Introducción y conceptos básicos de energía solar

Organic agriculture‘s productivity and potential contribution to feeding 9 billion people is not only a crucial question, but also one of its most contentious issues (de Ponti, Rijk, and van Ittersum, 2012). Statements on the feasibility of feeding the world with organic agriculture are often directly or indirectly based on comparisons of organic and conventional yields. Stanhill (1990), using mostly data from before 1985, was probably the first to conduct an extensive literature review of organic–conventional comparative yield data. Penning de Vries, Rabbinge and de Groot (1997) used crop growth simulations to conclude that organic agriculture can only produce enough food to feed 9 billion people at a global level (but not in every region) assuming diets with modest amounts or no animal proteins. On the basis of a review of comparisons of organic and conventional empirical yield data and simulations Lotter (2003) argues that, if meat consumption is reduced, large-scale conversion to organic agriculture is feasible without resulting in food shortages.

More recently, Badgley, Moghtader, Quintero, Zakem, Chappell, Avile´s-Va´zquez, Samulon and Perfecto (2007) also used comparative yield data to argue that organic production can ‗contribute substantially‘ to feeding the current and future world population, and that it may even be possible to reduce the agricultural land base. A result that was heavily disputed by Cassman (2007), Connor (2008) and Goulding, Trewavas and Giller (2009) as they argued that the yield data used by Badgley et al. (2007) and the assumptions made on nutrient availability in organic systems, particularly nitrogen, were too optimistic. Connor (2008) argues that organic agriculture does not need the ability to feed the world to contribute within agricultural production but provides some with justification to solve perceived and real production and environmental challenges in agriculture and food supply in a single step by large-scale transformation to organic agriculture. An important issue to the acceptance of organic agriculture is found in the question of its productivity. Existing analyses have put the carrying capacity of organic agriculture at 3–4 billion, well below the present world population of 6.2 billion (Connor, 2008).

The principal objections to the proposition that organic agriculture can contribute significantly to the global food supply are low yields and insufficient quantities of organically acceptable fertilizers. de Ponti et al. (2012) did a review and meta-analysis of 362 yield datasets, comparing organic and conventional agriculture. The results showed that currently organic yields of individual crops are on average 80% of conventional yields. These findings are an intermediate position between empirical data presented by other authors. Stanhill (1990) found a value of 91% using data from the 1970s, Goulding et al. (2009) found a value of 65% on the basis of 25 data entries for wheat, while Badgley et al. (2007) found a value of 130%. de Ponti et al. (2012) suggests that the latter value, however, was distorted as many of the data for developing countries had relative yields far greater than 100%. Connor (2008) agrees. The author further states that while the analysis of yield gaps for organic and conventional agriculture was at crop and field level. The results cannot readily be up scaled to higher system levels.

The other aspect of the debate is the unavailability of sufficient organic fertilizers. Organic agriculture relies for its crop nutrients on natural soil fertility, legume crops, compost and manure. When legumes are grown as a green manure crop instead of a food or fodder crop to add

nitrogen to the system, the average yield of food and fodder crops over the entire rotation is reduced. Yield data of the entire cropping or farming system should in those cases be adjusted accordingly, as was done for instance by Korsaeth (2008) and Taube, Loges, Kelm and Latacz- Lohmann (2005). Quite often, however, legumes serve as food or fodder crop, in which case nitrogen fixation is not at the cost of overall food production and part of the fixed nitrogen is made available to other crops in the rotation (Wander, Yun, Goldstein, Aref and Khan, 2007; Welsh, Tenuta, Flaten, Thiessen-Martens and Entz, 2009). In those cases, still additional nitrogen (and other nutrients) must be added to the cropping or farming system through other sources to make it possible to attain relatively high yields for the non-legume crops in the rotation.

Another important source of nutrients in organic farming is manure. Some of the successful organic production systems are dependent on relatively large manure applications imported from outside the farming system (Clark, Horwath, Shennan, Scow, Lanini and Ferris, 1999; Jaim and Al Kader, 1998). If these systems were to be adopted more widely in a given region, manure may become a limiting resource, thus reducing overall organic food crop yields (Jaim and Al Kader, 1998). By applying manure to food crops, rather than returning manure to fodder-producing areas, fodder yields may also decrease due to soil nutrient depletion.

According to Naegeli and Torrico (2009), while organically produced food seems not to be able to feed the World‘s Population, there are strong evidences that organic agriculture might help to alleviate the number of people suffering from hunger especially in developing countries. Given the strong negative externalities of conventional agriculture, the diversification of production as a basic principal of organic agriculture can contribute to the improvement of food security (Zundel and Klicher, 2007) which may improve the nutritional level in rural communities. The expanding global market for organic products (Connor, 2008; Badgley et al., 2007) and the possibilities for smallholder farmers in developing countries to access markets (Naegeli and Torrico, 2009) can have very positive effects on the rural economies, triggering rural development. The increasing awareness of what people consume also has positive effects on organic agriculture as an alternative option for agricultural production. Organic agriculture may thus be an option in some areas to support strongly rural development.