Influence of organic manures on carrot (Daucus carota L.) crops grown in a long-term field experiment in Sweden Lars Kjellenberg 1 *, Eva Johansson 1 , Karl-Erik Gustavsson 1 , Artur Granstedt 2 and Marie E. Olsson 1 1 Department of Plant Breeding, Swedish University of Agricultural Science, P.O. Box 101, SE-230 53 Alnarp, Sweden 2 Biodynamic Research Institute, Skilleby, SE 153 91 Järna, Sweden *Corresponding author: lars.kjellenberg@slu.se Accepted 8 May 2015; First published online 22 June 2015 Research Paper Abstract This study evaluated the effects of organic agriculture manuring systems on carrot (Daucus carota) root morphology and sugar and polyacetylene content. Carrots were harvested three times per season 2006–2007 in a long-term field experi- ment at Skilleby research farm, Sweden. The effects of pelleted chicken manure, fresh farmyard manure and composted farmyard manure (COM) were compared against control plots left unmanured since the field experiment started in 1991. The carrotswere analyzed for root size, root shape, amount of soluble sugars and amount of falcarinol-type polyacety- lenes. Differences between manuring systems were found to be smaller than the variation between harvest years and harvest occasions, probably due to the grass-clover ley included in the crop rotation system. On an average for the six harvests, manuring with COM increased root length by 6% compared with fertilizing with pelleted chicken manure. Carrots fertilized with pelleted chicken manure also had 6–7% lower total soluble sugar content than carrots manured with 50 t ha -1 of composted or fresh manure. The falcarinol to total falcarinol-type polyacetylenes ratio was 15.4% in carrots manured with 50 t ha -1 of composted or fresh manure and 14.7% in carrots fertilized with pelleted chicken manure. Seasonal fluctuations in falcarinol-type polyacetylenes were more pronounced in carrots manured with fresh or composted manure than in carrots fertilized with pelleted chicken manure. The results suggest that manuring organic carrots with compost may be the most beneficial strategy, at least in systems where fertilizer is applied only once per crop rotation, whether directly to the carrot crop or in the preceding crop. Key word: carrot, Daucus carota, soluble sugar, root morphology, Falcarinol-type polyacetylenes, organic manuring systems Introduction Choice of manuring strategy is an essential factor in all farming systems. The strategy has to consider climate, crop rotation and both short-term and long-term objec- tives. Within the framework of organic agriculture (OA), many different manuring systems have evolved to meet different demands. Previous studies on the long-term con- sequences of OA systems have shown that differences between manuring strategies are mainly manifested in soil properties rather than in crop quality traits (Mäder et al., 2002). Some differences in crop quality have been reported (Woese et al., 1997), but these are not consistent. Increased understanding of how different OA manuring systems influence crop development and quality is important when striving to grow nutritious and tasty organic crops to an acceptable yield. The carrot plant is biennial and root growth continues as long as external circumstances permit, with no distinct period of winter dormancy (Nilsson, 1987). Deciding when to harvest a carrot crop is thus a balance between yield, nutritive quality, storability and harvesting conditions. The morphological features of the carrot taproot are sug- gested to be correlated to different quality and chemical parameters (Rosenfeld, 2003). The shape of the carrot taproot is mainly determined by the relationship between its extension in length and its increase in thickness. This gives rise to different root types, from round to conical or more cylindrical. The initial length increase of the taproot Renewable Agriculture and Food Systems: 31(3); 258–268 doi:10.1017/S1742170515000174 © Cambridge University Press 2015