Absorption of Soil-Applied Herbicides
Soil-applied (preemergence) herbicides are used to control germinating weeds in a variety of settings (agronomic and horticultural crops, turf, industrial weed management). Herbicide dissolved in the soil water moves into seeds or seedlings as these structures absorb water from the soil – thus, absorption is a passive process. A large portion of herbicide present in the soil is bound to soil colloids (clay, organic matter), and this herbicide is less readily available to plants than the herbicide present in the soil solution. Conditions which favor movement of the herbicide into soil solution tend to increase absorption by plants. This article will discuss factors that influence the balance between free and bound herbicide and how these factors affect herbicide performance.
Adsorption of herbicides to soil colloids occurs due to the attraction between charges on soil colloid surfaces and herbicide molecule. In most situations, the charges are relatively weak and thus the process is reversible. An equilibrium is reached between the amount of herbicide bound to colloids and that found in solution. The ratio of bound to free herbicide is influenced by several factors, including chemical properties of the herbicide, soil characteristics and soil water content. Herbicides are more active under conditions that favor movement into the soil solution.
To be effective and safe, a preemergence herbicide must have properties that result in the majority of herbicide being bound to soil colloids with only a small amount remaining in solution. If the majority of herbicide remained in solution the herbicide would rapidly leach through the soil profile or leave the field with runoff. Rapid leaching of herbicides can cause two problems: 1) since the majority of weed seedlings germinate in the upper inch of soil, movement out of this zone results in ineffective weed control; and 2) rapid leaching could result in movement of the herbicide into groundwater. The majority of herbicide applied to the soil surface stays within the upper 2 to 3 inches of soil (Table 1). In this research, less than 100% of the alachlor and metribuzin was recovered due to herbicide degradation between the time of application and sampling. While most of the herbicide remains near the soil surface, under certain situations small quantities of some herbicides can leach through the profile and reach groundwater.
Table 1. Distribution of herbicide residues within the soil profile 20 days after
application. Six inches of rain occurred between application and sampling.
|Soil Depth (in)||Alachlor (Lasso)||Metribuzin (Sencor)|
|% of applied||% of applied|
Jones et al. 1990. Weed Sci.
Chemical characteristics affecting availability
Each herbicide has a unique set of chemical characteristics that influences its behavior in soil. Three properties that can help predict availability and mobility include: 1) water solubility; 2) adsorbtivity and 3) herbicide half-life. The first two properties determine how much of the herbicide will be bound versus free, whereas the half-life relates to the persistence of the herbicide.
Water solubility is a measurement of how much of a chemical will dissolve in water, and typically is expressed in parts per million. The greater the solubility, the more of the chemical that dissolves in water. Adsorptivity is a measure of a compound’s tendency to bind to soil particle surfaces. One common method of determining adsorptivity is to place equal parts of soil and water in a container, then add a small quantity of herbicide and thoroughly shake. The K value represents the ratio of herbicide bound to soil collids versus what is free in the water. Thus, the higher the K value the greater the adsorption to soil colloids. Figure 1 shows the relationship between bound and free herbicide for two theoretical compounds. While the two herbicides are present at the same concentration (24 molecules), three times more of the herbicide with a K of 7 is available to plants compared to the product with a K of 24. It is important to remember that this relationship is an equilibrium. As herbicide is lost from one phase (degradation,absorption, etc.), herbicide will move from the other phase to maintain the equilibrium.
With most herbicides, adsorptivity and solubility are inversely related. Thus, as solubility increases, binding to soil decreases. There are a few exceptions to this rule, including paraquat and glyphosate. Both of these products are highly water soluble yet they bind extremely tightly to soil colloids. With most herbicides, the adsorptivity coefficient (K) is more closely associated with its availability to plants than the water solubility, but it is important to consider both characteristics.
The half-life of a herbicide describes the length of time it takes for 50% of the herbicide to break down to secondary compounds. For example, if one pound of a product with a half life of 90 days is applied, we would expect 0.5 lb to remain 90 days after application. After another 90 days, 0.25 lb should be left in the field. The half-life of a herbicide varies with soil characteristics and environment. For example, the half-life of atrazine in Georgia on a soil with a pH of 6.8 was reported to be 39 days, whereas in Minnesota the half-life was 261 days on a soil with 7.9 pH . When comparing half-lives of different herbicides, it is important to insure the half-lives were determined under similar conditions.
Table 2 lists chemical properties of several common soil-applied herbicides. As one can see, a wide range of values occurs among these chemicals, yet all are effective preemergence herbicides. The one exception, paraquat, binds so tightly that it has no soil activity. The Koc value of paraquat is estimated to be 1,000,000, whereas the other products range from 2 to 24,300. Manufacturers of acetamide herbicides have attempted to differentiate their products based on soil availability (Amide Wars). While there are differences in the adsorptivity and solubility of these herbicides, these differences are relatively minor. For example, the Koc‘s range from 170 to 213 among the acetamide herbicides currently on the market. Trifluralin has a K value of 7000, more than 30 times greater than any acetamide herbicide, yet trifluralin is a highly effective herbicide. The two growth regulators are the most mobile herbicides listed. Picloram (Tordon) is classified as a Restricted Use Pesticide due to the potential for off-target movement, either leaching or runoff. While dicamba would be expected to be as mobile as picloram, its shorter persistence reduces the likelihood of problems associated with movement in the soil.
Table 2. Chemical properties of several soil-applied herbicides. 1
|Herbicide||Koc||Water Solubility (ppm)||Half-Life (days)|
1 The majority of values obtained from 7th edition of Herbicide Handbook, Weed Science Society of America.
Factors affecting performance of soil applied herbicides
Several soil factors influence the availability of preemergence herbicides. One important factor is the cation exchange capacity (CEC) of the soil. The CEC is a measure of the quantity of adsorptive sites present in a soil, and is based primarily on the clay and organic matter content. For most midwest soils, the organic matter content influences adsorptivity more than clay content. As CEC increases, more herbicide is bound to soil colloids and less is available in the soil solution. This is the reason why recommended rates for most soil-applied herbicides is based on soil type. By increasing herbicide rates on soils with a high CEC, the concentration of herbicide in solution can be maintained at toxic concentrations.
Soil pH can have a significant effect on the adsorption of many herbicides. pH is a measure of the availability of hydrogen ions (H + ) in a solution. As the pH decreases below 7 (acid conditions) the concentration of hydrogen ions found in the solution increases. Many herbicides can incorporate hydrogen ions into their molecular structure, therefore changing the charge of the herbicide molecule. At soil pH’s below 7, atrazine may pick up hydrogen ions from the soil solution causing the atrazine to take on a positive charge. The positive charge on the atrazine molecule under acid conditions increases the attraction between the herbicide molecule and negatively charged soil colloids. At soil pH’s above 7 most of the atrazine maintains a neutral charge and thus the herbicide is less tightly adsorbed and more available to plants. The greater persistence of atrazine at high pH’s is due to the herbicide being more susceptible to degradation when it is bound to soil colloids than when it is in free solution. This is the reason why atrazine carryover is a bigger problem in areas of northern Iowa with high pH soils than in southern Iowa. The adsorption and persistence of several sulfonylurea herbicides is also strongly influenced by soil pH.
Soil moisture plays two important roles in herbicide performance. First, the amount of herbicide in solution is directly related to soil moisture content (Figure 2). The amount of ‘space’ available for herbicides to go into solution decreases as soils dry out, thus less ‘free’ herbicide is present in dry soils. Under dry conditions, plants are exposed to less herbicide and therefore are less likely to absorb toxic herbicide concentrations. Corn injury from cyanazine (Bladex) was greater at all rates in a soil maintained at field capacity compared to soil at a lower moisture level (Table 3). Weed control failures frequently occur in years when soil moisture is limited during the first several weeks after planting due to the reduction in available herbicide. When soil moisture is replenished, herbicide will desorb from the colloids and reenter the soil solution. Herbicides that are readily translocated in the xylem and active in leaves (photosynthesis and pigment inhibitors) may control established weeds, or injure the crop, shortly after rainfall events due to the release of herbicide into solution where they can be absorbed by plants. This process is easily observed with the bleaching herbicides Command and Balance.
Table 3. Reduction in corn dry weight due to cyanazine injury at two soil moisture levels.
|Soil Moisture||Cyanazine Rate|
|1 lb/A||2 lb/A||3 lb/A||4 lb/A||5 lb/A|
Kern et al. 1975. Weed Science.
To be effective, the herbicide must also be present in the zone of the soil profile where the majority of weed seeds germinate. In the 1970’s placement of herbicide within this zone typically was accomplished using mechanical incorporation. As energy costs increased and conservation tillage systems were adopted, the majority of farmers began to rely on rainfall to move the herbicide off the soil surface. A common question is how much rain is needed and whether rain requirements differ among herbicides? Typically a 0.5 inch of rain is sufficient to ‘activate’ most herbicides, although the amount varies among soil types and the soil moisture content prior to the rainfall event. A dry soil requires more rain than a dry soil since the initial rainfall must wet a dry soil before significant movement of the herbicide will occur.
There are relatively small differences among herbicides in the amount of rain needed to mobilize them within the profile. Bill Simmons at the University of Illinois conducted numerous experiments in the mid-1990’s comparing rainfall requirements of three acetamide herbicides (Table 4). The only significant difference among the herbicides occurred with 0.25 inch of rain. While the activity of dimethenamid and acetochlor was increased more with this amount of rain than metolachlor, it is important to note that the level of control with any product was not commercially acceptable with only 0.25 inch of rain. At lower or higher rainfall amounts there were no differences in giant foxtail control among the herbicides. The soil type and soil moisture condition will determine the rain requirement more than herbicide characteristics in most situations.
Table 4. Influence of rainfall for activating acetamide herbicides on percent giant foxtail control. Numbers
in bold are significantly different from other herbicides within a column.
Simmons, 1997. Univ. Ill.
Factors that influence the availability of a herbicide in the soil determine how effective a treatment will be. Therefore, an understanding of herbicide behavior in soil is useful in diagnosing performance problems in the field. Generally, the differences in chemical characteristics among herbicides are relatively small, and therefore soil type and environment will have a greater impact on performance than does the specific herbicide applied.
Mulch for Weed Control and Soil Health
Mulch provides several benefits to the garden. Luckily, adding mulch is one of the easiest gardening tasks you can undertake. The question isn’t whether you should use it or not (the answer to that is a resounding yes!) The question is, which type of mulch should you choose, and how should you apply it?
Benefits of Mulch
- Mulch is a great weed suppressant. A nice, thick layer of mulch inhibits weeds in two important ways. First, by thoroughly covering the soil and depriving weed seeds of the light they need to germinate, mulch prevents them from gaining a foothold in the first place. Secondly, bare dirt is the perfect place for weed seeds to land and germinate. By covering all of your bare soil with mulch, most weeds will never be able to come into contact with the soil.
- Mulch helps retain soil moisture. Maintaining a consistent level of moisture in your soil is a big part of growing healthy plants. A plant that has a constant level of adequate moisture is less likely to become stressed, which means that it will be better able to resist insects and diseases. In tandem with keeping the soil moist, mulch also keeps the soil cooler in hot weather, which will prolong the amount of time it takes for many plants to bolt or go dormant. Some plants bloom best in cooler conditions, and mulch will aid in keeping these plants blooming longer.
- Mulch feeds the soil. Organic mulches (as opposed to inorganic ones such as glass, plastic, or rubber) will break down over time, adding nutrients and organic matter to your soil. The worms and microbes in the soil will break down organic mulches, which will result in healthier soil life.
Popular Types of Mulch
The following mulches are all organic, so they will nourish your soil while suppressing weeds and maintaining soil moisture. Each type has its strengths and weaknesses and will work better in some situations than in others.
Shredded or chipped bark doesn't break down as readily, which means that it won't provide as much nutrition to your soil, but also doesn't need to be replenished as often. Some popular woods for mulch are cedar, pine, and cypress. Bark mulches work well in many settings but are especially useful around trees and shrubs and on pathways.
Chopped leaves are plentiful and free if you have enough trees. They can be shredded by running over them a couple of times with a lawnmower or running them through a chipper/shredder. They work well on perennial beds, in vegetable gardens, and mixed borders. They break down fairly quickly and provide plenty of nutrition to the soil.
Straw is a popular choice for vegetable gardens as well as informal paths. It has a very utilitarian look, so it probably wouldn’t work in perennial borders or foundation plantings. It breaks down fairly quickly.
Grass clippings are another plentiful, free mulch. The only caveat here is to make sure that the grass clippings haven’t been treated with chemicals—you don’t want to introduce pesticides and herbicides into your organic gardens. They tend to break down very quickly, and, because they break down so fast, can heat up the soil rather than cooling it down. Grass clippings work well in vegetable gardens, informal mixed borders, or under a more attractive mulch, such as shredded bark or cocoa hulls.
Cocoa hulls are the most expensive of the popular mulches, but the look it provides for your garden is well worth it. Cocoa hulls have a dark brown, Earth-like appearance, so you don't even notice the mulch. However, they tend to develop mold in humid, wet weather. This mold doesn't harm your plants or soil, but it is unsightly.
Pine needles are another informal, and possibly free, mulch material. They look great in gardens of all kinds.
Pine needles can be a bit acidic, so it’s best to avoid using them near plants that don’t tolerate acid soils very well.
Compost (including leaf mold), like cocoa hulls, just fades into the plantings, so you don’t even notice that it’s there. Besides looking great, it provides plenty of nutrients to your soil and increases microbial activity. It will need to be replenished fairly often (at least once a year) but if you have your compost pile, you’ll have a steady supply of black gold ready to use.
How to Apply Mulch
There is a right way and a wrong way to apply mulch. The biggest mistake people make when adding mulch is that they don't apply enough. To smother weeds and retain soil moisture, a 2 to 3-inch layer of mulch is necessary. Less than 2 inches of mulch will let enough light through to allow weed seeds to germinate.
In addition to applying the right amount of mulch, you also need to make sure that it isn’t pushed against your plants. Pull the mulch back from tree trunks, shrubs, and the crowns of your annuals, perennials, and vegetables. Give your plants an inch or so of space. When mulch is applied up to a plant, it can hold moisture and cause the plant to rot.
Organic mulches, because they break down and improve your soil, need to be replenished from time to time. Plan on adding an inch of mulch to your gardens every year, either in spring or fall.
A layer of mulch can benefit every single area of your garden. Whether you have a ready-made supply, such as grass clippings or leaves, or whether you choose to order some in bulk or buy it in bags at your local garden center, the important thing is to make sure you use it. Your plants and your soil will appreciate it.