Yielding ability and weed suppression of potato and wheat under organic nitrogen management

Keywords: chickweed, early growth, leaf area expansion, light interception, light use efficiency, manure, mineralisation, modelling, organic farming, organic matter, soil nitrogen content , Solanum tuberosum L., specific leaf area , Stellaria media (L.) Vill. , Triticum aestivum L, weed suppression .Understanding how to obtain good yields and farm profits in arable organic farming systems is useful for conventional and integrated farming to decrease the current reliance on pesticides and mineral fertilisers. Two issues are of particular importance for organic farming: organic nitrogen (N) management and weed management.Optimisation of organic N management is complex because N directly affects crop growth and yield, and indirectly affects pests, diseases and weeds. Yields in organic farming are known to be 0-50% lower than in conventional farming, but it is unclear to what extent this is due to direct effects of N on growth. N management may also influence growth and reproduction of late-emerging weeds. Although they do not directly damage the crop, they may cause long-term weed management problems due to replenishment of the seed bank.The direct effects of N management on crop growth and yield of the target crops potato and wheat, and on establishment and reproduction of late-emerging weeds were investigated in a series of field experiments. Yield under organic N management strategies was explored with a model for crop growth and N dynamics in crop and soil. Nitrogen applied by slurry in amounts currently used in organic farming was found to limit growth of potato and wheat from emergence onwards. Early N limitations caused a linear decrease in the rate of early foliar expansion, while the light use efficiencies of the crops were hardly affected. Early foliar expansion rates with sufficient N were studied in more detail because they are an important base to calculate N-limited expansion rates. A combination of experiments and simulations showed that early foliar expansion rates with sufficient N could best be predicted when driven by temperature from emergence onwards up to a leaf area index of 1 for potato and 1.5 for wheat, after which they were driven by radiation. Model explorations showed that potato tuber yields under organic N management varied considerably with timing of slurry application, weather conditions and crop maturity class, and that low yields were associated with early N shortages. Increased N supply decreased reproduction of late-emerging Stellaria media in potato, but increased that in wheat. It is concluded that a basic dressing of an organic N source with a large proportion of mineral N could improve crop yields in organic farming, while row application of such manure may favour the crop relative to the weed.