Ok .. Preen is mostly the herbicide Trifluralin. Below is an excerpt of an article, Practical Applications of Herbicide Physiology. Here is the link to it:
http://passel.unl.edu/pages/in...
Trifluralin is a seedling growth inhibiting herbicide that is absorbed efficiently by the roots and shoots of germinating seedlings but is very immobile in the plant and soil. Trifluralin inhibits cell division in root and shoot meristems. Trifluralin is registered for control of unemerged weeds in corn when applied after the corn has developed two leaves. By the two leaf stage the shoots of corn and weeds have a well developed outer layer that prevents aborption of any trifluralin that may contact the shoot. Since trifluralin is very immobile in the soil it does not move deeply enough into the soil to come in contact with the immature (meristematic) portions of the root system of established corn or weeds. Emerged corn and weeds are not harmed from this type of trifluralin application because the tissue effective in absorbing trifluralin is not exposed to the herbicide. In contrast the shoots of germinating weed seedlings absorb trifluralin from the surface soil and are controlled.
What Kent said is correct. In reading further I'm seeing conflicting information about how long it persists in the soil. I need to look into that further.
