200 or lower 200-2000 2000-20000 20000+
Dermal LD50
(mg/kg)
200 or lower 200-2000 2000-20000 20000+
Eye effect Irreversible corneal
opacity at 7 days
Corneal opacity or
reversible within 7 days, irritation foe 7 days
No corneal opacity,
irritation gone within 7 days
No irritation
Skin irritation Severe
irritation or damage at 72 hours
Moderate
irritation at 72 hours
Mild
irritation at 72 hours
No irritation at 72 hours
Signal word DANGER
(Poison)
WARNING CAUTION CAUTION
Colour code Red label Yellow label Blue label Blue label
The fate of herbicide in soil can be classified as follows;
(I) Transport by adsorption and movement (volatilization, leaching, runoff etc.)
Transport of herbicides within the soil compartment can occur downward into the soil profile (leaching), across the soil surface (runoff), or into the air (volatilization) each can be a combination of more fundamental processes including adsorption, convection and diffusion.
The difference processes are discussed below:
(i) Adsorption
Adsorption is the attraction of ions or molecules to the surface of a solid. After application, many herbicides adsorb (bind) to the clay and organic-matter fractions of soils. However, herbicides adsorb poorly to the sand and silt fractions of soil. Therefore, the extent of herbicide adsorption increases as the percentage of organic matter and clay increases. The dinitroaniline herbicides, dithiopyr, oxadiazon and most other pre-emergence herbicides readily bind to soils.
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 favour movement into the soil solution. With most herbicides, adsorptivity and solubility are inversely related. Thus, as solubility increases, binding capacity to soil decreases. There are a few exceptions to this rule, including paraquat and glyphosate.
Both 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.
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.
Soil pH can have a significant effect on the adsorption of many herbicides. As the pH decreases below 7, 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. The adsorption and persistence of several sulfonylurea herbicides is also strongly influenced by soil pH. Adsorption of pesticides by soils has frequently been found to be correlated with organic matter and clay contents. It is generally accepted that this effect is due to the high adsorptive capacity of these soil constituents for herbicides. Adsorption of herbicides, therefore, is basic to understanding the behavior of herbicides in soil.
Certain members of the sulfonylurea group (chlorsulfuron and chlorimuron) can also persist in higher-pH soils because rates of chemical breakdown are decreased. Imidazolinone herbicides tend to be more adsorbed under acidic or low soil pH which reduces their availability for microbial degradation. They are broken down primarily by chemical hydrolysis and this process occurs much quicker under acidic conditions. Therefore, they tend to degrade faster when soil pH is low.
Soil moisture plays two important roles in herbicide performance. The amount of herbicide in solution is directly related to soil moisture content. 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 be less likely to absorb toxic herbicide concentrations. When soil moisture is replenished, herbicide will desorb from the colloids and re-enter the soil solution.
(ii) Volatilization
Volatilization is a process wherein a condensed phase such as liquid or solid is transformed into a vapour by elevation of temperature or reduction of external pressure. The term vapour describes a substance in the gaseous stage below its critical temperature. The tendency of a substance to volatilize is expressed by its vapour pressure. The vapour pressure is usually expressed in millimeters of mercury (mmHg) or occasionally in micron of mercury. Herbicides with vapour pressure of more than 1 x 10-5 mmHg at 200C are generally considered volatile.
Volatilization is considered one of the primary pathways for herbicide dissipation from the site of a herbicide application. Under unfavorable conditions, losses that results from volatilization can reach 80 to 90 % within a few days although such rates are dependent upon climate and microclimatic conditions (Taylor and Glotfelty, 1988). The process of volatilization is controlled by two mechanisms: the evaporation of the active ingredient from the soil or plant surface to which it has been applied or has migrated and dispersion of the resulting vapour through the overlaying atmosphere by diffusion and turbulent mixing. Though these two processes are different in character and are controlled by different chemicals and environmental factors, they are not dependent of one another and in general should be considered as an integrated whole.
The first process of evaporation involves a phase change from its initial state of solids or liquid to its corresponding vapour. Thus one of the main factors controlling the rate of loss by volatilization is the rate at which surface residues evaporate. The rate of evaporation in turn is controlled by the vapour pressure of the exposed residues and is either the vapour pressure of the active ingredient or lower values to which it reduced by interaction of the compound with surface. For residues no longer on the surface but drawn away to underlying by capillary action or diffusion after
application, the rate of volatilization becomes dependent upon the rate of back diffusion of the material to the upper layers (Taylor and Glotfelty, 1988).
The second process of dispersion into the atmosphere acts through the turbulent flow of air over the evaporating surface. The flow of air continuously replaces the air around the evaporating surface and mixes and dilutes the herbicides vapour in the surrounding air. The movement and dispersion of vapour within the turbulent layer are considered rapid to those at the evaporative surface. The flow of air over plant and soil surfaces is generally turbulent except for a shallow layer of air directly above the evaporating surface where flow is regarded as laminar. The actual rate of volatilization away from the evaporative surface through the stagnant layer is proportional to the molecular diffusion coefficient and the vapour pressure of the herbicides at the evaporating surface. The depth of the stagnant layer in turn is affected by both the surface geometry and the air flow of the system. Surface geometry or roughness alters the turbulent flow of air over the evaporative surface. Thus an increase in either air flow or turbulent increases the rate of vaporization (Spencer et al., 1982) as vapour are removed from the evaporative surface.
Vaporization depends on number of factors;
(a) Effect of soil organic matter
Once a herbicides has been applied to a soil, the material either remains on the surface, diffuse into the soil by capillary action or becomes sorbed to the soil matrix. The extent of sorption to the soil matrix is influenced by the carbon content of the soil, which in turn affects the rate of volatilization from the soil. Soil carbon content controls the concentration of pesticide present in the soil solution. Spencer (1970) has demonstrated that vapour density of a herbicide over a soil is proportional to its concentration in the soil solution. In general, the higher the soil carbon content the lower the concentration of herbicide in the soil solution. As a result of its reduced concentration in soil solution, the rate of volatilization for a herbicide from a soil with a high organic matter content is lower than that from one with low organic matter content (Jury et al., 1980; Getzin 1981; Glotfelty et al., 1984).
Empirically the concentration of a herbicide in the soil solution is equal to the ratio of its vapour pressure and water solubility. This concept has been further developed by Swan and coworkers (1982) as an empirical rate to its vapour pressure (P), water solubility (S) and soil adsorption coefficient (Koc) by
Kv = Q(P/Koc S)
Where Q is an empirically determined coefficient (b) Effect of soil moisture
The moisture content of soil has been determined to be one of the most significant environmental parameters which influence the rate of herbicide volatilization. In general herbicides volatilize more rapidly from moist than dry soils. The effect of low soil moisture content can be dramatic, effectively halting volatilization of a herbicide from a given soil.
Increased binding of a herbicide to a soil has been estimated to occur once the soil moisture content decreases to one to three molecular layers on the surface of a soil particle. However the point at which increased herbicide binding occurs varies and is also dependent upon soil composition which in turn, influences soil moisture capacity. The binding process is revertible and with increased soil moisture content such as after rainfall or dew fall, an increase in herbicide flux from the soil can be detected (Harper et al., 1976; Taylor et al., 1976; White et al., 1977; Grover et al., 1985). In the field, similar influences of soil moisture on herbicide flux have been observed. The flux of trifluralin from soil (Harper et al., 1976) decreased
shortly however when soil moisture is no longer limiting, volatilization increases proportionally with increased energy input in the form of sunlight (Glotfelty et al., 1989).
(c) Effect of temperature
The vapour pressure of a herbicide varies proportionally with temperature. Thus with an increase in ambient temperature a corresponding increase in volatilization from soil or plant surfaces is anticipated. When sorption sites on the soil are saturated with herbicide the vapour density or vapour pressure for that herbicide above the soil may be equivalent to that of the pure active compound (Spencer and Cliath 1969) and varies proportionally with temperature.
At lower concentrations, this temperature effect on the vapour pressure for a herbicide no longer occurs but is directly influenced by the energy of sorption to the soil matrix, as a result becoming less well defined of the phenomenon (Spencer 1970).
(d) Effect of formulation
The herbicides formulation also affects the rate of volatilization from a site. Use of emulsifiable concentrations (ECs) results in direct application of the active ingredient to the soil, allowing for sorption to the soil matrix. In Wettable formulation (WP) active ingredient remains on the soil with limited sorption to the soil matrix. Glotfelty et al., (1989) reported that simazine and atrazine applied as WP formulations were displayed from the soil on dry days by the wind as compared to toxaphene and alachlor applied as EC formulations. A change in formulation can be utilized to reduce volatilizations of a herbicide (Turner et al.
1978).
(iii) Leaching
Leaching of herbicides is their downward movement in soil as solute with soil water.
Herbicides may move in soil by the physical process of diffusion or mass flow in the liquid or vapor phase. Transport by diffusion is slow relative to mass transport by soil water movement but it may be excessively important over short distance. Soil and other factors that discourage adsorption aid leaching of herbicides in soils. High adsorption herbicides are relatively immobile. When a surface applied herbicide fails to leach even 2-3 cm depth in soil, it will bring about negligible amount or very poor pre-emergence control of weeds. On the other hand, if herbicide leaching is excessive, crop seedling may be injured. Very deep leaching may be desirable for controlling deep rooted weeds. Usually leached herbicides reside in sub- surface soil for a long period in active forms because here the conditions for degradation of herbicides are less favorable. Eventually, from sub-surface either a herbicide may return to soil surface during deep ploughing or it may contribute to ground table. Herbicide leaching in soil water can move herbicides out of the tillage and root zone of subsequent crops. Herbicide leaching is greatest in coarse-textured soils with low levels of organic matter. Highly soluble herbicides are prone to leaching.
It is observed that the intrinsic mobility of herbicides in soil is inversely related to its degree of sorption to soil surface (Gustafson, 1995). It was observed that top 10 cm soil layer showed higher retention of herbicides. Further it is reported that herbicide mobility was inversely related to clay and organic matter content and the Freundlich sorption constant. Herbicides have been known to move upwards in the soil. If water evaporates from the soil surface, water may move slowly upward and may carry with it soluble herbicides. As the water evaporates, the herbicide is deposited on the soil surface. In arid areas where irrigation is practical, lateral movement of herbicides on soils also occurs.
The extent to which a herbicide is leached is determined principally by:
(1) Adsorptive relationship between the herbicide and the soil (2) Solubility of the herbicide in water.
(3) Amount of water passing downward through a soil.
Solubility is sometimes cited as the principal factor affecting the leaching of a herbicide.
The salts of 2, 4 –D are water soluble and readily leach through porous sandy soil. Sondhia and Yaduraju, (2004) reported high mobility of atrazine and metribuzin in clay loam soil and reported that atrazine and metribuzin could leach upto the depth of 52 cm in the soil column.
Herbicides that are strongly adsorbed to soil particles, like glyphosate and paraquat, cannot be leached unless the soil particles are moved by the water. . Sandy soil would have a higher leaching potential than a clay soil due to larger pore spaces and lower CEC. Sondhia (2006) found that sulfosulfuron and metsulfuron were mostly absorbed in the surface layer of the soil (0-10) but small amount was leached down upto 80 cm and clay loam soil retained maximum amount of sulfosulfuron and metsulfuron in the surface soil as compared to loamy sand soil.
Sondhia (2004) found that 80 % of applied pendimethalin was found distributed in 0-12 cm soil depth and only 0.2 % could leach to the depth of 48-52 cm in soil column and indicating slow mobility of pendimethalin in clay loam soil. However approximately 45 % applied butachlor was found distributed in 0-4 and 7 % butachlor could leach to the 20-24 cm soil depths which showed greater adsorption of butachlor at the surface soil as compare to subsurface soil in clay loam soil.
(II) Degradation or decomposition by biological (microbial) and chemical processes and photodecomposition
Degradation processes include biological degradation by soil organisms and abiotic chemical and photochemical transformations. Degradation is the process of destruction of the original herbicides molecule and usually loss of herbicidal activity. After degradation parts of the original herbicides structure remain as different molecule, these breakdown products ultimately may be decomposed further to simple organic molecules, but more complex breakdown products may be incorporated into organic residues. Often degradation process occurs in the soil.
(i) Microbial decomposition.
Microbial decomposition is one of the most important methods by which herbicides are decomposed in soil. Microorganisms in the soil metabolize organic herbicides either aerobically (with oxygen) or anaerobically (without oxygen). In most cases, the microorganisms consume the herbicide molecules and utilize them as a source of energy and nutrients for growth and reproduction. Microbes can also degrade herbicides by a process called co-metabolism, which occurs when the organic herbicide is not used by the microorganism for growth but is metabolized in conjunction with another substrate used for growth. Some herbicides are decomposed easily by microorganisms while others are not.
For example, microbial degradation of 2, 4-D occurs very quickly in the soil, whereas microbial degradation of atrazine is slow.
Microorganisms in the soil include fungi, actinomycetes, and bacteria. The population levels and activity of these microorganisms depend on food supply, temperature, soil moisture, oxygen, soil pH, and organic matter content. When a herbicide is applied to a soil, microorganisms may immediately attack it. The population of the particular microorganism that uses that herbicide for an energy source will increase (Hutzinger 1981). After the herbicide is degraded, the microbial population may return to the original level, or it may stabilize at a level greater than before application. The increased population could cause more rapid herbicide degradation upon subsequent herbicide applications. Different microbes can degrade different herbicides, and consequently, the rate of microbial degradation depends on the microbial community present in a given situation (Voos and Groffman 1997).
There is sometimes a lag time before microbial degradation proceeds. This may be because the populations of appropriate microbes or their supplies of necessary enzymes start small, and take time to build-up (Farmer and Aochi, 1987, Kearney and Karns 1960). If this lag time is long, other degradation processes may play more important roles in dissipation of the herbicide (Farmer and Aochi 1987). Degradation rates of co-metabolized herbicides tend to remain constant over time.
Soil conditions that maximize microbial degradation include warmth, moisture, and high organic content. The optimum temperature for microbial activity generally is 26–32°C. As soil temperatures decrease, soil microbial activity declines, with minimal activity below approximately 4.4°C. Therefore, maximum microbial herbicide breakdown occurs in the summer when soils are warm. The rate of microbial breakdown decreases in the fall as soils cool, and virtually ceases as soil temperatures drop below 4.4°C.
Soil moisture is essential for soil microbial activity. Soil moisture levels between 50-100 percent of field capacity are optimum for microbial activity and, therefore, herbicide breakdown. When soil moisture is limited throughout the sowing season, the rate of microbial degradation of a herbicide is reduced, and the herbicide is more likely to carry over and injure later rotational crops.
The majority of microbial degradation of herbicides is by aerobic organisms, which are very sensitive to the oxygen supply. Flooded and compacted soils with poor aeration will reduce microbial activity and herbicide breakdown.
Soil pH also affects microorganisms. Soil bacteria and primitive fungi called actinomycetes usually favor and are most active in soils with a pH above 5.5. Other fungi are less sensitive to soil pH and predominate at pH 5.5 and below. Soil organic matter content is important to soil microbial populations and activity. Organic matter is the primary source of energy and nutrients for soil microorganisms. The highest microbial populations and the majority of microbial herbicide breakdown will be in the surface foot of soil where organic matter content is highest. Small increases in soil organic matter content can increase microbial activity and the rate of herbicide breakdown.
A warm, moist, well-aerated soil with pH between 5.5 and 7.0 generally is most favorable for rapid microbial breakdown of herbicides. Any adverse condition, such as cold temperatures or dry soils, will reduce the rate of herbicide decomposition by microorganisms and lengthen the soil persistence (half-life) of a herbicide.