Soil organic matter (SOM) can supply much of your 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 represents11-300kg N ha-1 for a crop*. At $1500/tonne for nitrogen fertiliser, this transates intoa value of $16.50-$450/ha/yr.
How can this be? Well 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 leumes and stored in the soil in organic forms,ot be broken down by other microbes – via two processes: mineralisation an 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.°
The indiscriminant use of nitrogenous fertilisers is carbon retrograde. Text books acknowledge the problem. ‘The of superphosphate and the ley farming of the times left a legacy of their own, in term of shallow-rooted pastures, dominance increased soil acidity, new weed invasions and nutrient imbalanced in many soils.# It burns up soil organic matter and takes calcium out of the soil. It can damage microbial communities and destroy their capacity to release natural sources of nitrogen. Contamination of aquifers, as in the USA, is also a dangerous problem.
*Smith, J.L., Papendick, R.I., Bezdicek, D.F., and Lynch, J.M., Soil organic matter dynamics and crop residue management, in Soil Microbial Ecology, Metting, F.B., Jr., Editor, Marcel dekker, Inc., New York, 1993, pp65-94.
+Charman, P.E.V., Soil Nutrient Decline in Charman, P.E.V. & Murphy, B.W., Soils: Their Properties and Management, Oxford U Press, 2000
° The Land, 5 June 2008, p.8.
#Zimmer, Gary, The Biological Farmer, Acres USA, 200
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