DECOMPOSITION AND MOVEMENT OF HERBICIDES IN SOILS, AND EFFECTS ON SOIL MICROBIOLOGICAL ACTIVITY AND SUBSEQUENT CROP GROWTH. PART II

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Abstract
Further experiments at Edmonton to determine the effects on soils and on subsequent crop growth of copper sulphate, sodium chlorate, and sodium dichromate, are reported, together with new experiments with ammonium thiocyanate. The field plot experiments were limited to Edmonton black soil, but three typical Alberta soils, including Edmonton soil, were used in the laboratory experiments. Copper sulphate applied to a series of fallow plots in 1931 did not affect the yields of wheat on these plots significantly in 1932, or the yields of wheat, oats, rye and flax on these plots in 1933. The soil was not appreciably injured, even temporarily, by the copper sulphate. Sodium chlorate was applied to one series of fallow plots in 1930, and to another series in 1931. The effect of the heaviest applications (1300 lb. per acre) lasted for three years in one series, but serious injury to crops from such heavy applications did not last for more than two years in either series. Sodium dichromate was applied to a series of fallow plots and a series of wheat plots in 1932. It reduced the wheat yields very much in 1932, but did not reduce crop yields the following year in either series, as it decomposed and lost its toxicity in the soil rather quickly. Ammonium thiocyanate was applied to a series of fallow plots and a series of wheat plots in 1932, and it reduced the wheat yields even more than sodium dichromate in 1932. It retarded nitrification and did not decompose and lose its toxicity completely during the season of application, but even in the cases of the heavier applications (650 and 1300 lb. per acre) the toxic effect disappeared early in the following season. The total and straw yields of crops sown on these plots were generally increased by the lighter applications (160 and 325 lb. per acre) in 1933, but the grain yields were generally reduced by the heavier applications (650 and 1300 lb. per acre). Growth of certain annual weeds was considerably stimulated by this nitrogenous weed killer in 1933 and 1934, in the plots to which the heavier applications had been made. Laboratory experiments showed that the thiocyanate may be leached out of a soil with water; that it decomposes fairly rapidly in soils under favorable conditions of moisture and temperature and more rapidly in fertile soil rich in organic matter than in poorer soil; and that nitrification in soils is depressed for a time by the ammonium thiocyanate.

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