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The behavior of highly volatile pre-emergence herbicides in soil and their relative availability to plants under various conditions is poorly understood and pertinent information is much sought after by workers in chemical weed control. Ashton and Sheets reported an inverse relationship between injury to oats and soil adsorption of ethyl–N,N-di–n–propylthiolcarbamate (EPTC) in various soil types. Later, Sheets demonstrated a close relation between the initial toxicity of EPTC and soil types. EPTC was inactivated about one-third as rapidly in an autoclaved soil as in a corresponding unautoclaved one, from which he suggested that microbial breakdown may be a major pathway of EPTC inactivation when the chemical is incorporated in the soil.
The absorption of chemicals by the leaf of a plant is a problem of major importance in chemical weed control. The presence of a waxy cuticle on the surface presents a barrier to absorption. Interspersed in the cuticle are the stomata which may serve as a portal of entry for chemicals but even the interior chambers of the stoma are lined with a cuticle. It may be presumed that the more intimate the contact between a chemical and the leaf surface the greater the possibility for absorption. The water carrier commonly used in application of these chemicals makes but a poor contact with the cuticle; however, this deficiency may be overcome by the addition of agents to lower the surface tension of the solution. While intimacy of contact of the chemical with the leaf surface is important in absorption, the state, i.e. liquid or solid, of the chemical may also influence the ease of absorption. Consideration of the physical processes involved shows that a chemical will be absorbed with relative ease from a liquid phase in contrast to the same chemical deposited as a crystal on the surface of the leaf. Either humid conditions following application or the addition of hygroscopic agents may help to maintain a liquid state thus facilitating absorption. Many surface active agents used in sprays are also fair hygroscopic agents.
The herbicidal and desiccant properties of 1:1′–ethylene–2:2′–bipyridylium dibromide have been reported by Brian et al. (1). This substance appears to be a rapid acting translocated herbicide, effective on a wide range of plant species (1,2).
The present extensive use of herbicides in soil application has resulted in a great deal of interest in the breakdown of these chemicals in soil (1, 4, 6, 8). An increasing demand exists for information that will permit fairly accurate predictions of the residual life of these chemicals (4). The breakdown of chemicals in soil has been repeatedly shown to be the result of microbiological activity in part but other factors are also known to contribute to the disappearance (1, 4, 5, 7, 8, 9).
The biological effectiveness of any systemic herbicide is vitally dependent upon the absorption of the chemical by the plant. The process of absorption is of particular interest in the use of herbicides and it is only as this process is understood and turned to advantage that maximum effectiveness of applied chemical can be achieved. In foliar absorption, with which this paper is concerned, the cuticle plays a very important role (2). The thickness and general state of the cuticle and the attendant stomata have been shown to vary markedly with growth conditions (10). The submicroscopic structure of cuticles and the accretion of wax deposit on leaf surfaces have been studied (7, 8). These factors are of significance in the consideration of the intimacy of the contact of a particle impinging on the leaf. The chemical nature of the cuticle (6) is undoubtedly of great importance in the partitioning of certain herbicides between a particle of the chemical and the interior of this leaf.
One of the more common methods of analysis for 2,4–dichlorophenoxyacetic acid (2,4,–D) is the one using chromotropic acid as the color developing reagent (1). In this laboratory it was found that it was difficult to obtain relatively pure chromotropic acid or its disodium salt. As an approach to the preparation of a purer compound, several experiments were tried.
The distribution of 3 (p–chlorophenyl)–1,1–dimethyl urea (CMU) in the soil horizon is an important factor to consider in using the material since it is a soil active herbicide. It has been reported that the persistence and toxicity of CMU vary considerably with soil type, temperature, and moisture conditions.
The use of Isopropyl N-Phenyl Carbamate (IPC) as a residual pre-emergent herbicide depends on the effective concentration remaining in the soil for a period of time. The effectiveness of such a treatment has been shown to be dependent upon several factors.
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