抄録
l. The crab grass occurs as a dominant weed on both thes weed potato and soybean fields. Both crops are susceptible to 2, 4-D, but the crab grass is resistant to this herbicide. Post-emergence spray of 2, 4-D on these crops, therefore, cannot be expected as a practical means. 2. During 1948 to 1951, field tests of weed control with 2, 4-D were made: (a) pre-planting treatment, (b) pre-emergence treatment and (c) between-row spray. 3. 75 to 150 grams of 2, 4-D per l0 ares were sprayed on the ridged beds, and after a rain, the sweet potato slips were planted. All of the slips took root and the growth of the crop was not retarded. Its yield equalled that of the weeded plot, with the weeds reduced to 1/1O ∼1/3 of those in the untreated plot. 4. When sweet potato slips were planted immediately after 2, 4-D was applied, the crop was severely injured, particularly in the drought year. But, those roots that were moistened and dusted with active carbon were delivered to some extent from the 2, 4-D injury. 5. The pre-emergence spray of 150 grams of 2, 4-D per 10 ares was considerably effective for the weed without decreasing the yield of soybean. A spray of more than 150 grams was injurious for the growth of the soybean. Seeds coated with active carbon prior to sowing was not injured even by the spray of 450 grams of 2, 4-D. This was attlibuted to the protecting action of the active carbon. 6. The soil treatment with 2, 4-D between the rows, after tillage, cares being taken in avoiding splash of the herbicide on foliages of the crop, was more effective on the annual weeds than pre-emergence tratment. The soybean plant was not injured by the applications of l00 grams of 2, 4-D per l0 ares on dry soil and 200 grams on moist soil. The yield decreased as the amount of 2, 4-D given increased.
