Sithin Mathew, Weed science graduate student, Debalin Sarangi, Extension weed scientist, and Tom Peters, Extension sugarbeet agronomist and weed scientist
Using pre-emergence herbicides is a great way to stay ahead of weeds in corn and soybean production, but there is more to it than simply picking the right herbicide or spraying at correct time. How well these herbicides perform is dependent on your soil and how you farm.
Then there’s the imidazolinone family (like imazamox, imazethapyr, and imazapic). These herbicides are more available and active in low pH soils, but they also break down faster in these soils—so your weed control window can be shorter if timing is not right. The dinitroanilines (pendimethalin, trifluralin, etc.) and the active ingredient clomazone are somewhat influenced by pH, but degradation by light and/or volatility is more important than pH. That is why knowing your soil pH helps you make smarter herbicide decisions. If you have not checked your soil pH lately, it might be worth a look before your planting season begins.
Table 2. Herbicide families that depend on soil pH.
Soybeans are a bit more sensitive. You’ve got to be careful with products like metribuzin or flumioxazin—they work great, but only if applied before the soybeans poke out of the ground. If it’s too late or the weather is cold and wet, there’s a higher chance of crop injury. After you spray your PRE herbicide, rain or irrigation (0.5–1 inch) is needed within approximately ten days so the herbicide is "activated"—meaning it washes into the soil where the weed seeds are. Herbicides will break down or blow away if they just sit on top of the soil and that means poor weed control.
As the soybeans are emerging, heavy rains can splash herbicide, especially herbicide that has not been activated, onto young plants. That’s when you might see signs like crinkled leaves, brown or dead spots on cotyledons or hypocotyls and burned spots on leaves (Figure 1).
Factors including soil type, moisture conditions, and soil organic matter content can affect herbicide performance. For example, sandy soils often require a lower herbicide rate than heavy clay or soils with lots of organic matter. Moisture is also key—most pre-emerge herbicides need about half an inch to an inch of rain (or irrigation) within 10 days after spraying to activate and work properly. Soil temperature matters too. Keeping an eye on it can help you determine the best application timing. Don’t forget to follow the label instructions—some of these herbicides can injure your crop if they are applied too close to planting or after emergence or at a wrong rate. In short, knowing your soil and monitoring weather conditions can help you get the most from your herbicide while protecting your yield.
The more organic matter, the higher the CEC—and that’s not always a good thing when it comes to pre-emergence herbicides. High CEC soils (like heavy clay or high-OM fields) can bind herbicides like atrazine, S-metolachlor (Dual Magnum, Dual Magnum II), and pyroxasulfone (Zidua) and reduce their soil mobility. That means there’s less herbicide left in the soil solution to actually kill weeds. Herbicides like pendimethalin (Prowl), flufenacet and flumioxazin (Valor) also tend to bind tightly in high-OM soils (low or medium soil mobility), so labels often recommend using higher rates in those cases. You can check the soil mobility of different herbicides in Table 1. It is recommended to always read the label carefully and match your herbicide rate to your soil type before spraying.
The opposite can happen if your soil has low organic matter or if it is a sandy soil. Herbicides can move too freely, leaching deeper into the soil and missing their target. That not only reduces weed control but can also increase the risk of groundwater contamination. In these lighter soils, using a lower rate or splitting up applications might help reduce crop injury and protect the environment. Also, keep in mind that how you manage your land matters too. Using practices like cover crops or reduced tillage can leave more residue on the surface. This can stop herbicides from moving down into the soil right away, but it also increases microbial activity that can break down herbicides faster.
Table 1. Commonly used PRE herbicides in corn-soybean rotations with their soil mobility.
Matching herbicide rates to your soil: Why doesn't one size fit all?
Your soil plays a big role in how well pre-emergence herbicides work. One key factor is cation exchange capacity (CEC), which basically explains how well your soil can bind nutrients and chemicals like herbicides. CEC is mostly influenced by how much clay and organic matter (OM) are in your soil.The more organic matter, the higher the CEC—and that’s not always a good thing when it comes to pre-emergence herbicides. High CEC soils (like heavy clay or high-OM fields) can bind herbicides like atrazine, S-metolachlor (Dual Magnum, Dual Magnum II), and pyroxasulfone (Zidua) and reduce their soil mobility. That means there’s less herbicide left in the soil solution to actually kill weeds. Herbicides like pendimethalin (Prowl), flufenacet and flumioxazin (Valor) also tend to bind tightly in high-OM soils (low or medium soil mobility), so labels often recommend using higher rates in those cases. You can check the soil mobility of different herbicides in Table 1. It is recommended to always read the label carefully and match your herbicide rate to your soil type before spraying.
The opposite can happen if your soil has low organic matter or if it is a sandy soil. Herbicides can move too freely, leaching deeper into the soil and missing their target. That not only reduces weed control but can also increase the risk of groundwater contamination. In these lighter soils, using a lower rate or splitting up applications might help reduce crop injury and protect the environment. Also, keep in mind that how you manage your land matters too. Using practices like cover crops or reduced tillage can leave more residue on the surface. This can stop herbicides from moving down into the soil right away, but it also increases microbial activity that can break down herbicides faster.
What does this mean to you?
Don’t use the same herbicide rate across all your fields. Coarse-textured sandy soils usually need lower rates than fine-or medium-textured loams or clay soils because they don’t hold herbicide as tightly. Using rates that are too high on sandy soils can hurt your crop. However, using too little on heavier soils might not provide adequate weed control. Check the herbicide label to match the rate with your soil type. You might even need to adjust your rate zone by zone if you have variable soils in a single field. Going with the maximum labeled rate (if that’s still safe for your soil type) can improve weed control, especially in tough weather conditions.Table 1. Commonly used PRE herbicides in corn-soybean rotations with their soil mobility.
| Trade name | Active ingredient (SOA) |
Soil mobility |
|---|---|---|
| Valor® | flumioxazin (14) | Low |
| Fierce® | flumioxazin (14) + pyroxasulfone (15) |
Low |
| Prowl®H2O | pendimenthalin (3) | Low |
| Dual II Magnum | S-metolachlor | Medium |
| Component of TriVolt™ | flufenacet (15) | Medium |
| Callisto® | mesotrione (27) | Very high to high |
| Outlook® | dimethanamid-P (15) | High |
| Metribuzin | metribuzin (5) | High |
| Authority® | sulfentrazone (14) | High |
| Armezon® | topramezone (27) | High |
| Balance® Flexx | isoxaflutole (27) | High |
| Harness® | acetochlor (15) | High |
| Warrant® | encapsulated acetochlor | High |
| Atrazine | atrazine (5) | High |
| Zidua® SC | pyroxasulfone (15) | High |
| Sharpen | saflufenacil | Very high |
Source: BAYER crop science, Conditions affecting PRE herbicide activation. Accessed, 04/20/2025, http://www.cropscience.bayer.us/articles/bayer/; Shaner DL, Weed Science Society of America, eds. (2014) Herbicide Handbook Tenth Edition
How does soil pH affect herbicide performance?
Soil pH might not be something you think about every day, but it plays a significant role in how herbicides respond in fields. Many PRE herbicides, including triazines (like atrazine in corn) and certain sulfonylureas (ALS inhibitors), exhibit pH-dependent behavior (Table 2). Let’s start with atrazine—a common pre-emerge herbicide in corn. If your soil is acidic (pH under 6), atrazine tends to bind to soil particles and is not available in the soil solution. That means it may control weeds less, but is unlikely to injure next year’s crop, like soybeans (carry over from previous year). On the other hand, in alkaline soils (pH above 7), atrazine is in the soil solution and is available for uptake by crop and weeds. This can help with weed control but also increases the risk of carryover injury to sensitive crops.Then there’s the imidazolinone family (like imazamox, imazethapyr, and imazapic). These herbicides are more available and active in low pH soils, but they also break down faster in these soils—so your weed control window can be shorter if timing is not right. The dinitroanilines (pendimethalin, trifluralin, etc.) and the active ingredient clomazone are somewhat influenced by pH, but degradation by light and/or volatility is more important than pH. That is why knowing your soil pH helps you make smarter herbicide decisions. If you have not checked your soil pH lately, it might be worth a look before your planting season begins.
| Herbicide family | Example active ingredients |
In acidic soils (pH <6) |
In alkaline soils (pH>7) |
|---|---|---|---|
| Triazines | atrazine, simazine |
Binds more tightly to soil: less weed control, but less carryover risk |
More soluble: better weed control, but higher carryover risk |
| Sulfonylureas | chlorimuron, thifensulfuron |
Breaks down faster: shorter control window, lower injury risk |
Breaks down slowly: longer control, but higher carryover potential |
| Imidazolinones | imazethapyr, imazapic, imazamox |
More available: good activity, but higher carryover potential |
Less available: weaker control |
| Dinitroanilines | pendimethalin, trifluralin |
Mildly affected by pH: more influenced by volatility and sunlight |
Same as acidic soils |
| Isoxazolidinones | clomazone | Mildly affected: volatility and sunlight are bigger factors |
Same as acidic soils |
Source: Manuchehri and Arnall, 2018, How does soil pH impact herbicides? Oklahoma Cooperative Extension Fact Sheets; https://extension.okstate.edu/fact-sheets/print-publications/pss/how-does-soil-ph-impact-herbicides-pss-2788.pdf
Getting the timing right: When and how rain matters for herbicides
Timing plays a big role in efficacy when it comes to pre-emergence herbicides. These products can be applied before planting, at planting, or right after planting—as long as it’s before the crop emerges. PRE means preemergence to weeds but following planting. Preplant or early preplant refers to soil residual herbicides planted before planting—the difference is the duration. The closer to planting you apply, the better the chance for longer-lasting weed control. For corn, most pre-emerge herbicides go down before or at planting. That’s when the weed seeds are just getting started and the corn roots are still shallow—so there's less chance of hurting the crop. The herbicide remains in the zone from surface to one or two inches and takes care of those early weeds before they emerge.Too little rain?
Products that are less water soluble (eg. acetochlor or sulfentrazone) might not work well—especially in soybeans.Too much rain?
The herbicide might move below the weed zone or into tile drains, which doesn’t just reduce control, it can also increase the risk of polluting water sources. In addition, leaching dilutes the herbicide so the concentration is less in the activity zone.A big rainstorm?
 |
| Figure 1. Herbicide injury (PPO) associated with heavy rain at emergence. Photo: Extension and Outreach, Iowa State University |
Bottom line
Watch your weather, time your spray right, and check those labels—especially for sensitive crops like soybeans.
When the soil is warm (around 60 to 75°F), herbicides tend to activate faster and move into the top layer where weed seeds are starting to grow, which helps with early weed control. On the other hand, if the soil is cold (below 50°F), the herbicide may not activate properly and weeds might not emerge right away, leading to poor control.
How soil conditions affect herbicide performance
Soil moisture
Soil moisture plays a critical role in how well herbicides work. When the soil has enough moisture, more herbicide moves into the soil solution and is available for weeds to take up. In dry soil, however, herbicide are bound to soil, so it might not kill weeds as well. Weed control is often less consistent if it remains dry right after planting. However, once it rains herbicides become active and start working.
Soil temperature
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| Figure 2. Soybean treated with Fierce EZ (within recommended rate) showing both flumioxazin and pyroxasulfone injury due to wet and cool soil conditions. Photo Debalin Sarangi |
Warm, moist soil also speeds up the herbicide breakdown, which can shorten the weed control and allow more weeds to come up later in the season. In contrast, cooler soil slows this breakdown, so the herbicide may last longer—but this can also increase the chance of crop damage if it sticks around too long and affects the young crop. This is especially true for herbicides like flumioxazin, sulfentrazone, and saflufenacil (PPO herbicides) which can injure soybeans if applied when it’s too cold. When the plants can't metabolize the herbicide well, it can lead to damage like stunted growth, crinkled leaves, or dead spots (Figure 2).
Soil microbes
Soil microbes (mainly bacteria and fungi) are key to breaking down pre-emergence (PRE) herbicides. When your soil is warm, moist, and has plenty of organic matter, these microbes are more active and break down herbicides like atrazine or metribuzin faster, which can shorten the weed control period. On the other hand, if your soil is cool or dry, microbial activity slows down, so herbicides stay longer—sometimes helping with weed control, but also raising the risk of crop injury if the herbicide lasts into sensitive crop stages. High organic matter soils increase microbial activity and can hold more herbicide, affecting both breakdown and weed control. Practices like tillage, crop rotation, and cover crops also change the soil’s microbial combinations, which can impact how well your herbicides work.Picking the right herbicide combination
For the best results, use pre-emergence herbicides from different groups (sites of action). Fields might require different combinations of herbicides based on the weeds present. Using two herbicides also reduces the selection pressure exerted by a single site of action, slowing the evolution of herbicide resistance. Another important factor is the different chemical properties of herbicides. Some herbicides, like metribuzin, Callisto (mesotrione), and Outlook (dimethenamid-P), are more water soluble and move into the soil with less water, while others, like atrazine, Prowl (pendimethalin), and Zidua (pyroxasulfone), are less water soluble and require more water for activation. Using herbicides with different chemical characteristics can help manage varying environmental conditions, such as rainfall.
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