  Plant-soil negative feedback (PSNF) is the rise in soil of negative
conditions for plant performance induced by the plants themselves, limiting the
full potential yield and thus representing a loss for the agricultural
industry. It has been recently shown that detrimental effects the PSNF has on
the growth of plant's biomass can be mitigated by periodically intervening on
the plant/soil system, for example by washing the soil. The periodic control
inputs were computed by using an average model of the system and then applied
in open-loop. In this paper we present two feedback control strategies, namely
a PI and a MPC-based controllers, that, by adapting online the duty-cycle of
the periodic control input, guarantee precise regulation of the biomass yield
and at the same time robustness to unavoidable modeling errors and
perturbations acting on the system. The performance of the proposed control
strategies is then validated by means of extensive numerical simulations.
