I get wanting to separate N and O because they are not related to each other, but that seems dangerous in practice. If M occurs, then BOTH N and O must occur. They are triggered by the same sufficient condition. I prefer to use parentheses to indicate that M is sufficient for both:

M ---> ( N & O )

I'm new at this, but the way I understand it is that "contextual argument" implies that the context is crucial, not just to understanding the argument, but to the argument being an argument at all.

The conclusion is not just "scientists have recently discovered a new way by which the plant can absorb phosphorus, a much-needed nutrient" --- it can be understood as this statement's opposition to the context provided earlier. Placing "it seems unlikely there would be much left to learn about what makes it thrive" before the conclusion, you find that the author is making a point about the scientific observation. The point being the "subtext" that there IS something left to learn about what makes plants thrive. Thus, the premise "wheat that grows near deserts has been observed to extract phosphorous directly from the dust covering its leaves" is evidence toward the provided conclusion (which looks like a piece of context at its surface) but is ALSO evidence toward the conclusion that there is more to learn about what makes plants thrive. I hope this helps!