Throw herbicides and soils together and they'll develop chemistry between them. If the two hit it off, a crop won't compete with weeds. Should they fail to get along, the unwanted vegetation may take over or worse.

Before playing matchmaker, farmers should know the type of soil they're planting in and how a herbicide reacts to the soil's properties, said Glenn Nice, Purdue University Cooperative Extension Service weed specialist.

Many factors determine whether herbicides and soils are meant for each other. Soil types and pH levels, a herbicide's molecular makeup, herbicide release mechanisms, environmental conditions and the living organisms within the dirt all must be considered, Nice said.

"You're dealing with an extremely variable environment," he said. "Soils are very variable, and they can be different from one end of the field to the next.

"Added, and compounded, to that is you're dealing with several different types of molecules in these herbicides. And then to make it even more complicated, you're faced with a multitude of environmental conditions. There are so many things that come into play."

A good example is atrazine, a common corn herbicide. Atrazine should not beused on coarse soils because the herbicide retains its potency in moisture and is prone to move with water, a phenomenon known as leaching. Leaching can cause high levels of herbicide to collect in one place, damaging crops or contaminating groundwater.

"You may see herbicides reacting differently in different soils," Nice said. "In most cases, the need for concern will revolve around heavy muck or sand soils. Something in between (your loams and medium loams) generally don't have as many product restrictions because these herbicides are developed for general-type soils."

Soil pH (the measure of its acidity and alkalinity) has a direct impact on herbicide effectiveness. The pH scale goes from 0 to 14, with 7 being neutral. The optimum pH values for crop production lie between 5 and 7.

"The pH affects all the processes that affect herbicide activity in the soil," Nice said.

"Take, for instance, the ALS (acetolactate synthase) class of herbicides. If the soil pH is above 7.4 it decreases a process called hydrolysis. Hydrolysis breaks the herbicide down. If you decrease hydrolysis you increase the ability of that herbicide to persist, thus possibly leading to carryover and crop injury the following year."

Just as soil conditions can create harmful herbicide reactions, so, too, the interaction may render the chemical application ineffective. Through processes called adsorption and absorption, the herbicide can be prevented from reaching the weeds it is intended to eliminate.

"Adsorption and absorption can sometimes be confusing. However, they're two totally different processes," Nice said. "Adsorption is the process in which the herbicide molecule binds to the surface of the soil particles. Absorption is the taking in of the herbicide molecules by living organisms – a plant, for instance, absorbing it into the roots."

Both adsorption and absorption release the herbicide. The herbicide releases (desorption) may provide residual benefits but also could cause the product to carry over into the next crop season.

Farmers who have questions about whether a herbicide is right for a field should check product labels before applying.

"Most product labels will guide farmers on which herbicides to use on which soils," Nice said.

For more information on soil and herbicide interaction, including pH influences on specific products, read "Soil, So What?" by Nice and fellow Purdue weed specialist Thomas Bauman. The paper appeared in a recent issue of Purdue's Pest & Crop Newsletter. The newsletter is available online at

http://www.entm.purdue.edu/entomology/ext/targets/p&c/P&C2002/P&C4_2002.pdf