Wednesday, February 24, 2010
The accepted agricultural wisdom is that fertilized fields in which plant residue is left in the field will actually gain soil carbon. Unfortunately, it seems the reverse is true - a study by University of Illinois scientists has shown that in their long term (1876-) test fields, soil carbon levels increased steadily till around 1965, when animal manure was used as fertilizer, and started declining after that, with the use of synthetic nitrogen fertilizers.
With decreasing soil carbon (humus), the soil structure deteriorates, as does the capacity of the soil to hold on to nitrogen. As nitrogen leaches out with water, the land needs more fertilizers to stay productive, creating a vicious cycle.
From the paper Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Production
... Overwhelmingly, the evidence is diametrically opposed to the buildup concept and instead corroborates a view elaborated long ago by White (1927) and Albrecht (1938) that fertilizer N depletessoil organic matter by promoting microbial C utilization and N mineralization. An inexorable conclusion can be drawn: The scientific basis for input-intensive cereal production is seriously flawed. The long-term consequences of continued reliance on current production practices will be a decline in soil productivity that increases the need for synthetic N fertilization, threatens food security, and exacerbates environmental degradation.There is an informative article on Grist.org with comments from the authors.
This dilemma calls for an international effort by agricultural scientists to thoroughly review, evaluate, and revise current cereal production and management systems and policies. The immediate need is to use scientific and technological advances that can increase input efficiencies. One aspect of this strategy would be to more accurately match the input of ammoniacal N to crop N requirement by accounting for site-specific variations in soil N-supplying capacity and by synchronizing application with plant N uptake. In the long term, a transition may be required toward agricultural diversification using legume-based crop rotations, which provide a valuable means to reduce the intensity of ammoniacal fertilization with the input of less reactive organic N.
Hat tip to Doug on the Fukuoka Farming group for pointing out the Grist.org article.