Aims Plant available soil nitrogen (N) may affect deep root growth and soil N depletion by catch crops. We investigated the influence of topsoil N availability on root growth and uptake by fodder radish. Methods We conducted field and greenhouse experiments of root growth and late autumn N uptake at medium and high soil N availabilities, and root N inflow at medium and deep soil depths (15N injection) in sandy loam and loamy sand, using the minirhizotron method in the field and rhizotrons in the greenhouse. Results High soil N availability resulted in lower root intensity in the field, but higher root intensity in the greenhouse experiment under both soil types. Radish had deeper roots and higher root intensity in sandy loam than in loamy sand. High soil N availability caused lower 15N uptake at both soil depths in the field and lower N inflow rates at both soil depths in field and greenhouse. At medium soil N availability in the field, N inflow was twice as high in the deep than in the medium depth. Conclusions Higher soil N availability affects root growth and decrease N inflow, thus lowering late autumn N uptake and hampering deep N exploitation by fodder radish. At medium soil N availability, the potential for N uptake in deep soil is higher probably due to younger roots than at a medium soil depth. The shallower and less dense root growth in loamy sand is probably due to its lower clay and higher P contents.