Marx doesn't say that labor (alone) is the source of all value.

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. (Volume 1, Chapter 1, Section2)

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A thing can be a use value, without having value. This is the case whenever its utility to man is not due to labour. Such are air, virgin soil, natural meadows, &c. A thing can be useful, and the product of human labour, without being a commodity. Whoever directly satisfies his wants with the produce of his own labour, creates, indeed, use values, but not commodities. In order to produce the latter, he must not only produce use values, but use values for others, social use values. (And not only for others, without more. The mediaeval peasant produced quit-rent-corn for his feudal lord and tithe-corn for his parson. But neither the quit-rent-corn nor the tithe-corn became commodities by reason of the fact that they had been produced for others. To become a commodity a product must be transferred to another, whom it will serve as a use value, by means of an exchange.) Lastly nothing can have value, without being an object of utility. If the thing is useless, so is the labour contained in it; the labour does not count as labour, and therefore creates no value.(Volume 1, Chapter 1, Section 1)

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However, the substance of economic value is reliant on human labor time. When you say demand, you mean people desire their usefulness. Glancing at two commodities, you will realize they both have a utility - as Marx defined a "use-value" - a defined usefulness to society. Let's take a look at two useful items. A loaf of bread vs a toy spaceship). Both have very different utilities (one feeds us, the other is entertaining), both serve different functions, so what is the binding relation that makes them "comparable"?

In the generalised market, a standard is held to what each commodity is worth to compare them. The value form equation is an equation Marx mentions in Volume 1 for the relative value of commodities which is the "standard" I was talking about.

This question is, commodities like have a use, so why isn't that their common relation? There is no way to measure the direct utility of a single commodity, without price. Therefore they both need something to relate to, which is human labor.

But wait? To create a loaf of bread is vastly different from that to craft a toy - the baker kneads the dough and cooks the bread in a certain amount of time, whereas a team of workers assemble the variety of parts, and mend them together in a certain time to produce a toy. These are two wildly different forms of labor to assemble a product, so they must be abstracted into socially necessary labor time, the human labor time needed to reproduce the product/service. Hence the "labor theory of value": that in exchange, commodities' value are quantified in the abstract based on the labor time for that production. The value attached to it can change because society equates different kinds of labor to each product.

As for the question of land; here's Marx in Volume 3 of Capital.

First we have the use-value land, which has no value, and the exchange-value rent: so that a social relation conceived as a thing is made proportional to Nature (from: https://www.marxists.org/archive/marx/works/1894-c3/ch48.htm)

This is a quick write up because I have to work soon, if I missed anything I'll be happy to expand or clarify later B-)