Many weeds have small seeds that germinate primarily within the top two inches of the soil.
For soil-applied herbicides to be effective, they must be positioned from the soil surface to two inches deep. Herbicide placement within the germinating weed zone can be accomplished by mechanical incorporation or rainfall. Soil-applied herbicides are absorbed when roots or shoots of germinating weed seedlings come in contact with the herbicide in the soil solution. Plant roots continue to take up herbicide as long as root hairs remain in contact with the treated soil. As the roots penetrate deeper in the soil and move farther away from the treated soil, the herbicide absorption declines.