Opinion time: If you were going to block-plant hobby scale quantities of either bush P. vulgaris or soy beans, what spacing(s) would you use? I feel like most of the spacing online and in books has more to do with harvesting equipment in farm fields than actual plant needs.
My 3 C's (I always find it hard to stop at 2
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There are so many factors that go into spacing that IMO there is no one-size-fits-all answer. Among others, factors I would consider are the growth habit of the variety, whether the plants will be irrigated or not, weed pressure, endemic diseases, rodent pressure, harvest access & method, climate (especially during dry down) and whether the blocks will be tended or mostly neglected. If seed is in short supply, wider spacing may be required to get the best yield per plant.
For dry beans, in garden culture, I would prefer wide rows (with full access from the two sides) to wider blocks. In my climate (where more often than not, I'm picking pods as they dry down in between storms) I would not space rows closer than 12" apart, due to risk of disease or mold. I've used final spacing of 8" between plants, and 2 @ 12". In irrigated garden culture, the more rampant bush varieties may need 12" between plants each way. In drier climates, or if the blocks will be untended, you might be able to space plants more closely, to both conserve moisture & hold down weed growth. Closer spacing will tend toward the optimal yield per square foot; but increases the risk of "boom or bust", and is less efficient in terms of yield vs. seed planted.
IMO if pods will be harvested by hand (vice harvested by pulling entire plants) wide rows should be used. The same applies if weed pressure is high enough that weeding at least once will be necessary, or if rodent pressure will require intervention. If the beans will be neglected after planting & irrigated only by rainfall, then wider block planting may be most effective.
Most soybeans available commercially are edamame varieties. Those tend to be in the 24" +/- 6" height range, and generally have sturdy upright stems; but soybean height (and resistance to lodging) are strongly influenced by spacing. Wider spacing = shorter, stockier plants. Because edamame soybeans are generally harvested by cutting/pulling entire plants, they can be grown in either wide rows or blocks... but because seed size & seeds per pod are important for edamame, spacing is much more critical. In rows, I thin early edamame soybeans to 3-4", and later more vigorous varieties to 5-6". SSE appears to use about 12" between plants each way, which causes the plants to become short, bushy, well branched, and with a very heavy pod load. Not very efficient space-wise, but outstanding quality for edamame. I would do that if I had space.
Field soybeans tend to be well supported also, but most varieties now are GM; few non-GM varieties are available commercially (which is the reason I began collecting them). Some of the higher-yielding OP varieties can be TALL. Some may even be semi-twining around each other, which helps support them off the ground - but makes manual harvest challenging. For seed saving, I actually tie taller soybeans up for support, to prevent them from being knocked down to ground level (where rodents will harvest them). Since I seldom plant more than a double row (18-24" apart) and since plant spacing can vary widely, I have no experience in block plantings. in general though, earlier varieties will be shorter & can be spaced more closely (perhaps 6-8" each way) while later, taller varieties may need wider spacing.
As
@Pulsegleaner pointed out, pod shattering can be an issue with soybeans. Some varieties are more prone to this than others, and in my experience, the degree of shattering (or not) seems to vary year-to-year. In the worst case, a large percentage of seed can be expelled onto the ground; so regardless of how the plants are spaced, they should be watched closely when plants yellow & pods begin to dry down.
Rodents are a major issue with soybeans. In wide blocks (or even wide rows) they will nest under the dense center growth, and unless dealt with, will destroy much of the crop. If that applies where the soybeans will be grown, and pest control is necessary, blocks may not be an effective strategy for soybeans. Farmers can get away with a relatively small loss percentage wise; but in small lots, the loss can potentially be total. I have traps under my ripening soybeans now.
Did you know that they will remain dormant until Spring, but will become active if there are vibrations?
