Differences among six winter wheat varieties regarding N efficiency (NE) and its components were assessed in field trials in four locations over 3 years under conditions of organic farming (OF). N uptake and utilization efficiency, redistribution of N vs. direct N uptake during grain filling, N uptake in three periods (tillering, stem elongation/heading and grain filling) and the quantity of mineralized N during the same sub-periods were determined. Significant differences for these traits and significant interactions among varieties and environments could be detected for NE and its components. Limiting N availability during grain filling was typical for the more extensive organic environments. Under these conditions, differences of NE could be attributed to differences in preanthesis uptake and in translocation from vegetative tissues to the developing grain. Pre-anthesis uptake contributed more to N efficiency than translocation efficiency. Under more favourable conditions, differences became more evident and were mainly due to direct uptake during grain filling. This confirms, that different varieties are necessary in different environments and that breeding may contribute to improve baking quality to a certain extent. However, utilization of mineralized N is still unsatisfactory in OF systems in Germany. More N efficient varieties alone will help only little to resolve this problem; this can be achieved only by also improving the cropping systems.

For organic potato producers, the two main challenges are disease and nutrient management. Both are limited by regulations that on the one hand prohibit the use of chemical fertilizers, especially nitrogen, and on the other hand prohibit most synthetic pesticides. Late blight caused by Phytophthora infestans is commonly thought to be the most yield-reducing factor. However, because there is no really effective fungicide available to control late blight, there are virtually no yield loss data available for organic farming conditions. In this paper, the state of the art of organic potato tuber growth under on-farm conditions with respect to disease and nutrient management is summarized by field trials and on-farm surveys on commercial organic crops carried out in the years 1995-1998. Soil nitrogen (N) levels, plant N uptake, disease development of P. infestans and potato yield were measured. Results indicated that N availability was most important in limiting yields in organic potato crops. From on-farm data, a model including disease development, growth duration of the crops until foliage decay and different parameters related to N status of the crop could explain 73% of the observed variation in yield. Only 25% of this variation in yield could be attributed to the influence of late blight. Differences in N availability explained 48%. In conclusion, several points emerged from the results. In organic farming, yields are mainly limited by nutrient availability in spring and early summer. The effects of late blight on yields may often be overestimated and cannot be deduced from results in conventional farming because of the strong interaction with nutrient status. Depending on N availability, tubers stop growing between mid-July (70-90 kg N ha −1 uptake), the end of July (110-140 kg N ha −1 uptake) and mid-August (140-180 kg N ha −1 uptake) due to N limitations. The higher the N status of a potato crop, the longer the growing period needed to achieve the attainable yield and the higher the probability that late blight stops further tuber growth and becomes the key tuber-yield-limiting factor. In the second part of this paper, the interactive effects of soil N availability and the impact of P. infestans on yield in the presence and absence of fungicides from 1996 to 1998 for mid-early main crops are reported. An empirical schematic model of disease impact depending on N availability was developed.

In organic farming systems, it has been demonstrated that grain pulses such as peas often do not enhance soil N supply to the following crops. This may be due to large N removals via harvested grains as well as N‐leaching losses during winter. In two field‐trial series, the effects of legume (common vetch, hairy vetch, peas) and nonlegume (oil radish) cover crops (CC), and mixtures of both, sown after peas, on soil nitrate content, N uptake, and yield of following potatoes or winter wheat were studied. The overall objective of these experiments was to obtain detailed information on how to influence N availability after main‐crop peas by adapting cover‐cropping strategies. Cover crops accumulated 56 to 108 kg N ha–1 in aboveground biomass, and legume CC fixed 30–70 kg N ha–1 by N2 fixation, depending on the soil N supply and the length of the growing period of the CC. Nitrogen concentration in the aboveground biomass of legume CC was much higher and the C : N ratio much lower than in the nonlegume oil radish CC. At the time of CC incorporation (wheat series) as well as at the end of the growing season (potato series), soil nitrate content did not differ between the nonlegume CC species and mixtures, whereas pure stands of legume CC showed slightly increased soil nitrate content. When the CC were incorporated in autumn (beginning of October) nitrate leaching increased, especially from leguminous CC. However, most of the N leached only into soil layers down to 1.50 m and was recovered more or less by the following winter wheat. When CC were incorporated in late winter (February) no increase in nitrate leaching was observed. In spring, N availability for winter wheat or potatoes was much greater after legumes and, after mixtures containing legumes, resulting in significantly higher N uptake and yields in both crops. In conclusion, autumn‐incorporated CC mixtures of legumes and nonlegumes accomplished both: reduced nitrate leaching and larger N availability to the succeeding crop. When the CC were incorporated in winter and a spring‐sown main crop followed even pure stands of legume CC were able to achieve both goals.

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In order to optimize potato crop management in organic farming systems, knowledge of crop growth processes in relation to limitations and reductions by nitrogen (N) and disease is necessary. This paper examines the effect of different prevention measures (seed tuber pre-sprouting, choice of cultivars: resistance to Phytophthora infestans; earliness of tuber initiation) against disease-related constraints on yields, depending on the N-mediated growing conditions of organic potato crops. Under conditions of a relatively high N supply, accelerating the early development and tuber initiation of potato crops by seed tuber pre-sprouting (yield increase of c. 18-23%) or the selective choice of cultivars with an earlier tuber initiation (yield increase of c. 0-21%) are the most effective strategies in combating late blight. They bring forward the crop development c. 7-10 days, escaping the negative impact of the fungus on tuber growth. Under conditions of a relatively low N supply, these strategies do not affect final tuber yield. The reason for the differences of the effect depending on N supply of the crops is, that the lower the N supply, the shorter the period of time over which tuber growth takes place, independent of whether P. infestans becomes severe or not. Tuber growth of organic crops low in N has mostly ceased by the time late blight becomes potentially important for limiting yield, with the consequence that preventative measures against P. infestans are meaningless. Surprisingly, the level of cultivar haulm resistance to late blight did not affect tuber yields in years with an early and strong late blight development. Probably, the positive effect of a longer resistance to the fungus (c. 1 week) was counterbalanced by a later tuber initiation (also c. 1 week). In years with a moderate late blight incidence, less susceptible cultivars were able to produce higher tuber yields on fields with a lower N availability (c. 17-20%), probably due to higher N use efficiency and a better match of N mineralization and Potato Research (2007) 50:15-29