Herbicides kill harmful plants in a garden or on a farm. The herbicides should not remain in the soil so long that the herbicides harm future plants. Therefore, testers provide examinations to check on whether the herbicides leave the soil in a reasonable time. Herbicide testing also ensures that no chemicals in the soil interfere with the herbicide chemicals, diminishing their effectiveness. Testing is used to determine whether herbicides pose any risk for harm to people as well.
When performing the herbicide tests, the testers must take into consideration the chemical composition of the soil, the physical structure of the soil and the soil's microbial content. These factors can skew the results of the herbicide testing. Clay soils tend to hold herbicides longer than sandy soils because the soils hold water more, while water tends to leak through sandy soil more and carry the herbicide chemicals along with it. Soils with lower pH tend to break down the herbicides more than soils with high pH, so soils with higher pH have herbicides that remain in the soil for longer periods.
Testers must determine the water solubility of the herbicides to determine if the herbicides will easily dissolve into water, thus allowing water to leach the herbicides away from the soil area. Herbicides that tend to cling to the soil will remain in the soil longer than herbicides that remain free, unless the soil experiences significant soil erosion that blows or washes a significant portion away.
Herbicides have different levels of vapor pressure, which determines the conditions under which the herbicides turn from liquid to solid and gas. If the herbicide has high volatility, it might dissipate if subjected to hot weather.
As with many chemicals, herbicides break down as compounds when they come in contact with other herbicides. Not only should the herbicide initially be tested for potential chemical reactions, but the gardener should test the soil to ensure that the soil does not have chemicals that will interfere with the herbicides. Also, some of these herbicides and the chemicals they mix with can harm humans by producing fumes or by seeping into the water supply. This can have carcinogenic effects and also may cause headaches or other side effects.
Soil needs to be tested for carryover effects. When herbicides have carryover, they travel from one area to another, according to the University of Illinois. For example, certain herbicides leach into different areas during rain. These residual herbicides can have an adverse effect on plants grown in the carryover areas. When applying certain herbicides, such as aminopyralid, clopyralid, fluroxypyr, picloram and triclopyr, gardeners should test nearby areas to see if any herbicides carry over.
Testers determine how aquatic herbicides leave an area, so that the applicators know when the herbicides will lose their effectiveness and when the water body will need more herbicide. Some herbicides stick to the soil and remain at the bottom. Sunlight and microorganisms destroy other herbicides, according to the University of Florida.