Fertilizer materials are frequently combined, either by dry blending solid granules or by tank mixing liquids, to save time and labor during application. However, blending different products without a proper understanding of their chemical properties can lead to serious consequences. An incompatible mixture can result in immediate product failure, equipment damage, nutrient loss, or, in the worst cases, the release of toxic gases or a fire hazard. Therefore, before combining any two fertilizers, it is important to know which materials should never be mixed.
Chemical Principles Behind Incompatibility
Fertilizer incompatibility generally falls into three chemical categories: precipitation, gas release, and hazardous reactions. Precipitation, or physical incompatibility, occurs when two dissolved salts react to form a new, insoluble compound. This solid material, such as sludge or fine sediment, settles out of the solution, making nutrients unavailable to the plant. This insoluble material is the primary cause of clogs in sprayer nozzles, filters, and drip irrigation emitters.
Another common issue is a pH-driven reaction that results in the loss of nitrogen through volatilization. When an acidic fertilizer, such as ammonium sulfate, is mixed with an alkaline material like urea or lime, the solution’s pH rapidly increases. This abrupt shift converts the ammonium ions (NH₄⁺) into ammonia gas (NH₃), which is then released into the atmosphere. This conversion represents a significant loss of applied nitrogen and can also pose a respiratory hazard to the applicator due to the toxic nature of ammonia gas.
The most hazardous reactions involve strong oxidizing agents, which can generate fire or explosion risks. Oxidizers, like those containing nitrate compounds, should never be combined with organic or reducing materials. These mixtures are chemically unstable and can become explosive if exposed to heat, friction, or contamination. Understanding these underlying mechanisms is necessary to avoid product loss and safety hazards.
High-Risk Fertilizer Combinations to Avoid
One frequent compatibility error involves mixing calcium-containing fertilizers with products containing phosphates or sulfates. For example, when Calcium Nitrate is combined with a phosphate fertilizer like Monoammonium Phosphate (MAP) or Diammonium Phosphate (DAP), the calcium ions react with the phosphate ions. This reaction forms calcium phosphate, a highly insoluble solid that precipitates out of the solution. This sludge formation means both the calcium and the phosphorus become chemically locked up and unavailable for plant uptake, while simultaneously clogging application equipment.
A similar precipitation occurs when a calcium product, such as Calcium Nitrate, is mixed with a sulfate-containing fertilizer, like Magnesium Sulfate or Ammonium Sulfate. The calcium and sulfate ions combine to form calcium sulfate, commonly known as gypsum. Gypsum has low solubility in water, causing it to drop out of the solution as a fine white powder. To avoid this issue in liquid feeding systems, calcium fertilizers must always be stored and applied in a separate stock tank from those containing phosphates or sulfates.
High-nitrate fertilizers should not be mixed with organic substances or reducing agents. Ammonium Nitrate, a powerful oxidizer, can react dangerously with materials like powdered metals, sulfur, carbon, or any organic compound, including many forms of urea. This combination creates an unstable mixture with the potential for violent, exothermic reactions that can ignite or explode. Dry blending Ammonium Nitrate with Urea is problematic because the blend is highly hygroscopic, rapidly absorbing moisture, which leads to severe caking and possible decomposition.
Mixing strongly acidic and strongly alkaline fertilizers causes a rapid increase in pH. Combining an acidifying source, such as Ammonium Sulfate or Urea, with an alkaline source like potassium hydroxide or certain limestone-based products drives the conversion of ammonium to volatile ammonia gas. This immediate gassing can build pressure in a sealed tank, pose a severe respiratory hazard, and result in the visible loss of nitrogen to the air.
Safe Handling and Storage Practices
Consult the Safety Data Sheet (SDS) and product label for compatibility warnings and handling instructions before attempting any mixture. When dealing with concentrated or reactive materials, use Personal Protective Equipment (PPE), including chemical-resistant gloves, protective eyewear, and long sleeves, to prevent skin and eye contact. Dilution is important, as highly concentrated fertilizer solutions are more likely to react and form precipitates.
For checking physical compatibility, the small-scale “Jar Test” is the industry standard. This involves mixing small amounts of the intended products in the same proportions as the full tank mix and observing the result for at least 30 minutes. The appearance of cloudiness, sediment, flakes, or heat generation indicates an incompatible mixture that should not be applied.
When loading a spray tank, adding ingredients in the correct sequence ensures proper mixing and dissolution. The general order is:
- Water first
- Wettable powders
- Flowable liquids
- Soluble liquids
- Chelates
- Surfactants
Constant agitation throughout the process is necessary to prevent materials from settling or reacting in concentrated pockets. For storage, physically separate oxidizing agents, such as nitrate-based fertilizers, from any combustible or organic materials to prevent accidental contamination.