How soil-applied herbicides work

Soil-applied herbicides play an important role in controlling weeds before or slightly after they emerge from the seed bank. Timely use of soil-applied herbicides (often applied PRE) can accomplish a number of goals in a weed control program. The use of soil-applied (or, as I like to call them, "soil-active") herbicides is increasing as weed seeds extend their germination period and populations of weeds resistant to some POST herbicides expand. Early weed control sets up a POST application to be more successful, and adding a soil-active herbicide with a POST application can extend weed control until harvest.

In the early years of the Roundup Ready system, some abandonment of PRE herbicide use likely accelerated the development of herbicide-resistant weed populations.

📸: BASF Contributing Writer/Featuring the many advantages of using a PRE herbicide with soil activity.

How do soil-applied herbicides work?

Herbicides in the soil solution move to the weed seeds or root surface as water is imbibed into the expanding seedling. This process can take place before the weed emerges from the soil. Some herbicides travel through the young plant, while others never make it past the root system. Transport to the seedling shoot occurs through the xylem, the main conduit for water in the plant. Some soil-active herbicides move into the plant and ultimately are mobile in the xylem and the phloem, the pathways for photosynthate, and other goodies that move both up and down through the plant. There are four main categories for soil-applied herbicide behavior in plants.

📸: @Olko1975 via Canva/PRE herbicides can be effectively applied before planting, as part of a burndown treatment, or after planting prior to weed emergence.

There are several different categories of soil herbicide activity that have distinct modes of action.

1) Herbicides get into roots but do not move in the plant

These herbicides stick to soil organic matter and also to membranes in the root. They do not move much once in the soil. Weed seeds germinating in proximity to the herbicide move herbicide into the root but no further. Shoot absorption is minimal, so crop seeds planted below the zone of the herbicide grow through to the soil surface successfully. The Dinitroanaline family is the best example of this type of herbicide. Pendimethalin (Prowl^® Herbicide and Prowl^® H2O Herbicide) is the main member of this group. After entering the root, these herbicides suppress formation of cell structures, leading to seedling death.

2) Herbicides that hurt roots and shoots and control weeds before they emerge

Herbicides are absorbed by roots and also shoots and can move around some within the seedling. Weeds are typically killed before they emerge. The chloroacetamides, dimethenamid-P (Outlook^® Herbicide and a component of Armezon^® Pro Herbicide) flufenacet, and pyroxasulfone (Zidua^® Herbicide) are the key members of this group. The major site of action is to inhibit long-chain fatty acid formation, thus disrupting formation of cell walls and cuticles, among other cell structures. Weeds are typically killed below the soil surface and never emerge.

3) Herbicides that need sunlight to kill weeds

These herbicides may be accumulated and translocated in emerging weed seedlings but will not kill weeds until they reach sunlight and start photosynthesis. They flow into weed seedlings with soil water and travel upward through the plant in the xylem. Soil-active herbicides in this group may be pigment inhibitors (HPPD inhibitors, isoxaflutole, and toprmezone), cell membrane disrupters (PPO inhibitors, saflufenacil, and sulfentrazone), or photosynthesis inhibitors (triazines). A wide variety of BASF herbicide offerings include saflufenacil, including Verdict^® Herbicide, OpTill^® Pro Herbicide, Sharpen^® Herbicide, Surtain™ Herbicide, and Zidua^® Pro Herbicide. Pigment inhibitors lead to plant “bleaching” once the weed emerges. Cell membrane disruptors result in brown veins and tissue death. The triazines are also mobile in the xylem and cause tissue death, beginning with leaf margins of the oldest weed leaves.

4) Herbicide vapor kills weeds

These herbicides are not very soluble but have a high vapor pressure, meaning that they move as a gas in the soil. The herbicide vapor accumulates in soil pores and contacts the shoots of weed seedlings as they move to the soil surface. Most herbicides in this category require physical incorporation and have been declining in use. Thiocarbamates (EPTC and Buylate) are the main herbicides in this group. These herbicides are mobile in the xylem and inhibit lipid synthesis, which stops cell division in shoot and root tips.

📸: @Olko1975 via Canva/Featuring a sprayer applying herbicide after planting prior to weed emergence.

Other herbicides with soil activity

A host of herbicides that are used primarily as postemergence or burndown herbicides have some soil activity. The ALS and AHAS inhibitors are taken up with the soil solution and move into both the xylem and phloem. Some of the carotenoid inhibitors that “bleach” weeds can be used either PRE or POST (mesotrione). Burndown applications that include saflufenacil (Sharpen^® Herbicide) also provide some residual control of small-seeded broadleaves.

Coming next: Achieving "perfect" herbicide activation.

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