Do you recall what section that part on value from rare/valuable land was from? I'm still behind in my reading and it's been a while. I remember this part from ch 1:

The use values, coat, linen, &c., i.e., the bodies of commodities, are combinations of two elements – matter and labour. If we take away the useful labour expended upon them, a material substratum is always left, which is furnished by Nature without the help of man. The latter can work only as Nature does, that is by changing the form of matter. Nay more, in this work of changing the form he is constantly helped by natural forces. We see, then, that labour is not the only source of material wealth, of use values produced by labour. As William Petty puts it, labour is its father and the earth its mother.

And then for sure in Volume 3 there is a long discussion on differing qualities of land, but this fits into the rent part of the analysis (it's later and we are not there yet in Marx's build-up of concepts). Just to foreshadow that part, Marx describes a differential of rents based on how good the land is and how much value can be produced with it.

To me, as per the ch 1 quote above, Marx is describing a precondition for producing a use-value with concrete labor. Remember that producing a use-value is an inescapable requirement of also producing a value, since value must have some bodily form to possess. So Marx is making this simple trans-historical statement not only about labor under capitalism, but all human labor: in order for humans to make things, we need to find existing material things and alter them with our labor.

It's kind of mind-bending, but several parts of the argumentation in these chapters is starting from an end result, and then describing what things must have been necessary to reach this point. Rather than starting with ingredients and describing a recipe how to make a commodity, Marx takes the commodity and describes the preconditions necessary to make it. So what looks like he's making wild assumptions ("why can he assume it's socially necessary labor?"), is actually a result of this direction of the analysis.

For example, this is the direction of the first chapter: Wealth in present society takes the form of commodities. To have a commodity, you need a use-value which was exchanged for some other use-value. Ok, what is a use-value? Something which is the product of a definite type of labor, using physical matter found in the world. Ok, it's exchanged? Then it must have been socially necessary. He doesn't make moral or value judgments about WHY it's socially necessary, he is simply stating the fact that it was already exchanged, therefore socially necessary.

Contrast this with a "bottom-up" approach, trying to design a commodity by first starting with concrete labor. A baker bakes, a barber cuts hair, and a carpenter makes furniture. To try to back into Marx's analysis, you have to try to explain how the baker 1) is performing socially necessary labor 2) is performing labor in the abstract, not his specific labor 3) calculates the "value" in his product during exchange. This is basically a problem that classical economists ran into (and to whom Marx is responding), because they were trying to give all sorts of definitions for how value is calculated, but it's exactly backwards compared to how Marx is analyzing it. The problem is that the classical economists are starting the analysis already making the assumption that value exists. But this has the problem of making value into some ahistorical phenomenon, and therefore missing the entire history of capitalism and making it impossible to describe how capitalism develops and changes.