This paper describes a model to represent human activities in farming systems based on the situated action theory. The idea is to consider the environment as an intelligent entity that has the ability to decide locally, at given time and space, the actions to perform. Surrogates of real actors, called actuators, just execute the actions dictated by the agents embedded in the environment. The model is based on the affordance and stigmergy concepts as well as a multiagents modelling approach.