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iron content of soils

Dave Gomberg's comment prompted me to triple-check my references on the
iron content of soils. My numbers come from an article in TAG vol 6:4 in
by Diana Walstad entitled "Iron: The Limiting Nutrient for Algae?". She

     "Iron, silicates and aluminum are the major components of mineral
soils, with iron the fourth most abundatn element in the earth's crust.
(Bowen, Brand). The total iron content varies from 1 mg/g soil in some
leached sands to over 3000 mg/g in some tropical soils such as laterite
(Wild). Median values for iron for all soils and sediments are reported
to be 40 and 41 mg/g, repectively (Bowen). The amount of iron in all of
these soils and sediments is more than adequate to meet the nutrient
requirements of aquatic plants. Indeed, I calculated that if I were to
use ordinary soil containing median levels of iron (40 mg/g) for an
underlayer in one of my aquariums, there would be enough iron for my
plants for 33,000 years."

The references quoted:

Bowen HJM. 1979. Environmental Chemistry of the Elements. Academic Press
(New York)

Brand LE, Sunda WG and Guillard RRL. 1983. "Limitation of marine
phytoplankton reproductive rates by zinc, manganese and iron". Limnology
and Oceanolography 28:1182-1198.

Diana Walstad had some other excellent points to make in this article.
For example, in nature, algae growth is limited by P however in the
aquarium we typically have P concentrations 100 times or more that found
in unpolluted freshwater. There is strong evidence that the limiting
factors for algae growth in aquariums are typically light intensity and
available iron in the water. Repeatedly in the APD it has been stressed
that it is critical to maintain 0.1 ppm (or less) chelated iron in the
water to avoid algae problems in well lit aquariums.

My motivation for discussing and researching alternatives to The Optimum
Aquarium method designed for informed aquatic gardeners:

1) to reduce our reliance upon expensive, commercial products such as
test kits, laterite and fertilizers.
2) to shed light on the properties which make substrates (including
laterite substrates) work.