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Influence of 2,4-D on the soil microflora Westlake, Donald William Speck

Abstract

A rhizosphere effect was demonstrated for barley and corn plants at 4 to 6 weeks of age. No qualitative difference between the composition of the rhizosphere and indigenous flora was detected in the response of isolates to nutritional media of varying complexity. Examination of the rhizospheres of barley plants treated with 2,4-D at the rates of 0, 4 and 8 ounces per acre indicated that there was no difference between the total counts of treated and untreated plants. However, evidence was obtained suggesting a qualitative difference between the rhizospheres of the 8 ounce treated plants and those of the 0 and 4 ounce treated plants. On the other hand, corn plants treated with 2,4-D at the rates of 1 and 2 pounds per acre showed a temporary increase in their total rhizosphere count as compared to the total rhizosphere count of untreated plants. The increased total counts appeared to be due to the stimulation of those organisms requiring amino acids and yeast-soil extract for maximum growth. The direct application of 2,4-D to soil at the rate of 100 pounds per acre appears to result in a slight decrease in the total indigenous count. Although this lowering of the total count was due to a decrease in the number of organisms in all nutritional groups, some groups were affected more than others. By comparing the growth response of isolates from the indigenous and rhizosphere flora to 2,4-D at different pH levels, evidence was obtained which indicated that a physiological difference exists between these flora. A study of the rates of the decomposition of 2,4-D in different soils indicates that there is a marked variability in the detoxication rates. An organism was isolated which was capable of decomposing 2,4-D, This isolate showed morphological and cultural characteristics which suited members of the Corynbacterium and Achromobacter genera.

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