Procedure
Turn inexpensive galvanized metal tubs into planters by
drilling several holes in the bottom and putting a layer
of gravel before adding your soil and plants. As long as
your tub or trough is hot-dipped galvanized (with a spangled
pattern) the zinc will cover exposed steel and your holes
won’t rust. Otherwise, seal out corrosion with adding
a thin coat of clear or silver nail polish. Rocks and gravel
provide better drainage than just soil.
So, how safe is growing food and making soil from compost in galvanized
steel containers? First it’s handy to understand
how galvanized steel buckets, tubs and containers are made.
Here is what we know
Zinc is the metal contaminate of concern when planting in galvanized
steel. Is the zinc that leeches into the soil harmful to plants and
the humans consuming it? Zinc is used as a common cold preventative
and is apparently good for our body’s biology as an "essential
trace element" responsible for numerous functions. Can a person
to ingest too much zinc?
“Zinc-galvanized steel is an excellent material for a watering
can.” Robert Rose, a professor of materials science and engineering
at the Massachusetts Institute of Technology, in Cambridge, and a
longtime gardener and greenhouse hobbyist, explains why, “Plain
steel rusts and thus, a protective zinc finishing is key.” When
zinc interacts with air, known as oxidation, a small layer of tarnish
develops to shield the underlying metal from further corrosion. It
resists ruin better than most garden equipment, weathering extremes
of temperature and contact with moisture, soil, and acidity. Plus,
steel sheets are easily cut and shaped into desired forms
[3].
Food and Drug Association
The acidity of some foods dissolves zinc. Galvanized iron should
be avoided as a food contact surface because it is highly reactive
with acids, explains number 4-101.15 (chapter 4) of the Federal
Drug Administration’s 2009 Food Code. The FDA recognizes the
unsanitary threat of corrosive metal in serving containers.
The
FDA has approved the use of galvanized steel for food preparation
and conveyance for all applications with the exception of foods
that have a high acid content, such as tomatoes, oranges, limes,
and other fruits. The acid content in these foods will attack the
zinc coating and cause accelerated corrosion. Many food products
are stored on galvanized racks or in coolers with galvanized shelving.
The recommended daily allowances for zinc are listed in the table
below from the National
Institutes of Health.
Yes, there can be too much of a good thing. The tolerable upper
intake levels for zinc are in the following table. Although a requirement
for a healthy body, excess zinc can be harmful. Absorbing too much
zinc can suppress copper and iron absorption damaging proteins,
lipids and DNA [1].
Study of heavy metal concentrations in vegetables
An August 2014 article published in the Food and Chemical Toxicology
Journal investigated concentrations of four metals Zinc, Lead, Copper
and Chromium in vegetables. The study’s rationale questioned
the likelihood of heavy metal contamination from fruit and vegetables
grown in contaminated soils and irrigation water. The study presented
original data on the concentration of heavy metals in some selected
fruits and vegetables consumed in Algeria. The goal was to figure
out whether they met international requirements. Human health risks
were determined using the estimated daily intake (EDI) and the target
hazard quotient (TTHQ) of these metals through consuming them in fruits
and vegetables
[1].
Noteworthy Points
- Findings showed that Zinc and Lead are more likely to accumulate
in fruits and vegetables, their mean concentrations were two to
two and half times [2–2.5] those of Copper and Chromium.
Results show also that within the selected fruits and vegetables,
the highest concentrations of Lead and Chromium were obtained
from root vegetables (onions, carrots and potatoes respectively)
and tomatoes, while Zinc is more likely to accumulate in spinach
and artichokes and Copper in potatoes and cucumbers.
- The levels of metals in fruits and vegetables in the study
were compared with levels in some other parts of the world. It
seems that the levels are generally comparable for copper and
zinc.
Average consumption levels were determined and compared to Provisional
Tolerable Daily Intake (PTDI) levels established by FAO/WHO in
2001.
- The consumption of fruits and vegetables with the levels of
Copper, Zinc and Chromium found were largely below the PDTI values
(500–1000–1500 µg metal/kg body weight/day for
Copper, Zinc and Chromium respectively) of FAO/WHO.
- The study concluded that Lead is the major component contributing
to the potential health risk via consumption of fruits and vegetables.
- Using global target hazard quotient no correlation was found
for the levels of Copper, Zinc or Chromium recorded in the foods,
“suggesting that it is not risky for the citizens to consume
these elements when contained within fruits and vegetables. Some
obvious health risks related to lead were found associated with
the consumption of these vegetables[1]."
Galvanized pipe for drinking water….
Many municipalities across the globe use galvanized pipes to transport
water into buildings. A Journal of Water Supply 2007 article assessed
current water service in buildings from galvanized steel pipes and
determined water supply contamination by testing for harmful concentrations
of regulated substances. The researchers found that, “corrosion
of galvanized steel is more affected by aging than any other physical
properties,” and that the corrosion of galvanized steel pipe
over long period of time show, “little effect of chemical
composition on the deterioration of a water supply [2].”
References
- Abdelhamid Cherfi, Samira Abdoun,
Ouardia Gaci, Food survey: Levels and potential health risks of
chromium, lead, zinc and copper content in fruits and vegetables
consumed in Algeria, Food and Chemical Toxicology, Volume 70,
August 2014, Pages 48-53, ISSN 0278-6915
- Cheol-Ho, B., No-Suk, P., Sang-Young,
P., Hyun-Dong, L., & Seong-Ho, H. (2007). Assessment of galvanized
steel pipes for water service in buildings by direct diagnosis
method. Journal Of Water Supply: Research & Technology-AQUA,
56(5), 335-343.
- Davis, R. H. (1993, June-July).
A world of watering cans: humble tools that have stood the test
of time. Horticulture, The Magazine of American Gardening, 71(6),
62+.
WARNING: These products can expose you to chemicals which are known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov.
