Application of sugar beet processing by-products to agricultural land has become a common practice to reduce disposal costs. This study was conducted to estimate N mineralization in soils amended with sugar beet processing by-products. Field moist Bearden and Angus soils adjusted to 80% field capacity water content and amended with beet pulp, beet tailings, spoiled and fresh sugar beet roots or biosolids (standard) were incubated at 30°C for 120 days. A non-amended control was also included. Periodically, soil samples were removed and analyzed for mineral nitrogen (N) (NH4+-N + NO3--N). Addition of sugar beet by-products resulted in N immobilization before the start of net N mineralization for both soils. In contrast the biosolids showed net N mineralization over the entire incubation period. The data on N-immobilization and mineralization were fitted by a double exponential model. This model allowed the estimation of both potential N mineralization and potential N immobilization kinetics of sugar beet waste amended soils. The maximum amount of N immobilized varied from 25.6 to 60.3 and from 34.0 to 121.6mgkg-1 soil, respectively, for Bearden and Angus soils. Expressed as a percentage of total N added, the N mineralized (% availability index) varied from 0 to 17.3% for various sugar beet processing by-products in the two soils. The effect of different sized spoiled sugar beet pieces on N mineralization showed that as size decreased, N immobilization increased and N availability decreased.