We study an optimal control problem of crop irrigation and fertigation with treated wastewater that contains vital crop nutrients, with the objectives of maximizing crop production and minimizing environmental and farming costs. A double modelling method is proposed that allows to use both a modern detailed crop model-the simulation model-together with a low-order dynamical systems model-the control model. The method is based on the design of the control model which, after calibrating its parameters, is capable of reproducing simulations of the complex model for a range of controls. With an dedicated dynamic programming algorithm we can then solve the optimization problem on the control model. We show that irrigation and fertigation strategies obtained as optimal controls for the reduced crop model behave well on the simulation model. This methodology allows to obtain efficient and simple controls that could be applied in practice.