Q.What is meant by the "marginal" or "no-rent" land in Ricardo's theory of rent? Why is rent described as a "price-determined" income rather than a "price-determining" one?

The marginal land is the least fertile land actually under cultivation, and it earns zero rent by definition; this is exactly why Ricardo insisted rent could never be a CAUSE of a crop's price.

(worked out below)

As population grows and demand for food rises, cultivation is progressively extended from the most fertile land available to progressively LESS fertile land. The marginal land (also called the no-rent land) is the LAST piece of land brought under cultivation to meet this demand — it is just productive enough to cover its own cost of cultivation, with no surplus whatsoever left over, and therefore earns zero rent by definition.

Ricardo used this concept to make a deliberately counter-intuitive point about causation. Since the market price of the crop must be high enough to make it worthwhile to cultivate even this marginal, least-productive land, the PRICE is fixed by the marginal land's own (relatively high) cost of production. Rent, on every grade of land BETTER than the margin, is then simply the surplus that grade earns over and above what the margin earns, valued at this ALREADY-DETERMINED price. Since price comes first and rent is calculated only afterwards, as a consequence of price, Ricardo concluded that rent is price-DETERMINED — it never enters into, or causes, the price of the crop, which is the opposite of what casual observation might suggest (that high rents make food expensive).