The ability of herbicides to adsorb on soils and sediments and their tendency to desorb are the most important factors affecting soil and water contamination (Blasioli et al., 2011). Several factors affect the adsorption of herbicides by soils. These include: a) type of clay colloid,
b) soil organic matter, c) soil pH, d) moisture content, e) chemical nature of the herbicide and f) leaching.
Clay colloid refers to the microscopic (0.001 to 1 μ in diameter) inorganic and organic particles in the soil. These particles have an extremely large surface in proportion to a given volume. Clay particles have negative charges and hence can attract to their surface positive ions (cations).
There are three major groups of clays: montmorillonite, illite and kaolinite. Montmorillonite is an expanding lattice clay providing both external and internal adsorptive surfaces. It has three layers, with one layer of aluminium oxide lying between two layers of silicon oxides.
Illite is also a three-layered clay, but it lacks the expanding lattice character which makes it less adsorptive of herbicide molecules than montmorillonite clay. On the other hand, kaolinite is only a two-layered clay, with alternate layers of aluminium and silicon oxides.
There are few residual charges in kaolinite clay, making it the least adsorbent of the three clays. However, kaolinite has one hydroxyl surface, which makes it adsorb some organic chemicals more strongly than the other clays (Rao, 2011).
The strength of adsorption follows the order of montmorillonite>illite>kaolinite. Montmorillonite adsorbs considerably more of various herbicides than illite and kaolinite.
The most significant factor affecting adsorption and hence, the behaviour of herbicides in soils is soil organic matter. The organic matter is made up of humic materials, plant and animal residues, and soil microbes.
Soil humic materials consists of three components: a) humic acid, the alkaline soluble and acid insoluble fraction, b) fulvic acid, the alkaline soluble and acid soluble fraction, and c) humin, the alkaline insoluble and acid insoluble fraction (Harper, 1994).
The humic acids are responsible for stable bonding during herbicide adsorption. Plant residues, following decomposition in soil, have a much greater adsorption capacity than the soil itself. Dao (1991) reported that decaying wheat straw adsorbed metribuzin better than the undecayed wheat straw.
This increase in soptive capacity was attributed to a decline in cellulose and its concomitant proportional increase in lignin. The humic material has a primary influence in the adsorption of several herbicides including 2, 4-D, chlorsulfuron, picloram, linuron and metribuzin (Rao, 2011).
Soil pH affects the detoxification of herbicides by affecting the ionic or molecular character of the chemical, the ionic character and the CEC of soil colloids, and the inherent capacity of soil microorganisms to react with the herbicides.
Hydrogen ions have a positive electrical charge, indicating that they can be bound to the negatively charged soil and organic matter particles. The more free sites clay and organic matter particles have on them, the more hydrogen and other ions that can be bound to these particles.
These binding sites, also called exchange sites, indicate the cation exchange capacity (CEC) of the soil. Thus, soils with greater CEC have more exchange sites.
Additionally, the more exchange sites a soil has, the more hydrogen ions that can be held to the soil for eventual release into the soil solution (Rao, 2011). This is referred to as reserve acidity, which explains why an acid soil with high CEC needs more time for neutralization than an acid soil with low CEC.
Many herbicides are ionic which enables them, when in solution, to give off or attract hydrogen ions depending on the pH of the solution.
For example, 2,4-D, MCPA, dicamba, chloramben, picloram, etc., which are acidic in character, can release hydrogen ions in a neutral or basic solution, while herbicides such as s-triazines, substituted ureas, phenyl carbamates, amides, etc., which are chemically basic in nature, can accept hydrogen ions in an acidic solution.
Other herbicides, such as diquat and paraquat, are so basic that they are positively charged in virtually all soil pH values.
The soil moisture content has a pronounced effect on both the degree of adsorption and the phytotoxicity of herbicides present in the aqueous phase.
Herbicides reaching the soil are partitioned into adsorption and solution phases, moving through the soil either by molecular diffusion or by mass flow with the movement of water.
The amount of herbicide present in solution depends on the solubility of the herbicide in water and the amount adsorbed by soil colloids. The availability of a herbicide for plant uptake is related to its desorption into water solution.
Thus, if a herbicide moves with the water, it may be completely removed from the soil profile and leached down to groundwater and streams.
Most of the herbicides have lower phytotoxicity at lower soil moisture contents. This is related to the degree of competition of the organic compound for adsorption sites at different moisture levels. Water is a polar compound and is strongly adsorbed by mineral colloids.
At low moisture levels, the number of water molecules present to compete for adsorption sites is relatively small and fewer polar organic molecules may be able to compete more favourably for the available sites to be adsorbed.
As the moisture content increases, the number of water molecules increases, resulting in reduced adsorption of the organic molecules. If the organic molecules have been adsorbed under conditions of low moisture and then the moisture level is increased, the adsorbed organic molecules may be displaced by water molecules and made available in soil solution for plant absorption (Rao, 2011).
Chemical Nature of Herbicides
Herbicides are classified into several groups depending on base structure of the herbicide molecule. Herbicides within the principal group can be loosely categorized as permanently ionized (i.e. quaternary ammonium compounds), ionizable (i.e. triazines) or neutral (carbamothioates) (Harper, 1994).
Different functional groups on the base structure lead to a range of polarity and ionizability within a herbicide group.
Substitution of functional groups on the base chemical structure also brings about changes in water solubility, volatility, adsorption strength and adsorption mechanisms as also charges in herbicidal activity (Rao, 2011).
Generally, soil and organic matter particles have negative electrical charges. Herbicides having positive charges are attracted and bound to them. Most organic molecules ionize only under certain pH conditions.
The pH of ionization may range from -0.5 to 11.2 depending on the functional group. Compounds that ionize at these extremes would be unlikely to occur as ions in soils. Within the normal pH range of soils, 4.0 to 9.0, dissociation usually takes the form of H+ loss by acids and H+ gain by bases.
The weakly basic herbicides such as triazines and triazoles, which are less effective in soils of low pH, adsorb hydrogen ions in an acidic solution and become cationic.
More atrazine is adsorbed by a muck soil at pH 3.2 than at 5.3, as little atrazine (ionization constant, pKa, 1.85) would exist as cation at pH 5.3.
Absorption is generally more pronounced when the pH of the soil is near the pKa of the herbicide. In high pH soils, triazines are desorbed into the soil solution, which results in greater availability of the chemical for uptake by plants and possible risk of injury even at rates considered safe.
The strongly basic herbicides such as paraquat and diquat are so rapidly and tightly bound to montmorillonite clay and organic matter that they are virtually inactivated as soon as they come into contact with the soil (Rao, 2011).
The strongly acidic herbicide glyphosate is adsorbed more at low pH. It is readily bound to kaolinite, illite and bentonite clays, and to charcoal and muck. The strongly acidic herbicides such as benzoic acids, phenols, aliphatics and nitriles possess carboxyl, phenolic or phosphonic functional groups and ionize in soil solution to become anions.
The weakly acidic herbicides such as 2, 4-D, dicamba and dinoseb are less active at a soil pH 5.0 or below. They tend to be repelled by, rather than attracted to, the negatively charged soil and organic matter surfaces.
With the decrease in percentage of negatively charged herbicide molecules at low pH, adsorption increases, and hence their low activity at pH below 5.0.
The non-ionic herbicides such as diuron and other urea herbicdes, and trifluralin and other dinitroanilines, which do not ionize significantly in soil solution can also be affected by soil pH, but to a much lesser extent than the basic and acidic herbicides. These non-ionic herbicides are adsorbed through physical adsorption forces (Rao, 2011).
Leaching refers to the movement of herbicides with water within the soil profile. This is influenced by the chemical nature of a herbicide, the adsorptive capacity of the soil and the amount of water available for downward movement through the soil. These aspects have earlier been discussed.
Leaching affects the selectivity of herbicides and by extension their relative efficacy. Excessive leaching to the deeper soil layers may render the herbicide less effective on shallow-rooted weed species, but could make it effective on deep-rooted ones.
In such a situation, a shallow- rooted crop species may exhibit tolerance while deep-rooted crop plants become susceptible. Irrigation or rainfall following herbicide application has a profound effect on leaching and crop and weed tolerance to a herbicide.
Herbicide Transport in Soil
Herbicide absorption by plants occurs primarily from free herbicide content available in soil water. The processes that control the concentration of herbicide in soil water are: a) solubility of the herbicide, b) adsorptive capacity of the soil for the herbicide and c) water content of the soil.
However, there is no general correlation between the water solubility of herbicides and the concentration of herbicides that remain free in equilibrium soil solution, because the adsorption of herbicides by soil is the main factor controlling the concentration of the solution.
Soil water content affects the rate of transpiration, mass flow and molecular diffusion in the liquid phase, thus controlling the rate at which herbicide is transported in the liquid phase to the site of action in plants.
Soil water content also determine the pore space diffusion in the vapour phase, which affects the rate of herbicide uptake by roots (Rao, 2011).
Diffusion is the movement of nutrients to the root surface in response to a concentration gradient. Herbicides are distributed in soil among solid, air and liquid phases.
Herbicide molecules in the liquid or gas phase are in a state of random motion with more molecules moving out of the high concentration region to the dilute region along the concentration gradient.
The diffusion rate in a very dry soil increases rapidly as the soil moisture content increases, while the diffusion rate in moist soils increases as the temperature increases.
Herbicides diffusing primarily in soil air are not active in extremely dry soil. Hence, incorporation of herbicides such as trifluralin and triallate may not lead to satisfactory weed control in dry soils, with soil moisture content below the permanent wilting point, unless soil moisture content is increased by way of rainfall or irrigation.
The slight activity of herbicides at low vapour pressure in dry soils may be due to diffusion in the vapour phase. The efficacy of herbicides with low vapour pressures increases as the soil moisture content increases.
The lack of herbicidal efficacy in dry soil may be attributed to insufficient water content to move the herbicide into the root zone or due to reduced diffusion of some herbicides in soil water to the emerging roots (Rao, 2011).
Mass Flow Transport
Mass flow is the movement of dissolved nutrients into a plant as the plant absorbs water for transpiration. The process is responsible for most transport of nitrate, sulphate, calcium and magnesium.
The amount of herbicide that reaches the roots by mass flow is calculated by the volume of water transpired from leaves. The water flow through the soil-plant system occurs along a potential gradient, which must decrease continuously from the soil through the plant to the atmosphere for transpiration to occur.
However, the mass flow of water and dissolved herbicide to the plants varies greatly depending on soil and atmospheric conditions.
The resistance in plant and soil (Rp+Rs) increases greatly as the soil water content falls and as the relative transport rate also decreases (Rao, 2011).
Generally, water will be transported through the soil-plant system fast enough to meet the potential transpiration demands only when the soil water potential is at a very small absolute value.
The potential transpiration rate will depend on meteorological parameters such as incident radiation, temperature and wind speed.
Normally, there is good correlation between the uptake of herbicides by plants and the rate of transpiration at varying soil moisture contents. The concentration of a herbicide in plant leaves is related to total uptake and rate of plant growth.