No. 3/April 10, 1997
Principles of Soil-Applied Herbicides
Utilization of soil-applied herbicides in corn and soybean production systems continues to be a common method to achieve weed control. Early preplant, preplant incorporated, and preemergence surface are the most common categories of herbicide applications to soil. Regardless of when the herbicide is applied to the soil, the effectiveness of soil-applied herbicides is governed by several factors.
The primary factor that governs the efficacy of soil-applied herbicides is having the herbicide available for uptake by the germinating weed seedling. Soil-applied herbicides must be absorbed into the germinating weed seedlings to be effective. These herbicides do not prevent weed seed germination; rather, they are first absorbed by the seedling and then exert their phytotoxic action, generally before the seedlings emerge from the soil. For the herbicide to be absorbed by the germinating seedlings, the herbicide must be in the soil solution or vapor phase. How is this achieved? The most common method for herbicides to become dissolved into the soil solution is by mechanical incorporation or precipitation. Many early preplant applications in no-till systems attempt to increase the likelihood that sufficient precipitation will be received to incorporate the herbicide prior to planting. If, however, no precipitation is received between application and planting, mechanical incorporation, where feasible, will in most instances adequately move the herbicide into the soil solution.
Many weed species, particularly small-seeded species, germinate from fairly shallow depths in the soil. The top 1 to 2 inches of soil is the primary zone of weed seed germination and should thus be the target area for herbicide placement. Shallow incorporation can be achieved by mechanical methods or by precipitation. Which of these two methods is more consistent? Rainfall provides for a fairly uniform incorporation, but mechanical incorporation reduces the absolute dependence on receiving timely precipitation. How much precipitation is needed and how soon after application the precipitation should be received for optimal herbicide performance depends upon many factors, but generally 1/2 to 1 inch of precipitation within 7 to 10 days after application is sufficient.
Herbicides remaining on the soil surface, or those placed too deeply in the soil, may not be intercepted by the emerging weed seedlings. Herbicides on the soil surface are subjected to several processes that reduce their availability. Volatility (the change from a liquid to gaseous state) and photolysis (degradation due to absorption of sunlight) are two common processes that can reduce the availability of herbicides remaining on the soil surface.
Aaron Hager and Marshal McGlamery, Department of Crop Sciences, (217)333-4424