NUTRIENT MANAGEMENT IN ORGANIC FARMING SYSTEMS - THE CASE OF NITROGEN

Nitrogen management in organic agriculture is discussed focussing on s trategies maximizing nitrogen input (source: N-2-fixation) and minimiz ing nitrate losses on-site (field losses) and off-site (losses from ma nure heaps). Due to a strong relationship between grain yield/amount o f N in grains and N-2-fixation in pulse-crops or total dry matter yiel d and N-2-fixation in fodder legumes, e.g. grass/clover mixtures, nitr ogen fixation can be maximized by selecting the best site-adapted (bes t yielding) species, varieties or mixtures, respectively. Strategies t o reduce nitrate losses are based on N-sinks, that is, plants taking u p soil-borne and residual nitrogen during or after growth of legumes e fficiently, e.g, brassica-underseeds in faba beans, catch crops, or fo llowing crops with great sink capacity for nitrogen. In order to keep pre-winter mineralization as low as possible (minimizing N-source) tur n over of grass/clover can be postponed or tillage intensity can be re duced in depth or frequency. Gaseous losses of NH3 from manure heaps c an be reduced by shorter duration of storage, by covering heaps with c lay or stone meal having a high base exchange capacity, and by coverin g heaps with an additional layer of organic residues of high carbon co ntent. Field losses of ammonia can be reduced by injecting liquid manu res directly to the soil. Generally manures have to be turned under im mediately after distribution.