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Summary. Factors investigated included the susceptibility of Avena fatua L. at various stages of growth, the amount of moisture needed to activate di‐allate in Regina heavy clay, the vapour action of di‐allate in soil, the effect of organic matter on the availability of di‐allate, and the persistence of di‐allate and tri‐allate in Regina heavy clay. A. fatua sown at depths of 5–15 cm was killed by di‐allate or tri‐allate. as were seedlings treated at the 15–2‐leaf stage. Seedlings of spring wheat (Triticum aestivum L.) survived rates of 24 oz/ac if the shoots were 1–3 cm long prior to exposure. When di‐allate at 16 oz/ac was mixed with a 5 cm layer of Regina heavy clay (wilting point 19–6%) slightly less than 15% soil moisture was required to give complete control of A.fatua. Further experiments indicated that a large percentage of A. fatua seedlings were killed by the action of di‐allate vapour in the soil. In other tests the activity of di‐allate and tri‐allate was reduced when the percentage of organic matter in the soil was increased by adding peat. This could explain why there is little damage to wheat sown in treated soil of high organic content while similar rates can cause extensive damage in soils of low organic content. Under comparable conditions tri‐allate persisted for a longer period in the soil than di‐allate. Both chemicals exhibited a lag period after which their breakdown appeared to follow a first‐order reaction. Differences in the period of persistence were attributed to a much longer lag period and a slightly slower rate of biological breakdown for tri‐allate as compared to di‐allate. Facteurs affectant l'activité du di‐allate et du tri‐allate

A series of experiments on different soils and with varying concentrations demonstrated that the herbicidal activity of 2,3-DCDT (2,3-dichloroallyl diisopropylthiolcarbamate3) is primarily via the coleoptile rather than the root system of wild oats and wheat. The degree of this activity differed between the two species depending on the stage of development when they came in contact with treated soil, the concentration of the herbicide and the length of time that the coleoptile was in contact with treated soil. In wild oats there were two distinct periods of high sensitivity, namely, during the initial half-inch of coleoptile growth and a later stage which appeared to coincide with the initiation of the crown node. Wheat was highly sensitive at only the early period. This crop was tolerant to sufficiently high concentrations of 2,3-DCDT, when planted at least a half-inch below a surface layer of treated soil, to permit the selective control of wild oats in wheat. However, the relative placements were critical. The pattern of response was similar in each of the soils but the degree of response was distinctly higher on the heavy clay than on the loam soils.

In an attempt-to.determiue the persi

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