Mixing Two Products in a Half-Full Sprayer
You are spraying a 3 ha orchard block with a 1,000 L air-blast sprayer. The tank already contains 400 L of water and a wettable powder fungicide you added 20 minutes ago. You now need to add an emulsifiable concentrate insecticide and a soluble liquid foliar nutrient. The labels state: the fungicide is compatible with the insecticide; the nutrient label warns not to tank-mix with strongly alkaline or strongly acidic products and to add it last; the insecticide label requires a jar test if tank-mixing with unfamiliar products. You have no pH meter. The water source is a farm pond. The sprayer has a working pressure of 15 bar at the nozzles.
1. Describe the correct order of operations for completing this tank mix and explain the reasoning behind each step.
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Correct sequence and rationale:
- Check compatibility first. Run a jar test with the pond water and all three products in the proposed ratios before adding anything else to the tank. The insecticide label requires it, and the nutrient's pH warning makes compatibility uncertain.
- Assess water quality. Pond water can carry suspended solids and variable pH. Without a meter, use pH strips or a test kit; if the water is strongly alkaline or acidic, the nutrient may be incompatible or the insecticide may degrade. Consider a buffering agent only if the label permits it.
- Add products in the label-recommended order. General rule: fill with water, add wettable powders first (already done), then emulsifiable concentrates, then soluble liquids, and add the foliar nutrient last, as its label requires. Keep agitation running throughout.
- Add the insecticide slowly with agitation, watching for curdling, flocculation, or separation.
- Add the nutrient last, with agitation, and check again for incompatibility.
- Top up to the final volume with water and maintain agitation until spraying is complete.
If the jar test shows incompatibility (precipitate, oily film, heat, or separation), do not proceed - apply the products separately.
2. The jar test shows a slight oily film but no precipitate. What do you do, and what formulation and equipment factors explain the result?
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An oily film suggests incompatibility or poor emulsification, even without precipitate. Do not spray the mixture.
Explanation:
- Formulation chemistry: an emulsifiable concentrate relies on surfactants to disperse oil droplets in water. High electrolyte content or pH shifts from the pond water or the nutrient can break the emulsion, causing oil to separate and form a film.
- Wettable powder interaction: fine powder particles can adsorb emulsifiers, destabilising the emulsion.
- Nutrient effects: soluble nutrient salts can raise ionic strength and alter pH, worsening emulsion stability.
- Equipment factors: 15 bar at the nozzles is high; some EC formulations shear or separate under high pressure, and inadequate agitation allows separation in the tank.
Actions:
- Do not add the mixture to the sprayer. Empty and rinse the tank if any was added.
- Apply products separately in sequence, respecting intervals on the labels, or find a compatible formulation.
- Improve water quality: use a cleaner water source or a label-approved compatibility agent.
- Check agitation and pressure: ensure adequate bypass agitation and confirm nozzle pressure is within the label and nozzle manufacturer's range.
- Record the jar-test result and the decision.