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h specific type of herbicidal activity is shown by certain diquaternary salts oi 2,2'-, 2.4'-and 4,4'-dipyridyl : the diquaternary salts of 2,3'-and 3,3'-dipyridyl are devoid of this activity. The herbicidal compounds have redox properties, giving rise to stable watersoluble free radicals by uptake of a single electron on reduction. A comelation exists between the redox potential of this reduction and the degree of herbicidal action. Evidence is presented that the compounds are normally reduced in green tissues by energy ultimately derived from light, but that as activity is also present in the dark, though a t a reduced level, other reduction mechanisms are also possible.

Investigation of the adsorption of I : 1'-dimethyl 4 : 4'-dipyridytium (paraquat) dichloride by a range of soils, using a solution-equilibration technique, has shown that the adsorption is characterized by three factors :I. Up to a limiting value, defined as the Strong Adsorption Capacity (SAC), the solution concentration of paraquat is reduced below the level of chemical detection by suspended soil : this strongly adsorbed paraquat is preferentially held against 0.1 to 2.oN solutions of ammonium ion. A range of strengths of adsorption probably exists within the strong adsorption range, but the greater part of the strongly adsorbed material is de-activated herbicidally ; this accounts for the herbicidal de-activation of paraquat in soils.2. Removal of soil organic matter by treatment with hydrogen peroxide usually does not greatly change the SAC. Strong adsorption of paraquat is primarily a property of clay minerals, and the presence of expanding lattice minerals is of particular importance.3. For fortifications above the strong adsorption region, there is a region of weaker adsorption but the total adsorption capacities of soils at saturation are less than their cation exchange capacities for inorganic cations. Taken with the difficulty of displacement, this indicates that the adsorption of paraquat is strongly influenced by factors other than simple electrostatic interaction.

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