Free Nitrogen

Soil organic matter (SOM) can supply much of a farmer’s Nitrogen needs. “In cropping systems, as much as 50%-80% of the N can be supplied from SOM and nearly 100% of the N in native ecosystems,” writes Professor Charlie Rice in his book Soil Carbon Management. This percentage represents 11-300kg N ha-1 for a crop. Nitrogen, like Carbon, is mobile. It cycles. Most N in soils comes from the air and is absorbed by micro-organisms associated with legume plants. N is fixed by legumes and stored in the soil in organic forms, to be broken down by other microbes – via two processes: mineralisation and nitrification, via which it is transformed into ammonium and nitrate.
Former NSW Department of Agriculture agronomist Adam Wilson told The Land that the best way to build up a N bank is to add carbon to soils. Management that builds C also builds organic N because both processes rely upon interactions between rootmass and microbes. He recommends adding organic carbon via composts, green manures or planned grazing, avoiding highly alkaline fertilizers which burn up C and humus, minimum tillage, and a legume or pasture rotation.