Follow these simple steps to determine fertilizer compatibility

Water-soluble fertilizers are blends of highly soluble greenhouse-grade salts. Commercial fertilizer manufacturers blend constituent salts together to produce customized water-soluble fertilizer formulations. Greenhouse operations can blend their own water-soluble fertilizers but must determine whether the savings justify the effort or if a specialty formulation is beneficial to the crop or required for water quality issues.

Challenges during in-house fertilizer mixing can arise if salts are pushed past their solubility limits or if incompatible ingredients are combined. A simple jar test can help you confirm that a blend will stay clear and injector-friendly before it ever reaches the bench.

Why solubility matters

Blending constituent fertilizer salts is a science. Growers must be aware of solubility limits: the maximum concentration of a particular fertilizer salt (solute) that may be dissolved in water (solvent) to form a nutrient solution. When the appropriate concentration of a fertilizer salt is added to water, ions will dissolve and form an unsaturated solution. But when a high concentration of a fertilizer salt is added to water, a saturated solution is formed, and excess solute can be observed as solids at the bottom of a fertilizer stock container. Solution temperature will also dictate solubility. Increased solubility will typically occur at warmer than cooler solution temperatures.

Incompatibilities to avoid

Some salts do not play well together. Therefore, growers must be knowledgeable of constituent fertilizer salt compatibility. If a combination of fertilizer salts that are not compatible are dissolved together, precipitation or formation of a solid will occur. The classic example, but most common mistake, is magnesium sulfate (Epsom salt) mixed with calcium nitrate, which can form insoluble precipitates.

The most common precipitation incompatibilities occur between sulfates and calcium but can also occur with phosphates and calcium and phosphates and iron. Precipitations between calcium and sulfates is why Cal-Mag fertilizer formulations do not supply sulfur to the plant. The magnesium source in these formulations has been changed from magnesium sulfate (which provides sulfur) to magnesium nitrate (which does not provide sulfur).

Growers should also be aware that some acids used to neutralize irrigation water alkalinity can also form precipitates when placed in stock solution. Figure 1 illustrates common fertilizer salts and acids that are compatible, incompatible or have reduced solubility when mixed in a stock solution.

The jar test

Growers can implement the jar test to determine fertilizer compatibility and unpredicted chemical reactions from other water-soluble additives, minerals, pesticides and water treatments. Before conducting a jar test (Figure 2), you will need to gather clear 1-quart jars or containers. To perform a jar test, follow these simple steps to determine fertilizer compatibility:

• Calculate desired concentrations of fertilizer to be dissolved in 1 quart of water.
• Weigh each fertilizer.
• Add water and dissolve fertilizers together.
• Cap the jar and let stand for 12 to 24 hours.
• Observe the jar for any cloudiness or if precipitation or solids formed.

Practical mixing tips

• Temperature counts: Dissolve fertilizer salts in tempered water and keep stocks above the temperature at which you tested. Cold rooms increase the risk of crystallization.
• Add thoughtfully: Dissolve one ingredient at a time and ensure each is fully in solution before adding the next.
• Watch the usual suspects: Keep calcium separate from sulfates and phosphates. Do not pair magnesium sulfate with calcium nitrate in the same stock.
• Separate when in doubt: Use A and B tanks or multi-head injectors for known incompatibilities or when you need higher concentrations.
• Validate acids: Test your acidified stock with all intended fertilizer salts. Some acid combinations form precipitates.
• Label and log: Record ingredient weights, water temperature and observations from each jar test so you can reproduce success.

Troubleshooting what you see in the jar

Cloudiness that clears with stirring points to borderline solubility. Warm the solution slightly or lower the concentration.

Persistent haze or sediment suggests true incompatibility or an overloaded stock. Separate ingredients into different tanks or reformulate to avoid problematic pairs, like calcium with sulfates or phosphates.

Grit or crystals on the bottom usually means you crossed a solubility limit at that temperature. Reduce the stock strength or maintain a higher storage temperature.

Overall, a 24-hour jar test is low-cost quality control that protects injectors, keeps stocks clear and delivers consistent nutrition.