THE DOS AND DON'TS OF Tank-mixing herbicides and fertilizers

Applicators commonly mix nutrients with herbicides to increase efficiency when spraying turf areas. Unfortunately, the wrong combination can result in clogged nozzles, “scum” and precipitates in the tank, or even damaged turf. These are all indications of incompatibility among the materials being mixed. However, you can avoid this scenario if you follow a few basic guidelines.

Tank-mix incompatibility

Incompatibility occurs when one or more components of the mix chemically react, reducing the effectiveness of one or all of the pesticide components. In addition, you may see poor weed control, clogged spray nozzles and precipitates, surface scum, oily drops, gel, excessive foam or clumps in the tank. It is possible, however, that the only effect of the incompatibility will be lack of efficacy. Some materials are so reactive that even if they only come in contact with each other on the leaf surface, they still interact.

Incompatibility can also cause phytotoxicity to turf and ornamentals, which of course is even worse than mere lack of effectiveness.

Let's look at ways to prevent the “incompatibility factor.”


The first consideration is the pH of your water source. Well water and city water often contain high amounts of calcium, iron and magnesium. This is commonly known as “hard water.” It is not uncommon to see the pH range as high as from 9.6 to 10.4 in these water sources. This can reduce the effectiveness of emulsifiable concentrate (EC) herbicides, but will have minimal or no effect on aqueous herbicides. By contrast, low pH or acidic tank mixes will adversely affect aqueous herbicides.

Besides the water source, fertilizers also can affect the pH of your tank mix. Phosphate fertilizers tend to be acidic, while most commonly used nitrogen fertilizers, such as urea, are alkaline. When they dissolve, the solution can develop a pH of 8.0 to 8.5 and may become even more alkaline over time. Avoid letting tank mixes sit overnight because the pH can change and reduce the effectiveness of the spray mixture.

It is not difficult to check the pH of your tank mix. You can easily do this with pocket pH meters, pH test strips or water test kits that measure pH. A pH of 7.0 is neutral. Generally, anything close to this is acceptable. If pH is a major issue in product performance, it will be noted on the product label. There are several commercial products available that can be used to raise or lower the pH as necessary.

The jar test

The jar test is the most practical method to visually check for incompatibility of a specific herbicide and fertilizer mix. For this test, fill a quart jar with the correct ratio of the materials in the mix. To determine the correct ratio, you must first identify the spray volume per a set ground area (e.g. 1,000 square feet) and the correct rates of the fertilizer and pesticides for that area. Once you determine this information, proportion each material down to the volume of the jar, generally 1 quart.

For example, if the spray volume is 1 gallon per 1,000 square feet and you use a pint of test solution, you would divide the product rates for 1,000 square feet by eight, as there are 8 pints in a gallon. (See “Jar test instructions,” above.)

Common sense suggestions


Chemical and fertilizer companies spend millions of dollars researching and developing their products. Detailed, specific information, including mixing instructions, is included on the label. The label in the photo on page 58 includes the following information:

  1. The herbicide is an emulsifiable concentrate (EC);

  2. Mixing instructions with water and agitation requirement;

  3. A Notice that the product may separate if allowed to sit for a prolonged period;

  4. A Notice concerning the use of tank additives that alter the pH; and

  5. Mixing instructions for liquid fertilizers and recommendations for a compatibility jar test.

Notice that the label contains information regarding additives. Sometimes additives are necessary to correct compatibility problems when mixing herbicides with other products. Additives such as surfactants, which reduce surface tension, are already found in many herbicides. For example, if a herbicide is an emulsifiable concentrate (EC), it contains emulsifiers that keep it suspended in water. Wettable powders (WP) and flowables (F) often contain surfactants that help prevent herbicide particles clumping together. However, indiscriminate use of additives can negatively affect the herbicide's performance, reducing weed control or injuring turf. Do not underestimate the importance of following the label instructions.

Also included on the label is a telephone number for technical support and safety concerns. As with any pesticide or fertilizer, it is extremely important that the applicator read the entire label and follow the instructions when mixing and spraying the product.

Spray tank considerations

Start with a clean tank. If you clean the tank properly after each spraying, this should not be a problem. However, if the tank is not clean, even the jar test won't help. Vigorously agitate the tank contents while mixing and applying the products. Spray tanks should be equipped with agitators that keep the contents in suspension. I don't usually recommend the “paddle-type” or bypass-line agitators because these can let suspensions settle out. “Jet-type” or “venturi” agitators mounted in the bottom of the tank are better. If you take a break, be sure to leave the agitator running.

Making the application

Even with proper agitation, never start the spray on the desired target. On golf courses, for instance, turn your sprayer on in the rough before spraying the greens. On home lawns, if using a hand-held spray gun, turn it on in the tank before spraying the lawn.

If you have not previously used a product or product mix, apply it to a small test area prior to broadcasting. Overlap your application in some places to determine the margin of safety. Signs of spray ineffectiveness or phytotoxicity should appear after a few days on the test area.

Some “dos and don'ts”

  1. Always follow the correct filling order for products, starting with 50 percent of the water in the tank prior to adding products.

  2. Avoid mixing concentrated products together before mixing in the tank.

  3. Don't mix non-chelated iron sulfate products with amine formulations of certain phenoxy herbicides. This could cause a cottage cheese like precipitate to form. Remember READ THE LABEL, which should make it clear if this is a potential problem.

  4. Don't mix strongly acid materials with strongly alkaline materials. Again, the label should mention it if this is a potential problem.

  5. You should pre-dissolve dry flowable herbicides that are packaged in water-soluble film packets in water prior to adding to fluid fertilizers or nitrogen — these materials can prevent the packet from completely dissolving.

  6. Don't mix potassium fertilizers or other fertilizers that have a high salt load (ionic strength) with emulsifiable concentrates and liquid flowables. This can cause compatibility problems and phytotoxicity.

  7. Ensure that when you apply herbicide/fluid-fertilizer mixtures, you're applying the correct herbicide rate. Sprayer output may not be the same using large quantities of fluid fertilizer as when using water as the carrier. You should recalibrate sprayers for the fertilizer carrier.

Avoiding tank mix problems is primarily a matter of using common sense and reading the label. These two things will help ensure effective applications and save you a lot of aggravation.

David W. Fearis, CGCS, is golf course products marketing consultant for PBI/Gordon Corp. (Kansas City, Mo.).


  1. Pour 1 pint of water into a 1-quart jar.

  2. Add the correct proportion of fertilizer and wetable powders to the water and ensure that the materials are uniformly mixed.

  3. Add aqueous components individually, again ensuring each is mixed uniformly.

  4. Add EC compounds last.

  5. Invert (don't shake) the jar 10 times to mix, and let stand for at least one hour. Inspect for unusual signs such as participates, clumping or layering in the mixture.

It is important to remember that a jar test will only show physical incompatibilities and not phytotoxic incompatibilities. To check for the latter, test by spraying an inconspicuous spot.

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