Stinner and House (1989) suggested an inverse relationship between the levels of chemical input and the system sustainability, and their principle is widely, more or less implicitly, accepted; Zandstra (1994, as reported by Hansen 1996), however, proposed a different scheme, with insufficient chemical inputs leading to exhaustion of natural resources and excessive inputs leading to accumulation and eventually to pollution.
The two principles are not as opposed as it can appear at first sight and can be reconciled to some extent observing that Stinner and House suggest to reduce chemical inputs through information and biological control, which implies avoidance of exhaustion. Anyway it is worth reporting the conclusion in a paper of Shapiro and Sanders (1997): Everywhere else in the world that food crop yields have been substantially increased, inorganic fertilizers have been a principal component of those yield increases. The other soil-fertility measures, especially organic fertilizers and rock phosphate, are complements not substitutes for inorganic fertilizers. Some problems related only to nitrogen and phosphorus are briefly discussed below; however, we cannot omit to comment the obligation by the Codex Alimentarius (http://www.fao.org/DOCREP/005/Y2772E/y2772E0c.htm) to use for organic farming sulphate of potash obtained by physical procedures but not enriched by chemical processes to increase its solubility: this is a blatant example of single-mindedness putting an unnecessary limitation, conducive to physical pollution for the greater energy required in transporting and hauling more fertilizer to compensate for its lower solubility. Energy for transport cannot be overlooked, in fact: it is estimated that big container ships use about 0.04 MJ/t/km, and trailers
use 1.9 MJ/t/km (Refsgaard et al. 1998; PréConsultants 2004). The same consideration applies to other substances suggested by organic norms, such as phosphate rocks, peat and guano, just on the ground that they are 'natural', without the due consideration to pollution depending on shipping along thousands of miles.
Nitrogen is the most widely used fertilizing element and is also the most highly polluting. The principles of organic farming ban the use of synthetic fertilizers -which gave origin to a flourishing industry of 'organic' fertilizers of uncertain composition, dubious effects and extravagant cost - on the assumption that green or animal manures enrich soil in organic matter and reduce nitrogen leaching.
While enrichment in organic matter by animal or green manure is unquestionable - but with other so-called organic fertilizers it is highly dubious - avoiding nitrogen leaching has been demonstrated a wishful thinking, since the lack of synchronization between N release by organic matter and N uptake by crops can lead not only to an insufficient supply to crops in the critical phenophases (e.g. Myers et al. 1997; Pang and Letey 2000) but also as a consequence to the leaching of unused nitrogen (e.g. Bonde and RosswallT 1987; Yadvinder-Singh and Khind 1992; Kirchmann and Thorvaldson 2000; Russo et al. 2008).
Environmental considerations for animal manure include the criticism of Wilson (2003) who remarks that the potential or real negative aspects of animal traction include...the additional labour needed for feeding and care, degradation of land and vegetation due to heavy grazing pressure and major additions to global warming gases and suggests that costs can outweigh the benefits. Gapper (2006) reports that bacterial contamination of Escherichia coli by animal faeces was found under almost 10% of organically produced vegetables versus 2% of other produce.
Sieling and Kage (2006) highlight the possibility of achieving very reduced rates of N losses from mineral fertilizers with an appropriate management, while more serious problems may arise from the use of organic manures and slurries concluding that slurry, especially when applied in autumn, increased N leaching more than inorganic fertilizers.
More cautiously Tilman et al. (2002) wrote: Reliance on organic nutrient sources is a central feature of organic agriculture, but it is unclear whether the 'slow release' of nutrients from organic compost or green manures can be adequately controlled to match crop demand with nutrient supply to increase nitrogen-use efficiency in intensive cereal production systems, thereby decreasing losses to leaching and volatilization.
One further consideration is worth reporting: the type of fertilizer does not affect the quality of the crop since when up-taking nutrients, plants do not care if they are of organic or mineral origin, as reported among others by Evers for carrots (Evers 1989a, b, c), Stamatiadis et al. for broccoli (Stamatiadis et al. 1999), ATTRA for wheat (ATTRA 2006), Russo et al. for lettuce, chicory and celery (Russo et al. 2008). Further evidence is supplied by Colla et al. 2000; Bulluck III et al. 2002;
Williams 2002; European Commission, Directorate-General for Agriculture 2002; Tomassi and Gennaro 2002.
Ali (1999) lists several rice-producing countries, including Taiwan, the USA, Japan, India, Nepal, the Philippines and Pakistan where the adoption of green manures (GM) has been nearly abandoned in favour of the more economic mineral N. It is somewhat surprising his finding, referring to rice and supported by analogous findings in researches conducted at IRRI by Ventura and Watanabe (1993) and Cassman et al. (1996), that the hypothesis that the continuous use of GM enhances productive capacity of soil better than inorganic fertilizer cannot be accepted. Naturally it is expected that such results apply only to the tropical lowlands where they operated and not to other lands since an abundance of experimental work supports the utility of GM, but this discrepancy underlines once again the importance of abandoning any pre-constituted approach in favour of flexible solutions, fitting the particular conditions of specific areas. Referring to southern Africa, for instance, Abalu and Hassan (1999) comment that harvested crops mine the soil of its nutrients unless they are replaced with plant residues, manures or fertilizers. Southern Africa does not have and is unlike to have the capacity to produce the quantity of plant residues and manures that would be adequate to replace the mined nutrients. Indeed, as suggested by Borlaug (1995), raising the average use of fertilizers in southern Africa from its present low levels to something like 100 kg/ha cannot be an environmental problem, only part of an environmental solution.
• Economic: reducing nitrogen doses to the possible extent appears as a typically win-win solution, with reduced costs and reduced pollution; however, it is not 100% true because the reduced physical input should be at least in part economically balanced by the costs for monitoring, analysing soil and leaves and accurately managing the fertilization. Compared to organic manure, mineral fertilization is somewhat cheaper due also to the opportunity costs of green manures and permits a more targeted and time-efficient action, thus reducing the risk of temporary crop malnutrition. Several experiences in very different environments (e.g. Kenya: Tisdell 1996; USA: Larson et al. 1998; southern Africa: Snapp et al. 1998; southern India: Victor and Reuben 2000; Punjab, India: Aulakh et al. 2001; Germany: Hulsbergen et al. 2001) concur to demonstrate that the highest yields are obtained when a basic organic manure is integrated by a mineral fertilization in moderate doses at the right crop phase. This practice was already known in Europe by the nineteenth century, under the name of 'sideration' (Lampertico 1899).
• Environmental: the main environmental risk of nitrogen fertilization is depending on the pollution of water bodies, which equally applies to both organic and inorganic forms (Russo et al. 2008). Mineral N is notoriously one of the most energy-demanding factors in farming activity; however, recent technological progresses in fertilizer manufacturing have substantially reduced the energy required, passed from about 80 MJ/kg in 1972 to about 40 MJ/kg in 1997 (Uhlin 1999a). This makes energy requirement to obtain organic N from a green manure very similar to that for mineral N; however, of course, green manure has the additional advantage of enriching the soil in organic matter. Ammonia volatilization depends much more on organic manure than on mineral nitrogen while N leaching can be higher with green manure than with mineral N (Yadvinder-Singh and Khind 1992). Furthermore, Witter and Kirchmann (1989a, b) demonstrated that the addition of peat, basalt powder, magnesium and calcium failed to reduce appreciably ammonia losses from animal manure.
• Social: a mixed organic/mineral fertilization as described above, with appropriate doses of mineral N applied after controlling the nutrient level in the plant tissues and the soil, permits to achieve the safest results in terms of pollution avoidance; this in turn brings about a better fruition of recreational areas, fishing ponds and water courses, and a reduction in emissions. Exchanging large N applications for more analyses, monitoring and accuracy in management entails a more qualified and rewarding job for operators. From the standpoint of food nutritional quality the origin of N, whether mineral or organic, is not relevant (e.g. Tomassi and Gennaro 2002). From the standpoint of consumer health, it has been claimed, but not conclusively demonstrated, that animal manure can be dangerous due to the contamination of fresh-consumed vegetables.
Solid phosphoric fertilizers are available as mono-ammonium phosphate, di-ammonium phosphate, triple superphosphate and single superphosphate; additionally, high-grade liquid phosphoric acid is available.
Furthermore, phosphorus is available as phosphate rocks (PRs); Rajan et al. (1996) give a review of PRs use for direct application to soils, listing advantages and disadvantages as follows.
Interest in phosphate rocks (PRs) as direct application fertilizer stems from the facts that i. Per kilogram of P, PR is usually the cheapest fertilizer;
ii. Direct application, with or without amendments, enables utilization of PRs which are unsuitable for manufacturing phosphoric acid and other soluble fertilizers such as triple (TSP) or single superphosphate (SSP);
iii. Because PRs are natural minerals requiring minimum processing they are environmental benign (Schultz 1992); and iv. PRs could be more efficient than soluble fertilizers in terms of recovery of phosphate by plants, even for short term crops in soils where soluble P is readily leached, as in sandy soils (Yeates and Clarke 1993) and possibly for long-term crops also in other soils (Rajan et al. 1994).
In spite of this, PRs are not widely used as direct application fertilizers. The reasons are:
i. Not all soils and cropping situations are suitable for use of PRs from different sources;
ii. The large number of factors controlling their dissolution in soil and availability to plants coupled with the inability to predict their agronomic effectiveness in a given soil climatic and crop situation; and iii. Their lower P content compared with high-analysis fertilizers which makes PRs more expensive at the point of application if long-distance transportation is required
Total phosphorus content of phosphate rocks is relatively unimportant, since what really matters is its reactivity in the soil, which in turn depends on the soil itself, the rock mineralogy and the level of rock grinding.
Phosphate rocks are acknowledged as non-active in alkaline soils and in those soils on a calcareous matrix which are so common for example in the Mediterranean region; to alleviate this problem, it is suggested to apply them in combination with green manures or in the composting process. Grinding ('micronization') is supposed to enhance their reactivity to some extent; however, it was not possible to convert an unreactive to a reactive PR, even by ultrafine grinding to a size <0.02mm (Khasawneh and Doll 1978). Gosling and Shepherd (2005) reporting the results of a research conducted in four arable soils in England where organic farming had been practiced for 15-54 years conclude: [T]he results ... indicate that soils in England under mixed organic arable rotations are able to maintain concentrations of total soil organic matter and N at similar levels to those found under typical conventional systems, though there was no evidence of the increase reported by other authors. However, the results do offer support to the argument that organic farming is mining reserves of P and K built up by conventional management. This situation is not sustainable in the long-term.
• Economic: assessing a priori which is the most economic form of P fertilizer is difficult. Excluding the use of phosphate rocks which can indeed be considered as inert rocks in most areas and are a support at best (in alkaline soils they can be only modestly reactive when combined to organic matter, which entails an additional cost for hauling and handling), the selection is limited to the highgrade or low-grade, more or less soluble solid forms. Of course liquid phosphoric acid, which is the most costly, is only used in those cases where a permanent irrigation system, particularly a microirrigation system, permits to distribute it to the crops uniformly and inexpensively; it has in this case the additional advantage of cleaning pipelines and emitters and discouraging the entry of insects into the emitters while not being polluting because it is closely controlled.
• Environmental: cadmium accumulation through the application of mineral P has been a matter of concern; however, through novel manufacturing and refinement processes, the Cd concentration has been reduced to <5 ppm P (HydroAgri 1998), whereas untreated phosphate rocks keep intact their Cd content. Energy considerations are one further factor against the adoption of phosphate rocks: in fact although considerable savings in energy are achievable in their manufacturing process compared to soluble forms, the required fine grinding and their transport and application are highly energy-demanding, since a much higher quantity of rocks is required compared to soluble forms. Edwards-Jones and Howells (2001), referring to phosphate rocks approved for organic agriculture in the UK, state that evidently their use is not sustainable. • Social: once again, environmental considerations are closely interlocked with social aspects. All those practices and technologies permitting to mitigate fertilizer environmental impact are simultaneously of benefit under social aspects. Like in the case of nitrogen, the mineral or organic origin of phosphorus does not impact the quality and nutrient value of food.
Synthesis of Subsection 3.2.2 - Environmental pollution can be brought about by excessive or insufficient nutrient availability. Yield quality is not affected by the source of nutrients, organic or mineral. The best results are achieved through a combination of organic and mineral fertilizers. Organic nitrogen is potentially more polluting than mineral. Phosphate rocks are often useless as fertilizers and polluting due to their cadmium content and the energy required for their treatment and hauling.
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