Some effects of 2,4-D in representative Idaho soils

The results of the first year's cropping experiments indicate that beans, peas, red clover, and alfalfa are quite sensitive to low concentrations of 2,4-D in the soil, with peas and alfalfa being more seriously injured. However, the influence of 2,4-D on the leguminous plants in soils was not nearly so marked as when the plants were grown in sand (2). This might be explained by the colloidal nature of the soil, which could fix the 2,4-D salt in an unavailable form. Whatever the explanation, the leguminous plants grew in soil at six times the concentration of the 2,4-D salt needed to limit growth in sand cultures. Further, nodulation was apparently as good in the 6 pounds per acre 2,4-D concentrations as it was in the controls, while in sand solution 0.21 pound per acre was reported practically to inhibit nodulation. The results of the second year's cropping of the soils treated with the 2,4-D salt the previous year indicate that usual field applications of 1 to 3 pounds of 2,4-D salts on soils will not interfere with planting of leguminous crops the following year. No indication of residual action of the 2,4-D salt was noted in the warm-wet or warm-dry storage of soils. Slight residual action of the 2,4-D was noted only in the soils that were stored under cold-wet conditions at the 6 pounds per acre rate. The efficiency of nodulation was evidenced by the similar protein content of the lots of leguminous plant seed produced in both untreated and 2,4-D treated soils. However, the bean seed still showed a decrease in protein content in the second crop when grown in soil treated with 6 pounds of 2,4-D. Since other evidence points to the detoxification of the 2,4-D salt between the two plantings, the reason for the continued decrease in bean protein remains obscure. Examination of the action of the 2,4-D salt on soil microorganisms indicates that the inhibition of growth is quite transitory. Growth occurred in concentration of the 2,4-D salt greatly exceeding those which might be expected to be found in the soil solution. It was noted, however, that Gram positive organisms were inhibited to a greater extent than were the Gram negative organisms.