Tank Mix Precipitate in a 300-Gallon Sprayer
A 300-gallon pull-behind sprayer is being loaded for a 40-acre corn field. The water source tests pH 8.2 and hardness 280 ppm. The applicator adds a wettable-powder herbicide first, then a liquid 28-0-0 nitrogen solution, then a nonionic surfactant, and finally a copper fungicide. Within minutes, a curdy white precipitate forms, and the 50-mesh suction screen plugs. The herbicide label states: "Do not tank-mix with alkaline materials or with products containing free ammonia." The fungicide label prohibits mixing with strong oxidizing agents but is not otherwise restrictive. The applicator has already sprayed about 2 acres before noticing the clog and wants to dilute the remaining tank with more water and finish the field before rain arrives in six hours. The field is in tassel, and the forecast is for 0.5 inch of rain.
1. What is the most likely cause of the precipitate, and should the applicator spray the remaining tank?
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Most likely cause: a prohibited tank mix, not merely a mixing-order nuisance. The herbicide label bars alkaline materials and products containing free ammonia. The water is alkaline at pH 8.2, and the 28-0-0 nitrogen solution is a fertilizer source that the label specifically warns against. The copper fungicide adds another reactive component, and high hardness can worsen incompatibility. The curdy precipitate and plugged 50-mesh screen are visible evidence that the products are not staying in suspension.
Do not spray the remaining tank. Diluting with more water does not cure a label-prohibited mix, and it may not redissolve the precipitate. Spraying would risk:
- illegal application inconsistent with the label;
- uneven coverage and clogged nozzles;
- crop injury from a concentrated or altered mixture;
- equipment damage.
The applicator should stop, isolate the tank, and treat this as a compatibility and label problem-not as a race with the rain.
2. What should be done with the 2 acres already sprayed and the tank contents?
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For the 2 acres: stop further application and document what happened-time, area, products, calculated rates, water source, weather, and when the precipitate appeared. Notify the supervisor or owner. Assess the sprayed area for crop injury and whether the label permits any follow-up treatment. Do not automatically respray; the first pass may have received an illegal or uneven mixture, and a second pass could compound the problem.
For the tank: do not dump the mixture on the ground, into a ditch, or into a storm drain. Follow the product labels and Massachusetts disposal requirements for pesticide-containing waste. Because a precipitate is present, ordinary rinsing may not be enough; the tank may need professional cleaning or disposal. Triple-rinse only where the label and local rules allow, and contain rinsate for proper use or disposal. Clean the screens and nozzles separately, and keep the contaminated rinse material out of the environment.
3. How should the operation prevent this from recurring?
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Prevention starts before the tank is filled:
- Read every label’s mixing section. The herbicide label explicitly prohibited alkaline materials and free-ammonia products; that prohibition controls the entire mix.
- Run a jar test using the actual water and the same products, ratios, and order. Watch for precipitate, heat, or separation.
- Test water pH and hardness. Adjust pH only if the label permits it; do not assume an acidifier fixes a prohibited mix.
- Follow a written order of addition from the labels and product guidance. WP, liquids, surfactants, and fertilizers are not interchangeable in order.
- Use a compatibility agent only if labeled for the products involved.
- Train loaders and keep a mixing checklist at the fill station. The cost of a jar test is far lower than lost product, clogged equipment, and an illegal application.