Entropy and sustainability

Soil-plant systems exchange both energy and matter with their surroundings and are consequently open systems thermodynamically. They should therefore tend towards a steady state described by non-equilibrium thermodynamics and characterized by minimum production of entropy. The theory surrounding the principle of minimum entropy production provides a good analogue of the behaviour of natural and agricultural ecosystems subjected to perturbations. Entropy-increasing processes are those that degrade complex, ordered structures of large molecular weight to small molecules such as CO2, NH3 and H2O. Processes such as photosynthesis that build small molecules into larger ones lessen entropy. These ordering processes are permitted by thermodynamic work performed when heat is transferred from the sun. They depend critically on the capacity of the system for self-organization, which is identified with its biological potential. Several of the small molecules are environmentally undesirable in excess. This, together with the theoretical considerations above, suggests that minimum production of entropy should be a criterion of sustainability. It implies that agricultural systems should be allowed to become steady states where possible and that maintaining the biological potential is essential. An 'audit of small molecules' is suggested as a way of assessing sustainability.