Introduction
Edamame or vegetable soybean (Glycine max [L.] Merrill) is a specialty soybean that is harvested as a vegetable when the seeds are still immature. It is usually sold as pods or as whole harvested stems but occasionally as shelled beans. The seeds are usually boiled in the pod, shelled out, and eaten as a snack, as a vegetable with meals, or added to soups or confections. When eaten as a vegetable, the seeds are added to salads, stir-fried, or combined into vegetable medleys. As a sweet, the beans are ground to a paste, sweetened, and used as a topping for sticky rice. Most production and consumption is in China, Japan, Taiwan, and Korea. Most production research, variety development, and production guides originate in Taiwan and Japan.
Washington State University has conducted variety trials and is involved in a breeding program for improved varieties of edamame for the Pacific Northwest. Very little information on production techniques for the coastal areas of the Northwest is available. Information derived from the Asian literature is often an inadequate guide to production in our area. For example, the Japanese literature recommends application of 35 to 55 pounds N/acre in addition to 8 tons/acre of well-decomposed animal-waste compost which provides an unknown amount of available nutrients (O'Rourke, 1994). Recommendations for plant populations are not consistent. If this crop is to be grown in the Pacific Northwest, which appears to have a good climate for production of high yields and quality, we will need to develop recommendations based on the soil types and cultural methods likely to be used in this region. The purpose of this trial was to investigate the effects of two, between-row spacings and three rates of applied N on three edamame cultivars previously determined by Carol Miles to have the potential for high yield west of the Cascades.
Methods
Non-inoculated 'Butterbeans', 'Shironomai', and 'White Lion' edamame were seeded to a winter-fallowed Willamette silt loam, pH 5.8, at the NWREC on 3 June. Plot preparation included plowing, disking, a broadcast and incorporated application of triple superphosphate and sulfate of potash, each at 200 pounds/acre, and harrowing to form a seedbed. The three cultivars were seeded on 20 and 30-inch rows, with 3-inch in-row spacing. Metolachlor, 1.5 pounds/acre, was applied after planting for weed control. Escaped weeds, mostly sowthistle, prickly lettuce, groundsel, and red-root pigweed, were controlled by hand-hoeing. Nitrogen rates applied were 0, 36, and 72 pounds/acre as ammonium nitrate, with half the N applied one week after seeding and the remainder on 8 July, near first bloom. The experimental design was a randomized complete block split plot, with cultivar-spacing combinations as main plots and N rates as subplots. Subplot size was 15 - 20 feet. Plots were sprinkler irrigated as necessary, usually approximately one inch/week. Yields were estimated by harvest of a 10-foot section of one of the centermost rows of each subplot. Pods were stripped by hand, separated into categories of two or more beans/pod, one bean/pod, and unmarketable (mostly lacking developed beans). Cultivars differed in maturity with 'White Lion' harvested on 8 September, 'Shironomai' on 16 September, and 'Butterbeans' on 21 September.
Results
Cultivar. The cultivars differed significantly in plant development (Table 1). Stand of 'Shironomai' was nearly 30 percent less than that of 'Butterbeans' or 'White Lion'. 'Butterbeans' and 'White Lion' produced plants from about 63 percent of the planted seed, while the establishment rate for 'Shironomai' was only 44 percent. 'White Lion' produced a slightly shorter and narrower canopy (measured 30 July) and bloomed earlier than the other two cultivars. Canopy height of 'White Lion' did not increase with increasing rate of N to the same extent as for the other cultivars (Table 2).
Despite its relatively poor stand, 'Shironomai' produced a higher yield/acre, higher yield of large pods (2 or more beans), and larger pod weight/plant than did the other cultivars (Table 3). A greater pod weight/plant more than offset the reduced stand. 'Shironomai' also tended to have the heaviest pods. 'White Lion' produced the smallest total and marketable yield.
Although there were significant interactions of cultivar and N rate affecting total pod weight/plant and marketable pod weight/plant (Table 4), 'Shironomai' was the highest yielding cultivar at each N rate. The interaction of between-row spacing and cultivar also affected total and marketable pod weight/plant (Table 5) but 'Shironomai' was the highest yielding at both spacings.
Spacing. The 20-inch spacing produced a greater stand per meter of row than did the 30-inch spacing (Table 1). This is because one of the two planters per bed was not properly covering seed and stand was reduced on this row for the 30-inch spacing. With 20-inch spacing, only one of three planters did not adequately cover the seed, so mean stand was greater. The main effect of spacing on canopy development on 30 July and on bloom was not significant. However, spacing and N rate interacted to affect bloom (Table 6). Bloom was advanced at the lowest rate of N for 20-inch but not for 30-inch spacing.
The 30-inch spacing produced a higher yield/plot (per meter of row) but a lower yield on an area basis (Table 3). Mean weight of individual pods was higher at 30-inch spacing as was total and marketable pod weight/plant. But the nearly 50 percent higher number of plants/unit area at the 20-inch spacing offset the greater per- plant yield obtained with the wider spacing.
N Rate. Rate of N had no effect on stand, but canopy height and width increased linearly with increasing N (Table 1). Increasing N also delayed flowering.
Total weight harvested/plot, yield/unit area, weight of marketable pods/plot, mean pod weight/plant, and mean marketable pod weight/plant increased linearly with increasing rate of N. However, mean weight of the individual pod did not increase from the intermediate to the highest rate of N. The number of beans/pod, mean bean weight, and number of plants harvested per plot did not vary with N rate (Table 3).
Although the main effect of N rate on total and marketable pod weight/plant was a linear increase with increasing rate of N, not all cultivars were consistent in their response to N (Table 4).
There were no 3-way interactions of N rate, spacing, and cultivar affecting plant development or yield.
Conclusions and Discussion
For the purposes of this experiment, marketable yield was defined as those pods acceptable for fresh marketing in-pod. However, pods with only one bean may be marketable as shelled green edamame or as seed for planting. Yields of both one-bean pods and pods with 2 or more beans responded similarly to treatment.
Given that complete canopy closure was not obtained with the 30-inch spacing, that mean bean weight was not affected by spacing, and that mean pod weight was only slightly lower at the 20-inch spacing, 20 inches appears to be a reasonable between-row spacing for these cultivars on this Willamette soil. Although mean pod weight, number of beans/pod, and mean bean weight did not increase between the intermediate and high rates of N, yield/plant and per unit area did increase between the intermediate and high rates, indicating that 72 pounds/acre may not be sufficient for maximum production in a situation where the seed was not inoculated, the soil had not recently been used for production of legumes, and there was not a history of application of manures or composts. Although not quantified, we noticed a strong tendency for a high proportion of the plants to have nodules at the zero N rate, but not at other N rates. These plants, although dark green in color, did not exceed non-nodulated plants in size. Inoculated seed might have responded differently to N rate.
Literature Cited
O'Rourke, A.D. 1994. Edamame: the vegetable soybean. p. 173-199. In: A.D. O'Rourke (ed.). Understanding the Japanese food and agrimarket, a multifaceted opportunity. Haworth Press, New York.
Table 1. Main effects of cultivar, between-row spacing, and rate of applied N on stand, plant height and width, and flower development in edamame, NWREC, 1998. Seedlingsz/ Canopy heighty Canopy widthy Plants in bloom 6 m inches inches 23 July 30 July 6 Aug. Cultivar ----------%---------- Butterbeans 49 19 17 0 32 96 Shironomai 35 19 17 26 15 97 White Lion 50 17 15 0 73 99 Significance *** *** *** *** *** NS Spacing 20 inches 48 18 16 9 44 98 30 inches 42 18 17 8 35 98 Significance *** NS NS NS NS NS N rate, lb/acre 0 45 17 15 12 52 99 36 44 18 17 7 35 98 72 45 19 18 8 32 96 Significance NS *** *** NS *** NS ***,NSSignificant at 0.1% level and nonsignificant, respectively. zCounts made 18 June. yMeasured 30 July. Table 2. Interaction of cultivar and rate of applied N, averaged over two spacings, on canopy height of edamame, NWREC, 1998. Cultivar N rate, lb/acre Canopy height, inches Butterbeans 0 16.1 36 19.5 72 19.9 Shironomai 0 17.9 36 18.9 72 19.4 White Lion 0 16.4 36 16.5 72 17.0 LSD, Significance 1.0*** ***Significant at 0.1% level of probability. Table 3. Main effects of cultivar, between-row spacing, and rate of applied N fertilizer on edamame yields, NWREC, 1998. Total wt. Unfilled 1 bean/ 2+ beans/ Wt. 100 No. of Mean No. of Pod wt./ Marketable g/ tons/ pods pod pod pods beans/ bean wt. plants/ plant pods/plant plot acre g/plot g/plot g/plot g 25 pods g 3 m g g Cultivar Butterbeans 1353 3.1 56 433 864 259 57 0.60 26.5 53 34 Shironomai 1551 3.6 137 431 974 278 54 0.52 17.7 91 58 White Lion 1136 2.6 56 340 718 251 NR NR 25.0 46 29 Significance *** *** *** ** *** NS * * *** *** *** Spacing 20 inches 1154 3.3 60 338 745 249 56 0.55 22.1 54 35 30 inches 1539 2.9 106 464 958 276 56 0.58 24.0 72 45 Significance *** ** *** *** *** * NS NS NS *** ** N Rate, lb/acre 0 1101 2.5 42 325 726 251 57 0.55 21.8 53 35 36 1400 3.2 86 433 874 269 55 0.57 24.5 65 41 72 1539 3.5 120 446 956 269 56 0.56 22.9 71 45 Significance *** *** ** *** ** * NS NS NS ** * *,**,***,NSSignificant at 5, 1, and 0.1% probability level, and non-significant, respectively. Table 4. Interaction of rate of applied N and cultivar, averaged over two spacings, on mean pod weight per edamame plant, NWREC, 1998. Applied N, lb/acre Cultivar Total pod wt./plant Marketable pod wt./plant ---------------------g----------------------- 0 Butterbeans 51 36 Shironomai 70 44 White Lion 38 24 36 Butterbeans 44 27 Shironomai 105 65 White Lion 48 30 72 Butterbeans 63 38 Shironomai 100 63 White Lion 51 32 LSD, Significance 16* 11* *Significant at 5% level. Table 5. Interaction of between-row spacing and cultivar, averaged over three rates of applied N, on the mean pod weight per edamame plant, NWREC, 1998. Row spacing, inches Cultivar Total pod wt./plant Marketable pod wt./plant ---------------------g----------------------- 20 Butterbeans 47 31 Shironomai 72 45 White Lion 44 29 30 Butterbeans 59 37 Shironomai 111 70 White Lion 47 29 LSD, Significance 13** 9* **,*Significant at 1 and 5% levels, respectively. Table 6. Interaction of between-row spacing and rate of applied N, averaged over three cultivars, on flower development of edamame, NWREC, 30 July, 1998. Spacing, inches N rate, lb/acre Plants in bloom, % 20 0 64 36 33 72 35 30 0 39 36 37 72 29 LSD, Significance 15* *Significant at 5% level.