Foliar applied herbicides almost always have more uniform activity on plants than do soil applied herbicides. This is largely because the conditions that affect their activity are more uniform above the soil than they are below ground. It is not uncommon to see inconsistent activity with soil applied herbicides. It is also difficult to predict where and for how long the effects of these herbicides will occur. Many factors affect soil applied herbicides.

In this desert region where crops are intensively irrigated and the soils are low in organic matter, the most important variables affecting soil applied herbicides are adsorption and volatility. Adsorption is the process whereby herbicides are bound to soil particles. All herbicides have electrical charges that cause them to bind to the positive or negative charges in soil or organic matter. This is similar to how metal is bound to a magnet. It is reported as the absorptivity coefficient (K) and is generally more important than water solubility in determining how herbicides move in the soil. Many factors can affect adsorption. These include soil texture, pH, organic matter and soil moisture. Although the soils here are generally low in organic matter, crop residues from crops grown previously in rotation can temporarily tie up many herbicides. Soil moisture can also have a significant effect. Herbicides compete with water for binding sites on the soil and bind stronger to dry rather that wet soils.

Absorptivity and water solubility are usually inversely related. As solubility increases, binding to the soil decreases. There are some exceptions to this. Notable among these are Gramoxone (Paraquat) and Glyphosate. Both of these herbicides are highly water soluble but they bind tightly to the soil.

Herbicide volatility occurs when a herbicide changes from a liquid or solid form to a vapor. This is affected primarily by vapor pressure and varies considerably for various herbicides. The higher the vapor pressure, the higher the volatility. Some of the most volatile herbicides used here are Eptam, Banvel, MCPA and 2,4-D. Some of the least volatile are Glyphosate, Gramoxone, Karmex and Pursuit.

To contact Barry Tickes go to: btickes@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.