FIELD OBSERVATIONS ON NITROGEN CATCH CROPS .1. POTENTIAL AND ACTUAL GROWTH AND NITROGEN ACCUMULATION IN RELATION TO SEWING DATE AND CROP SPECIES

In temperate climates with a precipitation surplus during autumn and w inter, nitrogen catch crops can help to reduce nitrogen losses from cr opping systems by absorbing nitrogen from the soil and transfer it to a following main crop. The actual and potential accumulation of dry ma tter and nitrogen in catch crops were studied in the field during four seasons with winter rye (Secale cereale) and forage rape (Brassica na pus ssp. oleifera (Metzg.) Sinsk) or oil radish (Raphanus sativus spp. oleiferus (DC.) Metzg.). Sowing dates were end of August and three an d six weeks later. Potential nitrogen accumulation, Y (g m(-2)), could be summarized with Y = 96 - 0.34 X, where X is the day number in the year of the sowing date (range: late August till end of September). Sp ecies were compared in their performance, looking at differences in sp ecific leaf area, leaf weight ratio, leaf area ratio, light extinction and persistence during frost. The rate of dry matter accumulation in intervals of 14 days appeared to be determined primarily by the amount of radiation intercepted. A regression, forced through the origin, ga ve as a common slope 1.12 g dry matter accumulated per MJ intercepted global radiation, irrespective of season, species, sowing date or nitr ogen treatment (period from ca. day 250 to day 310). From this result the inference is made that leaf expansion is a key process, determinin g the performance of catch crop species under varying environmental co nditions.