Illustrating the Nitrogen Cycle in Organic Farming.

    A figure depicting the practical and cultural aspects of the Nitrogen (N) cycle in organic farming was drawn for the new ASA book: Ecology in Organic Farming Systems.  Organic farming focuses on building and maintaining an appropriate level of soil organic matter which provides a reservoir for the slow release of N and other nutrients to crops.  Diversified organic farming systems are designed to use crop rotation, perennial crops, legumes, livestock, waste recycling and composting to build soil organic matter content and N fertility.  Solar energy and biological processes drive the N cycle in an idealized organic farming system.   The transformations within the N cycle are essentially the same as in natural ecosystems and conventional farming, but they are modified by organic cultural practices.  The N in organic matter is slowly converted by microbial activity into forms useable by plants at rates that vary with soil temperature and moisture.  Most of the N taken up by crops is in the mineral forms ammonium and nitrate, but plants can also utilize some low molecular weight organic compounds.  The incorporation of plant residues low in N concentration can cause immobilization by which mineral forms of N are removed from the soil solution for use by microorganisms.  Well managed organic farms can be self-sufficient in N by use of a combination of crop rotation, waste recycling, composting, integrated crop and livestock production, cover cropping, and cultural practices that minimize nutrient losses via erosion, leaching, and volatilization.  Regulations do not allow use of industrially manufactured N fertilizers or sewage sludge.  Crop residues and manures produced on conventional farms that use either synthetic N fertilizers or sewage sludge, however, are currently allowed on organic farms.  A limited amount of Chilean nitrate fertilizer is allowed on organic farms in USA, but not in Europe.