In order to optimize N management in organic farming systems, knowledge of crop growth processes in relation to N limitation is necessary. The present paper examines the response of potato (Solanum tuberosum L.) and wheat (Triticum aestivum L.) to N with respect to intercepted photosynthetically active radiation (PAR), light use efficiency (LUE), and leaf N concentration ([N]). Potato and wheat cultivars were grown in field experiments (1997 and 1998) at three N levels: no N (N1), cattle (Bos taurus) slurry (N2), and cattle slurry supplemented by mineral N fertilizers (N3). Estimated available N from the soil (0-0.9 m) plus added fertilizer was 80 (N1), 150 (N2), and 320 (N3) kg ha-1 for potato and 115 (N1), 160 (N2), and 230 (N3) kg ha-1 for wheat. Nitrogen deficiency was quantified by an N nutrition index (NNI; 1 = hardly limited, 0 = severely limited). Nitrogen deficiency increased in the N1 and N2 treatments up to 20 (potato) and 50 (wheat) d after emergence, with small changes thereafter. An increasing N limitation in potato (NNI = 1-0.55) resulted in a linear decrease in crop dry weight and cumulative intercepted PAR and in a linear increase of the harvest index, whereas the LUE decreased only at NNI values below 0.65. Crop dry weight and cumulative intercepted PAR for wheat decreased linearly with N limitation (NNI = 0.9-0.6), but the harvest index and LUE were unaffected. For both crops, N limitation to 0.55 caused a linear decrease in maximum leaf area index, the rate of foliar expansion, leaf area duration, and to a lesser extent, leaf [N]. In conclusion, both crops respond to N limitation by reducing light interception while maximizing the LUE and leaf [N].