A study in Iowa (Larson et al. 1972) with continuous corn produced on a silty clay loam over a twelve-year period indicated that 6 t/ha (2.7 tons/ac) of residue (dry weight basis) was required to maintain the organic matter level in the soil. Under any cropping system, organic matter levels reach a state of equilibrium, but whether or not a state of equilibrium was achieved in the above experiment would be determined only if the experiment was conducted or a very long time.