Growing catch crops is an effective measure for reducing nitrate-N leaching during autumn and winter. In this study we investigated 1) how effective different catch crop types are over time, 2) how deep they can extract N with their rooting system and 3) the effect of field conditions. The two catch crops under investigation were Italian ryegrass (Lolium multiflorum Lam.) and white mustard (Sinapis alba L.). They were sown the 29th of August 2008 at three different locations (headland, normal and wet conditions) in a sandy loam field plot (Terric Anthrosol) in Merelbeke (Belgium) on wheat stubble. At each location, a randomised block design was established with three replicates. Apart from the catch crops, there was a third treatment with bare plots. Mineral N in the soil profile (0-90 or 0-210 cm) and above and below ground biomass parameters were measured in September, October, November and February. N-uptake in October was higher for white mustard than for ryegrass because of the fast early crop development. This difference had disappeared in November. The nitrate-N content in the 0-90 cm soil profile (residual N) did not change between October and November for white mustard and fallow, but decreased for ryegrass in that period. The maximum rooting depth for both crop types was 50-60 cm, except for the headland where, depending on the precise location, the crops could only extract N from the 0-25 or 0-45 cm top soil. Remineralisation from white mustard in winter was demonstrated.