Plant nutrient use efficiency in organic farming - consequences of exclusive use of organic manures and untreated minerals.
2007
Kirchmann, H. | Ryan, M. H. | Bergström, L.
On organic farms, nutrients are applied in organic or sparingly soluble inorganic form, with plants receiving the nutrients they require indirectly through the action of soil microbes. This review examines the implications of exclusive use of organic fertilizers for the sustainability of farming systems, primarily using examples from two contrasting regions, Europe and Australia. In both these regions, mean yields are generally 25-65% lower on organic farms than on conventional farms, primarily because of decreases in plant-available nutrients that cannot be overcome by enhancement of the soil biological community. Overall nutrient inputs are lower on organic farms, although organic farms in Europe are increasingly applying approved commercial fertilizers. However, these inputs simply allow organic farms to acquire nutrients originating from conventional systems. If organic crop production were to be widely adopted, lower yields would require more land (33-100%) to maintain current production levels. In Europe, organic farming increases nitrate leaching, especially if expressed per unit of food produced, because of lower N use efficiency by plants. Despite their aim of maximizing nutrient recycling, organic farming systems recycle only on-farm waste and approved food waste, with most types of municipal waste excluded due to concerns about pollutants. Readily soluble inorganic fertilizers can be extracted from municipal wastes through new nutrient recovery technologies, but current regulations do not allow their use in organic cropping systems. In conclusion, promotion of organic principles does not improve use and cycling of nutrients and does not reduce leaching of nutrients, but excludes other more effective solutions for nutrient use in agricultural systems.
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