The absolute hardest sci-fi I've ever seen is probably near-term cyberpunk or stuff like The Martian. That stuff is pretty rare. Even most "hard" sci-fi requires some suspension of disbelief. The difference between hard and soft is whether one must go entirely into the realm of fantasy to suspend that disbelief, or whether it just requires the assumption of non-linear advances in technology or superhuman intellect.

The Martian's take on potatoes is not very hard at all. Not only is Martian soil loaded with percholrates, it would have taken far longer than shown to prime the soil with enough microbes to support the growth of so many potatoes.