Humectants: the drier it is, the more you need them
By John Garr and Jerry Green of GarrCo Products
Humectants are used widely, but often the unspoken component in spray mixtures. Humectants do the essential task of keeping water in the spray deposit and can even rehydrate spray deposits with water from rain or dew and from moisture in the air. Humectants keep the spray deposit in a liquid or gel that increases retention, pesticide uptake into the leaf and contact with insects on the surface. When a deposit is dry, it does not make good contact with the leaf surface and can readily be washed, blown, or shaken off. Humectancy makes the deposits stay moist, which is particularly helpful improving the uptake of water-soluble herbicides like glyphosate and glufosinate.
Officially, humectants are defined as materials that increase the water content and slow the drying time of spray deposits. Since water is the usual carrier for pesticide applications, humectants help ensure that some of that water is retained in the spray deposit and thus maintain a uniform mixture. Humectants are needed more under dry conditions than conditions with humidity. Humectants slow the evaporation of spray droplets in dry air and thus theoretically can help maintain droplet size and reduce drift.
Many humectants have other adjuvant properties than prolonging droplet drying. Some humectants help partially insoluble pesticide mix with water. Glucose and molasses have been used as humectants since 1894 and also act as stickers to increase retention on the leaf surface. Salts have humectant activity, which is part of the justification for the use of fertilizers as adjuvants. Fertilizer salts such as ammonium sulfate (AMS) and urea can strongly attract water and even absorb moisture from the air. AMS acts both as a water conditioner and a humectant to increase pesticide performance. Other common humectants include sugars such as fructose, glycerol and glycol derivatives. Many surfactants also act as humectants. So do some of the polymers used to mitigate drift.
Oils are liquids but are not humectants. Oils never actually become part of the water and under a microscope look like little marbles floating around in the water. The oil helps the spray deposit remain liquid, but will only help the uptake of pesticides in the oil portion. Fifty years ago, researchers found some benefits when oil was the spray carrier at extremely low spray volumes, but the disadvantages outweigh the advantages, so oil sprays were never widely adopted. Safety issues and the potential to increase the production of driftable fine spray droplets were serious concerns. Even spray mixtures containing just one percent or less crop oil concentrate (COC or MSO) commonly applied today can significantly increase the number of driftable spray droplets.
Humectants are essential under hot and dry conditions when spray droplets evaporate the fastest. High relative humidity will extend the drying time of spray droplets but is not a good parameter to rely on. You will probably know the relative humidity in the air at your application site, but the relative humidity in the crop canopy and on the leaf surface will be very different. Humectants will help protect against uncontrollable variations that influence drying time. Having too much humectant in a formulation is not always good. Companies can use inexpensive humectants as the major formulation component instead of surfactant to reduce costs, but that will also reduce the performance of the adjuvant.
When used appropriately and not as a formulation filler, humectants are an important class of adjuvants. In some ways, they are the Rodney Dangerfield of adjuvants and need to get more respect. Humectants should not just be the unspoken component in a spray mixture. The practical solution is to use drift reduction polymers and certain types of surfactants that provide humectancy while also mitigating drift and improving uptake.