Soil Active Herbicides
Some soil active herbicides are absorbed by plant roots but not shoots. Other soil active herbicides are absorbed by plant shoots and less by roots. Placement of the herbicide in the soil with respect to the site of uptake will influence herbicide effect and is an important management consideration (See animation).
Soil active herbicides come in contact with the plant through one of three processes, mass flow, contact, or diffusion. With mass flow, herbicide molecules are carried along with soil moisture as the plant absorbs water. An analogy would be a piece of wood floating with the current in a stream. Contact simply implies that plants come in contact with herbicides in the soil by the roots or shoots 'growing into' the herbicide. The diffusion process is one in which molecules move from an area of high concentration to an area of lower concentration. Diffusion can take place in both a liquid and a gaseous medium although with the exception of fumigants like methyl bromide most soil applied herbicides move very little in the gaseous phase (See animation).
For any of these three mechanisms to function adequate soil moisture is required. With mass flow water is the ’vehicle’ carrying the herbicide molecule. Moisture is required for the plant to be actively growing and absorbing water. For the plant to 'grow into' the herbicide soil moisture is required to support plant growth. For diffusion to function as a mechanism of contact, soil moisture is required to serve as the phase to dissolve the herbicide. For these reasons activity of soil applied herbicides is enhanced by good soil moisture conditions and reduced by limited soil moisture.
The roots and shoots of plants are the primary absorptive tissues of soil active herbicides. Some herbicides are absorbed primarily by the roots while others are absorbed by the shoots. In either case, the effective absorptive tissue is the relatively young/immature tissue. In the case of the roots, it is the root hairs that are particularly effective in herbicide absorption. As either shoot or root tissue matures the outer layers become thickened and form a barrier to absorption of herbicides as well as other compounds. For absorption to be efficient the tissue must come in contact with the herbicide when it is still in a relatively immature stage. Mature portions of roots and shoots do not absorb herbicides readily.
Some classes of soil active herbicides including the triazines are absorbed by the roots of plants but not the shoots (Figure 2).
For root absorbed herbicides to be effective they must be in the root zone of plants. Applied to the soil surface root absorbed herbicides must be moved into the soil with rainfall or tillage to be effective. For practical purposes soil applied herbicides are rarely found in phytotoxic concentrations in soil deeper than approximately 3 inches. Plants with the bulk of their root systems deeper than 3 inches normally are not greatly affected by these herbicides.
Soil active herbicides that are absorbed primarily by the shoots (grass coleoptile, broadleaf hypocotyl or epicotyl) of germinating seedlings must be in the zone of soil above the seed when the shoots grow through this zone to be effective. If shoot absorbed herbicides are below the shoot depth they will have less effect (Figure 3).
Differential placement of a herbicide with respect to the absorptive tissue of plants is a technique used to achieve selectivity. The herbicide simazine is widely used to provide selective weed control in many species of established trees even though most trees are biochemically sensitive to simazine. Simazine is a root absorbed herbicide and is relatively immobile in the soil. Simazine placed on the soil surface in tree plantings is moved into the top few inches of soil with rainfall where it is absorbed by the shallow root system of germinating weeds resulting in their control.
The bulk of the root system of established trees is much deeper in the soil than one or two inches and hence the tree root system does not come in contact with the simazine and therefore there is no absorption of the herbicide and no damage. A similar selectivity mechanism allows Sencor (metribuzin ) to be used safely on dormant established alfalfa but not on dormant newly seeded alfalfa. The root system of established alfalfa is deeper than newly seeded alfalfa and therefore the established plants absorb very little Sencor.
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 absorption 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.
Environmental conditions influence the potential for crop injury from soil active herbicides. In general crop injury from soil active herbicides is greatest under cool moist conditions due to several factors. Under cool conditions plant metabolism and therefore herbicide degradation is slowed resulting in greater concentrations of herbicide persisting in the plant for a longer period of time. This results in increased injury potential. (Figure 4)
Under cool conditions all plant processes are slowed including vegetative growth. As a result under cool conditions plant roots and shoots do not grow rapidly through and out of a soil zone containing herbicide. This extended period of contact of the absorptive tissue with the herbicide results in increased herbicide uptake and therefore increased injury potential. Moist soil facilitates plant contact with herbicides through either mass flow or diffusion. Other things being equal greater herbicide uptake and injury potential occurs under high moisture conditions.
Soil active herbicides with the exception of fumigants have no effect on dormant seeds. Moisture absorption and therefore herbicide absorption by dormant seeds is minimal and therefore exposure of the living embryo of the seed to most soil applied herbicides is minimal. Soil moisture facilitates seed germination and herbicide absorption, both requirements for activity of soil active herbicides.