The retention of spray liquids has been measured on the surface of leaves of various plants. Various spray liquids, having a range of wetting abilities, were used and the retention was measured both by spraying the surfa.ce up to the point of spray run-off, thus giving a measure of the maximum retention, and by spraying the surface t o beyond run-off conditions.For any given surface, the maximum retention was related to the theoretical retention factor, which was calculated from the values of the dynamic advancing and receding contact angles of the spray liquid on the solid surface and from the surface tension of the spray liquid. After spraying beyond run-off, the theoretical retention factor will predict the relative levels of retention with reasonable accuracy.This agreement between measured and theoretical retention has enabled the retention theory to be used to study the importance of many of the variables that govern spray retention in practice. These variables are discussed.
A survey has been made of some of the physical, chemical and biological interactions that influence the persistence and availability of herbicides applied to the soil. The most important physical interaction between herbicides and soil is considered to be sorption of the pesticide to the soil surface. This influences not only the rate of leaching of the herbicide through the soil and its movement in the vapour phase, but also the rate of chemical and microbial decomposition. An attempt has been made to construct a model for the persistence of some hydrolytically sensitive herbicides to enable their persistence in soils to be deduced from hydrolysis rate‐constants and adsorption data. The formulation used and the method of application of the herbicide to the soil can also have an important influence on its persistence.
The phytotoxicity and wetting ability of 61 surface-active agents of known chemical structure have been tested on the leaves of two varieties of apple and two of plum.Surface-active materials containing a branched alkyl chain in the hydrophobic portion of their molecule are better wetters than corresponding materials containing a straight alkyl chain. In any homologous series of materials, wetting ability increases with increase in size of the alkyl group. With the non-ionic ethylene oxide condensates, maximum wetting occurs with those materials containing the minimum number of ethylene oxide groups conferring water solubility.Phytotoxicity appears to be governed by the nature of the ionic charge, by the physical size of the molecules or ions and to a lesser extent by the nature of the gegenion. All nonionic materials cause little damage on apple and plum leaves, anionic and cationic materials cause variable damage depending on their chemical structure and on the nature of the leaf surface. In any homologous series of surface-active agents the phytotoxicity appears to pass through a maximum as the molecular size is progressively increased. Since this maximum usually occurs a t fairly short alkyl chain lengths (dodecyl or below), for most practical purposes phytotoxicity decreases as the size of the alkyl group increases.These results are discussed in the light of the conclusions reached in Part I1 of this series, and the physical and chemical factors governing phytotoxicity are considered. IntroductionIn Part 111 of this series of papers it was shown that considerable damage to apple and plum leaves was produced by certain surface-active agents. From the limited number of agents used in that study it appeared that phytotoxicity was influenced by the presence or absence of an electric charge on the surface-active molecule and also by the chemical structure of the material. Many physical and chemical properties of surface-active agents are governed to a large extent by the size and structure of the hydrophobic portion of their molecule, which is usually a paraffinic chain or an alkyl-substituted benzene or naphthalene ring. Thus, in a study of the germicidal properties of some alkyltrimethylsulphonium iodides Kuhn & Dann2 found that the maximum effect against Escherichia coli was obtained with a C,,-alkyl chain whilst optimum activity against staphylococci was found with a C,,-alkyl chain. Ross & Ludwig3 have recently studied the fungitoxicity and phytotoxicity of an homologous series of N-n-alkylethylenethioureas with the alkyl group ranging from ethyl to dodecyl. The fungitoxicity increased on moving up the series to octyl, and then declined sharply ; the phytotoxicity also increased up to at least the octyl homologue. On the other hand Dills & Menusan4 found that the phytotoxicity of the potassium salts of fatty acids decreased on ascending the homologous series, the caproate producing considerably more damage than the stearate.The work described in this paper is an attempt to find out how far the phytotoxicity ...
The phytotoxicity of solutions of anionic, cationic and non-ionic surface-active agents to the leaves of several varieties of apple and plum trees is described. The leaf damage is shown to be dependent largely on the chemical nature and the concentration of the surfaceactive agent and also on the species of tree. Other factors which affect the damage to a more limited extent are the variety and manurial status of the tree, the age of leaf and weather conditions. Many of the ionic materials cause considerable leaf damage, particularly cationic materials on plum leaves, but the non-ionic materials are relatively safe to use. The damage is considered to result from the disorganisation of the cell permeability barriers following the penetration of the surface-active agent through the leaf cuticle. Possible routes for such penetration are discussed.
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