the bilge under a potash cargo is significant: it means either the hull is leaking, condensation is draining into the bilge in quantity, or potash-laden water is accumulating at the bilge zone. Any water in the bilge under potash should be pumped out promptly. Don’t leave it: KCl solution in the bilge attacks uncoated steel and bilge pump internals, and a bilge pump failure on a long voyage creates a progressive accumulation problem.

Cargo temperature log. The log should record hold air temperature and ambient temperature daily. Wide divergence between hold and ambient temperature can drive condensation, as noted earlier. Where the vessel crosses a temperature gradient (tropics to temperate, or vice versa), monitoring in the first 48 hours of the transition is worthwhile.

No hold entry without atmosphere check. Even on a Group C potash cargo, any hold entry during voyage requires an atmosphere check for oxygen content. There’s no ammonia risk from potash, and no toxic gas generation specific to the cargo. But enclosed spaces on bulk carriers can develop oxygen-deficient atmospheres from a variety of mechanisms unrelated to cargo chemistry (rusting of hold steel consumes oxygen, for example), and the enclosed-space entry requirement is absolute regardless of cargo type.

Discharge operations

Discharge of potash at receiving terminals uses grab cranes at most ports. Pneumatic discharge systems are used at some specialized fertilizer terminals, particularly for fine-grade or soluble products. Standard bulk grabbing is the default for granular and standard MOP.

Grab discharge rates. A modern bulk terminal grab crane with a 20-tonne capacity grab cycling at 6 to 8 grabs per hour achieves approximately 120 to 160 tonnes per hour per crane. With four cranes operating simultaneously on a 70,000-tonne Panamax, total discharge rate is approximately 480 to 640 tonnes per hour, giving a total discharge duration of approximately 110 to 145 hours of effective crane time. With hatch changes, draft surveys, and port delays, a four-crane operation on a Panamax potash discharge typically completes in 3 to 5 calendar days.

Caked cargo. Where the top layer has caked due to moisture exposure during voyage, the grab can’t directly penetrate the crust. The sequence for caked cargo: atmosphere clearance for enclosed-space entry, then a bulldozer or pneumatic breaker enters the hold to fragment the crust, then grab discharge proceeds on the broken fragments. The cost of hold entry and mechanical crust breaking (mobilization of equipment, labor, time) falls on whoever bears liability for the caking under the charter party. Establishing whether caking was present at loading (shipper’s fault, potentially), developed during voyage due to hatch cover leakage (ship’s fault), or arose from normal hygroscopic exposure within the voyage’s humidity conditions (shared risk) requires the surveyor’s evidence trail.

Hold inspection during discharge. The first officer should inspect the hold from deck level or from the hatch access ladders at the midpoint of each hold’s discharge. The purpose is to confirm no significant caking, no unusual odor, no visible hull damage (cracks in the tank top, for example), and no gross bilge accumulation. The discharge record should include the time of inspection and the findings.

Cargo residues after discharge. After grab discharge, fines remain in structural crevices, bilge well areas, and the corners of tank tops and frames. These residues must be swept and then washed out before the hold can be presented as clean for the next cargo. See the post-discharge washing section above.

Regulatory framework

The IMSBC Code is the primary instrument. SOLAS Chapter VI Regulation 1-1 made the Code mandatory for solid bulk cargoes on 1 January 2011. The Code’s biennial amendment cycle means that individual schedules, including POTASH and MURIATE OF POTASH, are updated when new data or experience warrants. The 2023 edition (incorporating MSC.500(105)) and the subsequent MSC.539(107) amendments adopted at MSC 107 are the current versions as of 2026.

SOLAS Chapter XII (additional safety measures for bulk carriers) applies to bulk carriers constructed on or after 1 July 1998 and governs structural requirements including hold flooding detection, double-bottom spaces, and hatch cover integrity standards. The hatch cover watertightness requirements under SOLAS XII are directly relevant to potash carriage: a hatch cover that passes the SOLAS XII standard is by definition adequate for potash from a water-tightness standpoint, and the hatch cover hose test records required by SOLAS XII should be maintained as part of the potash voyage file.

Port State Control inspections under the Paris MOU (European flag and port states), Tokyo MOU (Asia-Pacific), and other regional MOUs verify compliance with the IMSBC Code schedule requirements. For a potash shipment, inspectors may check that the shipper’s declaration is present and complete, that the IMSBC Code individual schedule is aboard and referenced, and that the hold preparation survey records are available. A vessel without the shipper’s declaration, without hold survey records, or with obviously contaminated holds at a loading port can be detained.

The IMSBC Group C cargoes classification also intersects with MARPOL Annex V, which governs garbage and cargo residue discharge at sea. Potash is not listed as a harmful substance for MARPOL Annex V purposes, so cargo residues from potash washing can be discharged at sea outside special areas, subject to the distance-from-land requirements. However, the concentrated KCl wash water from the first hold wash is often retained until a shore reception facility is available, since the environmental loading of a high-chloride solution close to port or in an estuarine environment is significant.

Potash versus salt: the key carriage comparison

The parallel with salt carriage (salt: IMSBC Code schedule and carriage) runs through this entire article because both are chloride salts classified Group C, both cake hygroscopically, and both require post-discharge washing for corrosion control. The practical differences are worth stating explicitly for cargo planners who allocate holds and vessels across both cargoes:

Corrosivity. Both corrode steel, but the mechanism differs slightly. NaCl solution at a given concentration is a somewhat more aggressive steel corrosive than KCl solution at equivalent concentration, because the sodium ion is more mobile in solution and the solution conductivity (which governs electrochemical corrosion rate) is slightly higher. But MOP cargoes are typically denser and fill holds more completely than typical solar salt loads, meaning the surface area of steel in contact with KCl solution (during any moisture event) is proportionally larger. In practice, vessels alternating between salt and potash cargoes accumulate corrosion damage at similar rates.

Hygroscopic threshold. KCl’s ERH is approximately 84% at 20 degrees Celsius; NaCl’s ERH is approximately 75% at the same temperature. This means salt starts absorbing moisture from the air approximately 9 percentage points of relative humidity sooner than MOP does. For port conditions with ambient humidity of 80%, MOP is below its ERH threshold and salt is above it. This difference is meaningful in tropical humid ports and makes salt somewhat more prone to initial caking than MOP.

Cargo density and hold load. Both have bulk densities in the range of 1.0 to 1.4 t/m3. Solar salt tends toward the higher end (1.2 to 1.4 t/m3) due to larger, denser crystals. MOP (1.0 to 1.2 t/m3) is somewhat lighter. A vessel loading maximum deadweight in solar salt will be operating at higher specific hold loading (tonnes per m2 of tank top) than the same vessel loading the same DWT of MOP.

Contamination sensitivity. Potash is far more sensitive to cargo residue contamination than road salt. A road salt cargo that arrives with 0.5% grain contamination is still acceptable for de-icing purposes. A white MOP cargo that arrives with 0.1% grain residue may be downgraded or rejected by an agricultural receiver.

Post-discharge washing requirement. Both require it. For salt, the primary purpose is to prevent corrosion of the hold structure. For potash, the purpose is both corrosion prevention and preparation of the hold for a contamination-sensitive next cargo.

Charter party and commercial considerations

A voyage charter party for a bulk potash shipment typically addresses several cargo-specific points that go beyond the IMSBC Code’s minimum requirements:

Hold preparation warranty. The vessel owner typically warrants that the holds are suitable for the cargo, clean, and watertight. The charterer or shipper typically arranges the hold inspection. The hold inspection certificate from the recognized inspector is the evidence that the warranty has been met. Where the inspector rejects a hold, the owner must arrange cleaning and re-inspection before loading: the vessel bears this cost and the associated laytime.

Post-discharge washing. Charter parties for potash regularly require the owner to perform hold washing after discharge to a specified standard (clean-swept, washed, dry) before redelivering the vessel. Failure to wash holds after potash discharge is a breach of the charter party and creates a direct liability exposure for the next cargo if corrosion or contamination from potash residues damages that cargo.

Caking liability. The charter party should specify who bears the cost and risk of caked cargo. Standard allocation: if moisture content at loading is above the agreed specification, caking risk is on the shipper. If hatch covers were defective and leaked, caking risk is on the vessel. If the cargo cakes within specification moisture and with no evidence of hatch leakage, the risk is shared or contested. A complete voyage file, including moisture samples at loading, hatch cover test records, and voyage weather logs, is the evidence base for any dispute.

Lay-can and loading rate. The major potash terminals operate on a fixed shiploader rate (stated in the charter party), and laytime is calculated against this rate. Where dust suppression water spray adds loading time (the spray system on some older terminals requires stopping and restarting the conveyor), the laytime treatment of these pauses should be agreed in the charter party.

Freight basis. Most potash voyages are quoted on a freight-per-tonne basis with a FIOST (free in and out, stowed, and trimmed) or FIO (free in and out) freight basis. Under FIOST, all cargo handling costs (stevedores, trimming, shore crane) are for the shipper’s and receiver’s account respectively, and the vessel’s freight earnings are a fixed per-tonne rate. The shipper’s declaration, cargo specification, and hold certificate all flow through this commercial structure.

Limitations

This article summarizes the IMSBC Code Group C schedule for potash (MURIATE OF POTASH and related entries) as published in the 2023 edition of the IMSBC Code incorporating MSC.500(105) and MSC.539(107), and industry practice as of 2026. It is not a substitute for the full IMSBC Code text, which mariners, terminal operators, and cargo surveyors must consult directly for every shipment. The Code is subject to ongoing amendment through the IMO’s biennial amendment cycle; always verify the current amendment state of any individual schedule against the IMO’s official publications.

Cargo physical properties (bulk density, moisture content, particle size, caking tendency) vary between production batches, producers, storage conditions, and loading port humidity. The nominal schedule values in this article are representative of the commercial range; any individual shipment must be assessed against the shipper’s declaration and the cargo samples drawn at loading.

The chloride corrosivity and hygroscopic caking characteristics described here represent documented industry experience. Actual corrosion rates and caking depth depend on hold coating condition, moisture exposure, voyage temperature profile, and cargo moisture content, all of which vary by voyage. No general description substitutes for hold survey on a per-voyage basis.

Charter party allocations for caking liability, hold washing obligations, and hold inspection costs described here reflect common market practice but are not universal. Terms vary between operators, trade routes, and market conditions. Legal and commercial review of specific charter party language is required for any specific transaction or dispute.

The potash export figures cited for Canada, Russia, Belarus, Germany, Israel, and Jordan are based on publicly available industry data from the International Fertilizer Association and related trade bodies; actual annual figures vary with market conditions, sanctions regimes, and weather-related crop demand changes.

See also

• IMSBC Code
• IMSBC Group C Cargoes
• Diammonium Phosphate: IMSBC Code Schedule and Carriage
• Urea: IMSBC Code Schedule and Carriage
• Salt: IMSBC Code Schedule and Carriage
• Phosphate Rock: IMSBC Code Schedule and Carriage
• Ammonium Nitrate Fertilizer: IMSBC Code Schedule and Carriage
• Cargo Hold Preparation Standards
• Marine Cargo Hold Ventilation
• Bulk Carrier

Related calculators

• IMSBC - Potash Muriate
• IMSBC - Potash (MOP)
• IMSBC Group A/B/C Classification