The demand for a factor which may be land, labour or capital is a derived demand. Land, say for instance, is demanded for its produce. The higher the produce, the greater is the demand for land. A firm will pay rent equal to the marginal revenue productively of land. The rent diminishes as more rand is used due to the operation of law of diminishing returns. The demand curve of a factor is, therefore, negatively sloped which means more land will be used only at lower rents, other things remaining the same