Herbicides can be applied in combination, using either water or liquid fertilizer as a carrier, to decrease trips over the field. Compatibility of these mixtures is critical, and should be tested prior to application even though product labels allow mixing. Most labels contain instructions for testing compatibility and tank-mixing herbicides with liquid fertilizer. Follow label directions closely when applying these combinations.
Testing for compatibility requires a glass jar and the herbicides and liquid fertilizer to be mixed. Place one pint of liquid fertilizer in the jar and add two teaspoons of the liquid herbicide. If the herbicide is a dry formulation, mix two teaspoons of herbicide with sufficient water to form a slurry and add the slurry to the fertilizer. Cover the jar, shake well, and observe the mixture for 30 seconds. Check the mixture again after 30 minutes. If the mixture does not separate, it is compatible; however, each batch of liquid fertilizer should be checked, as they vary in mixing properties. The pH and mineral content of water will also influence compatibility.
If more than one herbicide is to be mixed with water or liquid fertilizer, the herbicides should be premixed in liquid fertilizer or water and tested for compatibility by mixing appropriate proportions of all components. The combination should be thoroughly agitated before each additional herbicide is added, and a specific mixing order followed. Generally, unless label directions state otherwise, add the herbicide to water or fertilizer in the following order:
Spray tanks should be at least half filled with the carrier before the herbicides are added. Compatibility agents are available which may be added to improve compatibility. If the mixture foams excessively, separates, or becomes syrupy, do not apply. Even if all components appear compatible, the tank mixture will require constant agitation to prevent separation or poor distribution in the tank. Be sure the entire tank is agitated and mixed before spraying. Do not store tank mixtures of herbicides for long periods or overnight without constant agitation. Best results will be obtained by applying tank mixtures promptly.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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