We utilize Viability theory to evaluate the effects of CAP. A differential equation describes the dynamic development of soil productivity. If farmers do without entitlement, they are free in soil handling. And as we assume they are short-term profit maximizers, they miss to pay attention to soil conservation. Soil productivity is at risk; correspondingly, economic sustainability is at risk. But if farmers activate their entitlements, they become obliged to soil conserving measures. The model demonstrates that the decision to participate or not at the "entitlement & cross compliance"-program, depends on payment-level and the effects of the decision depend on the dynamics of the environmental system.