 For example, a poor harvest in a major agricultural region due to adverse weather conditions, or the monopolization of the supply of farmland, could have a big effect on world market prices for farm products.Marx extends his theory of agricultural rents to building rents and mine rents, and considers the effect of rent income on land prices.This theory is the least known part of Marx's economic writings, and among the more difficult ones, because earnings from farm work can be affected by many different variables, even at a highly abstract level of analysis.