Why there is imperfect substitutability in the second stage in law of variable proportions.

The variable factors are imperfect substitute for the fixed factor.This implies that additional unit of variable factor or labour cannot compensate for the unavailability of a unit of capital.Therefore, the firm cannot substitute labour for capital and as a result diminishing returns takes place. For example, If we increase the number of labours employed on a farming land, then, after a certain point the additional labours will have no contribution in production as the land on which the yield has to be grown i.e capital is limited. 

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