We think there are at least two distinct reasons that a long-term sod can improve soil quality without additions of off-farm organic matter. First, the constant growth and dying off of a large perennial root system adds organic matter to the soil on a continuous basis. Secondly, the combination of permanent ground cover and undisturbed soil protects this slow-but-steady accumulation of humus from oxidation. No wonder that perennial grass-legumes mixes have played an essential role in traditional farm systems.