Soil-acting herbicides

CgH,3BrN202. A soil-acting herbicide. White crystalline solid, m.p. 158-159" C. It is a non-selective inhibitor of photosynthesis used for weed control In citrus and cane fruit plantations. It is relatively non-toxic to animal life. 

Ureides (e.g., diuron, linuron) and triazines (e.g., atrazine, simazine, ametryne) all act as inhibitors of photosynthesis and are applied to soil (see Figure 14.1 for structures). They are toxic to seedling weeds, which they can absorb from the soil. Some of them (e.g., simazine) have very low water solubility and, consequently, are persistent and relatively immobile in soil (see Chapter 4, Section 4.3, which also mentions the question of depth selection when these soil-acting herbicides are used for selective weed control). 

A soil-acting herbicide used to control most germinating annual grasses and broad-leaved weeds... 

If great persistence is required in a soil-acting herbicide, it should have, in addition to stability, a low water solubility and a low vapour pressure. The mode of herbicide action is in many cases unknown, but it is likely to involve the inhibition of metabolic processes such as photosynthesis, respiration, DNA or lipid synthesis. 

Repeated shallow incorporatiorts can birild up high organic matter levels in the surface layers of some soils. These can be beneficial to the development of yoimg crop plants, bnt can also canse problems with the adsorption of soil-acting herbicides. If the straw is incorporated at a wet time, anaerobic conditions may develop and the decomposing straw will release toxic substances, such as acetic acid, which can check or kill seedling crop plants. 

A fairly rough autumn seedbed is adequate for winter wheat and helps prevent soil-capping in a mild, wet winter. When soil-acting herbicides are used a fine seedbed is required. In a difficult autumn, winter wheat may be successfully planted in a wet sticky seedbed and usually it still produces a satisfactory crop. Spring wheat should only be planted in a good seedbed. 

For herbicides with low vapor pressures, the major mechanism of transport within the soil is movement in the soil water. The ability of a soil-acting herbicide to provide effective weed control depends upon its ability to move to its target site within the soil profile. Some selectivity between weeds and crops can be achieved by preparation of molecules with appropriate penetration depth within the soil. For the shallow-rooted weeds, the herbicide only needs to move to approximately the top 5 cm of the soil profile further movement takes the chemical beyond its target site and increases the potential for environmental contamination. An understanding of water movement in soils is therefore fundamental in determining the transport of herbicides to their target sites. 

Uses Technical diquat dibromide is more than 95% pure and forms white to yellow crystals. It is used to desiccate potato vines and seed crops, to control flowering of sugarcane, and for industrial and aquatic weed control. It is not residual (i.e., it does not leave any trace of herbicide on or in plants, soil, or water). Diquat dibromide is a nonselective, quick-acting herbicide and plant growth regulator, causing injury only to the parts of the plant to which it is applied. Diquat dibromide is referred to as a desiccant because it causes a leaf or entire plant to dry out quickly. 

The bipyridinium herbicides were introduced in 1958. These are very quick-acting herbicides which are absorbed by the plants and translocated causing desiccation of the foliage. These herbicides are strongly absorbed to the clay components of the soil and become effectively inactivated. 

Besides some occasional gas or upset stomach, this plant is not very toxic to people. It is however a fast acting herbicide, very toxic to other plants. In nature the leaves and flowers drop from the plant and decay into the ground. This deposits the poisons from the plant into the soil to deter competition from other plants. 

Methazole is a selective herbicide which can be used both as a residual soil herbicide and as a contact foliage-acting herbicide for the control of certain grasses and many broad-leaved weeds. 

The triazines, such as simazine (4.62), are the most used of all soil-applied herbicides small changes in the substituents give excellent control over selectivity and persistence. These substances [and the phenylureas, such as diuron (4.63), which are also soil-types] act by interfering with the Hill reaction in photosynthesis (see Section 4.6). Both of these parent substances, simazine and diuron, have undergone extensive modification for particular purposes, but each is still extensively used. A newcomer, chlorsulfuron (6.73), combines molecular features from both parents, but is notable for the greater dilution at which it acts, and also for the speed (all cell division is halted within 1 hour). This substance, l-(2-chlorophenylsulfonyl)-3-(4-methoxy-6-methyl-l, 3,5-tri-azin-2-yl)urea, is taken up by wheat, oats, and barley plants which rapidly inactivate it. It is harmless to Man (Campion and Tichon, 1981). 

Weed control can be very difficult. Weeds grow very vigorously with the high nitrogen levels. Residital or soil-applied herbicides are not normally effective as the chemicals become adsorbed onto the particles of orgarric matter. Foliar-acting herbicides or mechanical methods are corrrmorrly rrsed for weed control on these soils. 

Soil-acting or residual herbicides. These chemicals act through the roots or other underground parts of the plant after being applied to the soil surface. Some examples are propyzamide, pendimethalin, triallate, flufenacet and prosuftbcarb. These herbicides vary in their persistency in the soil ftom weeks to months. Some residual herbicides also have some contact activity, such as metribuzin. 

Herbicides which are taken up by the root depend upon mass flow of soil water to the root and uptake of the chemical in the water removed from the soil by the plant. The concentration within the root has been shown by Briggs et to be related to the octanol/water partition coefficient. For compounds with a of less than zero, there is virtually no accumulation in the plant root as the rises, so the concentration in the root rises compared to the soil solution concentration. Briggs reported that the optimum for translocation to the leaves is 2. Herbicides with Xow values higher than 4 are accumulated in the roots but not translocated. Soil-acting compounds with systemic activity are, therefore, unlikely to have a Xow greater than 3.5. " Further details are discussed in Chapter 9. 

An internal standard method gives more reliable results when elaborate sample preparation is required, as in extraction of a drug substance from biological fluids, or extraction of pesticides and herbicides from soil and plant matter. The addition of internal standard (IS) to the sample and standard acts as a marker to give accurate values of the recovery of the desired compound(s). Since the determination of wt% involves the ratio of the detector responses in the two chromatograms, the injection volume is not critical as in an external standard method. 

The soil sterilizer metham (sodium methyldithiocarbamale) may act simultaneously as a fungicide, a nematodidde, and an herbicide [4, 5, 30, 41]. Metham itself is relatively toxic (the FD50 is 285 mg/ kg for white mice, and 820 mg/kg for... 

The compounds profiled in this book include solvents, herbicides, insecticides, fumigants, and other hazardous substances most coimnoidy found in the groundwater and soil environment the organic Priority Pollutants promulgated by the U.S. Enviromnental Protection Agency (U S. EPA) under the Clean Water Act of 1977 [40 Code of Federal Regulations (CFR) 136, 1977] and compounds most commonly found in the workplace. 

The production of compounds which may act as allelochemicals is not restricted to higher plants. For example, Heisey, DeFrank and Putnam (41) have discussed substances produced by soil microorganisms which may have herbicidal activity. Bacteria may also mediate allelopathic activity in economically important situations such as forest regeneration (Line, personal communication) crop/weed associations ( ), and reduced cultivation systems where plant residues are retained ( ). 

Weeds compete with crop plants for valuable nutrients. The traditional method for controlling weeds is to plow them under the soil, where in decomposing they release the nutrients they absorbed while they were alive. Plowing also aerates the soil, but it is either labor-intensive or energy-intensive and can lead to topsoil erosion. In the early 1900s, farmers noted that certain fertilizers, such as calcium cyanamide, CaNCN, selectively kill weeds while causing little harm to crops. This prompted a broad search for chemicals that act as herbicides. Today, a farmer can choose from hundreds of herbicides, many tailored for a specific weed. Farmers in the United States apply almost 600 million pounds of herbicides annually, which is about three times more than the amount of insecticides they apply. 

---