Summary of Counter-Arguments:
| - In fact, conventional farming leads to more nutrient losses than organic farming due to leaching of synthetic fertilizers, soil erosion, non-recycling of farm resources. - Organic farming builds soil fertility over the long-run instead of trying to accomplish an instant matching to crop demand of nutrients. - Leguminous plants can provide enough nitrogen to the following crops of the crop rotation. - Almost all current agricultural systems have a nutrient cycle leak to the consumer, whether organic or conventional. Ultimately, research will have to identify the most sustainable ways to help minimize the rural-urban nutrient cycle leak. |
Details of Counter-Arguments:
As shown by several studies, organic agriculture has the potential to improve soil fertility and help build both nutrient and carbon stocks in the soil. For example, a farming system trial on soybeans carried out at the Rodale Institute (USA) showed that yields were comparable in conventional and organic cropping systems (less than 1% difference), but a comparison of soil characteristics during a 15-year period found that soil fertility (including nitrogen content and organic matter levels) was enhanced in the organic system, while it decreased considerably in the conventional system. Moreover, the conventional system had the highest environmental impact, where 60 percent more nitrates were leached into the groundwater over a five-year period than in the organic system. [1]
Farming is no lab activity: there is no way for farmers to provide inputs (whether natural or synthetic) that exactly match crop demand in the short term (e.g., over a growing season). This is why it is important to build soil fertility and nutrient content over a longer period of time by bring in fertilizing inputs with diversified nutrient composition. In conventional agriculture, farmers tend to focus on the growing season timeframe because soils that have low levels of organic matter cannot easily store nutrients from one season to the other. However, in organic agriculture, it is much more possible to build up soil nutrient stocks over a longer period of time. When there is a serious disequilibrium between nutrient inputs and crop demands, soil and plant analysis or observations can help the farmer identify and correct the disequilibrium. This is true for both conventional and organic agriculture. The difference is that conventional farmers generally focus on only three nutrients: N, P and K, while organic farmers bring in organic matter composed of a complex combination of nutrients, less easily quantifiable, but more effective in maintaining soil fertility. An important input provided by organic matter to the soil is humus, which is not a nutrient per se, but is very necessary to the maintenance of good soil structure and to the availability of nutrients for the plants. The risk of creating a major nutrient deficiency is smaller in organic agriculture than in conventional agriculture. However, if it does occur, organic farmers can still correct the deficiency by bringing in natural fertilizers such as rock phosphate (for P), magnesium rock (for Mg deficiency) or mineral potassium (for K deficiency). Moreover, synthetic trace element fertilizers (Fe, B, Mn, Zn, Mo, Cu, F, Co, etc.) are allowed in organic agriculture provided there is a documented deficiency.
Green manure does not provide phosphorus, but can provide enough nitrogen to meet crop needs. While green manure/cover crops do not remove or provide phosphorus (it is absorbed by the plant, but returned to the soil afterwards unless the plant is harvested), they bring in atmospheric nitrogen through biological fixation, a process that accounts for about 70 percent of the total nitrogen fixed in the biosphere. They also have the potential to bring in 75 to 300 kilograms of N per ha, [2] which is comparable to the synthetic N fertilizer inputs usually brought in by conventional farmers. For instance, in Pennsylvania clover used as a winter cover crop can provide enough nitrogen for a wheat-maize-soy rotation without additional fertilizers.
Most current agricultural systems have a nutrient cycle leak to the consumer. This problem is not limited to organic agriculture, but rather results from population growth, as well as from the increasing dissociation between the place where the crops are produced and the place where the consumers eat them (leading to massive rural to urban nutrient flows). The waste of nutrients through soil erosion and through the misuse of animal waste, especially in conventional agriculture, also adds to the problem. These nutrients end up in rivers and ultimately in the sea. To replenish nutrients, agricultural systems rely on natural mineral reserves such as rocks (also used in organic agriculture). Although this solution is the most common to-date, natural reserves are not renewable resources, and can be depleted over the long run. Ultimately, agricultural systems will need to seek solutions that truly help limit the rural-urban nutrient cycle leaks. Some examples of potential solutions include recycling human and animal wastes and using sea and lake resources (e.g., algae, fish bones, and nutshells) as agricultural inputs. A pragmatic and radically new approach to sewage management would be needed, but, theoretically, the amount of plant nutrients and organic matter that could be recovered would almost be sufficient to fertilize the crops needed to feed the world’s population. [3]
[1] Drinkwater, L.E. et al, (1998) Legume-based cropping systems have reduced carbon and nitrogen losses, Nature, v. 396, 19.
[2] D.H. Hubbell and Gerald Kidder (2003) at http://edis.ifas.ufl.edu/pdffiles/SS/SS18000.pdf
[3] See “Possibilities for Closing the Urban–Rural Nutrient Cycles” in “Global development of organic agriculture” (2006) for a more detailed analysis of this possibility.