Soil type and herbicide characteristics affect pesticide movement, effectiveness, and the risk of causing problems, so the label for some preemergence herbicides specifies different application rates according to soil type. In comparison with clay soils, less herbicide sticks to sand particles, so more of the herbicide can contact weed seedling roots and be effective at lower rates. Sandy soils also pose a higher risk that excess chemical will leach downward, so a lower rate reduces the risk of polluting groundwater. Herbicide dissolved in water or bound to soil silt can move in runoff water and damage desirable plants along the drainage path or pollute waterways.