Highly productive agroecosystems need high inputs of plant nutrients (nitrogen, phosphorus, potassium, and other elements) to replenish the nutrients removed with the exported products. These inputs can be delivered either as commercial fertilizer, recirculated sewage sludge and ash from garbage burning, or manure from cattle, pigs, poultry, etc. All sources have their advantages and disadvantages. Commercial fertilizers are well defined, low in pollutants such as heavy metals (although exceptions exist), hygienically safe, and are concentrated, easy to transport, and rapidly available to the plant when applied in the field. However, production and longrange transport of fertilizers is energy consuming, and the concentrated product increases the risk for too high doses, leading to environmental pollution. An even greater problem is that a large part of the farmers of the world cannot afford to buy enough fertilizer to maintain soil fertility and obtain good yields. In all, world N fertilizer production in 2001 was slightly less than 90 Mt, very unevenly distributed. In sub-Saharan Africa, only 1.1 kg fertilizer nitrogen is used per person and year, whereas in China the corresponding value is 22 kg.
In theory, recirculation of nutrients from waste of the exported products seems to be ecologically sound. In practice, there are a number of problems. First, sewage sludge mainly consists of water, which either must be removed (requires energy) or transported, which is expensive and impractical. Second, sewage sludge contains harmful bacteria, human parasites, etc. and has to undergo hygienic treatment. Third, and most severe, is the problem with contaminants, such as heavy metals and organic toxins. Therefore recirculation of sewage sludge and garbage incineration ash is strictly regulated in most countries. In this perspective, replacement of nutrients using newly produced fertilizer can be a better solution from an environmental viewpoint.
Naturally, animal manure produced on the farm should be and is recycled to soil as much as possible. Compared with fertilizers, manure has the advantage of containing organic matter, which improves soil structure. On the other hand, manure contains mostly water (expensive storage and transportation, heavy machinery needed for spreading), and it will lose nitrogen through ammonia emission, both at storage and spreading.
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