If farming practices are degrading the land by enhancing salinsation and soil erosion, and the appropriate way to deal with the problem is by planting trees or regenerating native brush, farmers are faced with several questions. Assuming a rather homogenous area where cropping yields have been decreasing, when should a farmer stop cropping and start planting? Conversely, when should cropping be resumed? On a long term basis, what is the economically optimal pattern between cropping and land rehabilitation? How do differences in soil quality and related production and rehabilitation rates affect management outcomes? The basic question is first investigated in terms of optimal switching times, when switching incurs a cost, then optimality conditions are given the real situations farmers face. Namely, spatial synergies, such as alley-cropping, are likely to be preferred to temporal solutions. But the time dimension will never be absent from sustainable farm management.