Arnout van Delden*

Group of Crop and Weed Ecology, Wageningen Univ., P.O. Box 430,6708 AK Wageningen, the Netherlands, and Plant Res. Int., P.O. Box16, 6700AA Wageningen, the Netherlands. Current address, CentraalBureau voor de Statistiek, Kamer 451A, P.O. Box 4000, 2270 JMVoorburg, the Netherlands.

Abstract

In order to optimize N management in organic farming systems,knowledge of crop growth processes in relation to N limitation isnecessary. The present paper examines the response of potato (Solanum tuberosum L.) and wheat (Triticum aestivum L.) to N withrespect to intercepted photosynthetically active radiation (PAR), lightuse efficiency (LUE), and leaf N concentration ([N]). Potato andwheat 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 mineralNfertilizers (N3). Estimated availableN 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. Anincreasing N limitation in potato (NNI =1–0.55) resulted in a lineardecrease in crop dry weight and cumulative intercepted PAR and in a linear increase of the harvest index, whereas theLUE decreased onlyat NNI values below 0.65. Crop dry weight and cumulative interceptedPAR for wheat decreased linearly with N limitation (NNI =0.9–0.6),but the harvest index and LUE were unaffected. For both crops, Nlimitation 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].