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Misconception Number 10: Organic farming leads to significant nutrient losses. Organic farmers rely primarily on compost, animal manure, or green manure crops to supply soil fertility. The nutrients in these organic sources typically do not match crop demands. It is easy to over-apply nutrients, such as phosphorus, while nitrogen needs are just barely met. The nutrient cycles have a leak to the consumers, which are not replenished with fertilizers.
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.
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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.
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