 Rent as an economic category is regarded by Marx as one form of surplus value just like net interest income, net production taxes and industrial profits.In good part, Marx's theory is a critique of David Ricardo's Law of rent, and it examines with detailed numerical examples how the relative profitability of capital investments in agriculture is affected by the productivity, fertility, and location of farmland, as well as by capital expenditure on land improvements.Ricardo conceptualized rent income essentially as an "unearnt" income in excess of true production costs, and he analyzed how some farm owners could obtain such an extra profit because of farming conditions which were more favourable than elsewhere.