Aside from the 0.4 acres of soybeans possibility (or lack thereof) for growing old enough to yield forage and/or beans - and to beat a horse dead already done in several other threads regarding legume inoculation - since it is soybean planting season - let's repeat the beating.
1. Soil contains many different strains of bacteria of the kind not harmful to humans but that are beneficial in the breakdown of soil organic matter into useful forms.
2. Legumes are susceptible to being invaded by these bacteria. But, it's not a bad thing. It's a symbiotic relationship.
That is, this co-habitation is beneficial for both the bacteria and the legume. Legumes do NOT manufacture nitrogen, the attached bacteria does. It's a by-product used by the legume. The deal for the bacteria is a steady supply of carbs provided by the legume. You will see this coupling on the roots of the legume - the little lumps or nodules, hopefully, on the legume roots.
The inoculant contains the right species of bacteria because certain strains have a better relationship with specific legumes. If the soil your planting into has a population of the right kinds of bacteria (likely true) then adding inoculant to your planter box is unnecessary. But how do you know? You don't. To inoculate or not to inoculate. Use an ounce or 10 ounces or 10 lbs. Or use none and you'll probably be OK.
What's the worst outcome? If the plant doesn't nodulate, you'll just need to provide some N fertilizer or let the plant use what's becoming available in the soil as organic matter breaks down.