No idea what the GP means by marginal crop land, but yes, people tend to grow cattle on land that isn't very good for crop, for one reason or another (the most common reasons being a rocky terrain or a non-plane surface, poor soil is usually not such a reason as soil is always fixed anyway).

> My understanding is that rainforests take up so many nutrients that the soil is actually very poor and unsuitable for most kinds of agriculture.

That's a characteristic of the Amazon forest specifically (and not even all of it). The soil is poor because it's an old land with a lot of raining, that carries the nutrients away, it's not a fault of the forest. Other rain forests have different characteristics.