On Sat, 22 Mar 2003 08:53:58 GMT, Tony wrote:
I read somewhere that too much fertilizer can accumulate in leafy
vegetables to the point of being toxic for human consumption, does anyone
know where I can find out more about this, especially as it relates to
hydroponics? I read this in a book on green houses, it said something
about problems with nitrogen not being used up by the leafy vegetables in
certain times of the year and thus accumulating in the edible parts of the
vegetable.

Nitrogen is poisonous? In the quantities a plant can pick it up? Hmmmm [..]

Most likely he was talking about nitrogen in the form fertilizers supply
it, i.e., nitrates/ammonium.

here's some info on the proiblem which is quite real. you might also want to
consider the fact that mustard will accumulate arsenic in soils where it is
plentiful.

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[1]Web Site Index
[2]New Alchemy Quarterly, No. 33

Nitrate in Winter Greenhouse Leafy Vegetables

Fall, 1988

In recent years, people are eating more fresh vegetables than in the
past, and supermarkets have responded by providing a wide range of
vegetables year-round. During winter and early spring in the
Northeast, these vegetables must be trucked in from farms in
California or other distant locations. Freshness and nutritional
quality may suffer during the long trip, and the fuel consumed exceeds
several-fold the energy value of the food. Also, conventional
vegetable production often entails considerable use of pesticides,
which may leave potentially hazardous residues in the produce. For
example, the Food and Drug Administration detected pesticide residues
in one-third of the lettuce samples and one-half of the tomato samples
it tested between 1982 and 1985.1 Finally, some of our winter fresh
vegetables are grown in Chile and other developing countries where
land and labor are exploited to produce export crops while their
people go hungry.

For these reasons, growing winter vegetables locally in greenhouses
may be an important component of a sustainable regional agriculture
for the Northeast. Throughout its history, New Alchemy Institute (NAI)
has experimented with winter vegetable production in several solar
greenhouses and one compost-heated greenhouse. Leafy vegetables, such
as lettuce, spinach and Chinese cabbage appear most promising, since
they grow well under the low light and low temperature conditions of
the winter greenhouse. Several small commercial greenhouses in New
England are producing lettuce and other greens, and some other growers
are considering winter greenhouse vegetable production. Such produce
is much fresher and more nutritious than trucked-in vegetables, and
few Or no pesticides are used in growing them.

Unfortunately, leafy vegetables grown under winter greenhouse
conditions tend to accumulate nitrate to potentially unhealthful
concentrations. At New Alchemy Institute high nitrate concentrations
were first detected in leafy vegetables grown in our composting
greenhouse, where the soil had been grossly overfertilized by ammonia
from manure in the composting chamber.2 However, we also found high
nitrate levels in leafy vegetables from our two solar greenhouses, and
one commercial solar greenhouse, whose soils received only moderate
amounts of nitrogen from organic sources. Library research revealed
that nitrate accumulation in winter-grown leafy and root vegetables is
a widespread and well-known problem in Europe, where several countries
have legislated maximum allowable concentrations of nitrate in
produce^3 (see Table One).
[3]table 1
Excessive dietary nitrite may cause a life-threatening hemoglobin
malfunction in infants, and may be converted to carcinogenic
nitrosaminos during digestion, posing a possible cancer risk for all
ages.4 Although the extent of this cancer risk is unknown, vegetables
are the main source of dietary nitrate, and researchers agree that
efforts to lower nitrate levels in leafy vegetables are warranted.5

Studies in Europe^5 and in the U.S.6 have identified two major factors
in nitrate accumulation. First, low light levels, such as occur in the
winter greenhouse, retard nitrate utilization by crop growth
processes. Second, heavy application of nitrogen fertilizer increases
uptake of nitrate by the crop. In either case, the excess nitrate
accumulates in the leaves and stems.

During the winter of 1987-88, NAI undertook a nitrate study on
greenhouse lettuce to assess the extent of the problem in New England
and to attempt to identify crop varieties and/or cultural practices
that reduce nitrate levels. In late winter, we harvested six heads of
butterhead type lettuce from each of seven greenhouses in southern New
England, including our own composting greenhouse, and measured their
nitrate concentrations. With one exception, the greenhouse lettuces
contained mote nitrate than did California field-grown butterhead
lettuces purchased at a local supermarket (Table). The lettuces from
greenhouse 7 had not received a fertilizer application for several
weeks before harvest, and appeared nitrogen-deficient. The reason that
lettuce from greenhouses 5 and 6 had somewhat less nitrate than the
four hydroponic greenhouses is not clear, and no conclusion can be
drawn regarding the tendency of soil,' integrated aquaculture or
nutrient solution culture to cause nitrate accumulation. However, the
data do suggest that nitrate accumulation in winter greenhouse lettuce
may vary with the availability of nitrogen to the crop. Further
studies to identify a nitrogen fertility program that lowers nitrate
content of leafy vegetables without restricting crop production would
be worthwhile.

Another strategy for lowering nitrate concentration might be to
harvest crops on the afternoon of a sunny day, when the maximum amount
of nitrate assimilation has just occurred. In preliminary experiments
during winter at NAI, lettuce and other greens harvested late on a
sunny afternoon had 15 to 20 percent lower nitrate concentrations than
the same crops harvested early in the morning after a cloudy day.

Nitrate concentrations of lettuce from four of the greenhouses
exceeded the West German guideline maximum, whereas none exceeded the
maximum allowed in Austria and tho Nethorlands. In 1974, the World
Health Organization established an "acceptable daily intake" of 220 mg
nitrate for an adult,4 which might be consumed in a single large salad
containing two ounces of lettuce at 3,900 ppm nitrate. However, it is
difficult to compare the risk from winter greenhouse lettuce with that
of lettuce trucked in from California, which may be lower in nitrate
yet contain pesticide residues, and may have lost some nutrients in
transit. Researchers and growers should work together to identify
varieties and cultural practices that lower nitrate levels in winter
greenhouse vegetables. In the meantime, it may be advisable to eat
winter greenhouse-grown lettuce and other leafy vegetables in
moderation.

Mark Schonbeck

Mark is a research associate working on cover cropping and nitrogen in
vegetable production.

References
1. Lawrie Mott and Karen Snyder, 1988. Pesticide Alert. Organic
Gardening 35 (6): 70-78.
2. M. Schonbeck, 1987. "Nitrogen in the Composting Greenhouse." New
Alchemy Quarterly 29: 18-19.
3. C. Stopes, L. Woodward, G. Forde & H. Vogtmann, 1988. "The nitrate
content of vegetable and salad crops offered to the consumer as
from 'organic' or 'conventional' production systems." Biological
Agriculture and Horticulture 5 (3): 215-221.
4. H. Vogtmann 8c R. Biederman, 1985. The nitrate story: no end in
sight. Nutrition and Health 3: 217-239.
5. W.J. Corre & T. Breimer, 1979. Nitrate and Nitrite in Vegetables.
Center for Agricultural Publishing and Documentation, Wageningen,
Netherlands.
6. D. N. Maynard, A. V. Barker, P. L. Minotti Sc N. H. Peck, 1976.
Nitrate accumulation in vegetables. Advances in Agronomy 28:

[4]Web Site Index
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Original Article from:
New Alchemy Quarterly
Fall 1988, No. 33
© 1988, New Alchemy Institute, Inc.
237 Hatchville Road
East Falmouth, MA 02536
[5]http://www.fuzzylu.com/greencenter/
Scanning & HTML conversion: FuzzyLu MultiMedia.

References

1. http://www.fuzzylu.com/greencenter/index.htm
2. http://www.fuzzylu.com/greencenter/q33/toc.htm
3. http://www.fuzzylu.com/greencenter/q33/parts/table1.gif
4. http://www.fuzzylu.com/greencenter/index.htm
5. http://www.fuzzylu.com/greencenter/home.htm