Weighing Pesticides - Weigh it Before You Spray it

The old rule of thumb is that 1 lb/100 gal = 1 tablespoon/gallon.

This rule of thumb is inaccurate, costly, and hazardous to crop health. The physical properties of wettable powders from various sources can differ considerably. For example:

• 1 ounce of Brand X Captan 80WP = 2.4 tablespoons
• 1 ounce of Brand Y Captan 80WP = 5.2 tablespoons

Using weight, 1 lb/100 gal gives a concentration equal to 0.16 oz per gallon.

• 1 tablespoon of Brand X Captan 80WP would make a 0.42 oz per gallon mix
• 1 tablespoon of Brand Y Captan 80WP would make a 0.19 oz per gallon mix

In other words, 1 tablespoon per gallon of Brand X Captan 80WP would be equal to more than 2 lb/100 gal, and 1 tablespoon Brand Y Captan 80WP would be slightly more than 1 lb/100 gal. Using volume to estimate the weight of Captan 80WP could result in crop damage.

The lesson is clear: weigh it before you spray it. Considering the cost of pesticides, accuracy can save money. Considering the damage that can be done by pesticide overdoses, accuracy can save crops.

Purchase a weighing scale. There are many digital scale options that work well and are affordable.

References

Powell, C. C. 1981. Small batch WP fungicide guide and sample problems. Plant Pathology Notes, Ohio State University Cooperative Extension Service. Department of Plant Pathology Newsletter. July 27, 1981.

Simone, G. W. Extension Plant Pathology Report No. 31. Department of Plant Pathology, University of Florida.