Including seeds made by licensees, about 93 percent of soybean plantings last year contained Monsanto’s Roundup Ready trait.
The relationship between market share and monopoly is a tenuous one indeed. A company can have 100% market share for all soybeans and still not have a monopoly, because of the threat of entry. That is, if they raised the price even slightly, a slew of competitors could come in and it would lose all business. Market share does not equal market power.
Indeed, it isn't even necessary for market power--only being the marginal producer is required.
We might add that while it may have a monopoly over soybeans, it may not be able to generate any inefficiency or extra profits from that monopoly due to agricultural substitutes to soybeans.