The "soil-soup" method where soil is collected and water is slowly
added with much mixing until you have something like thick soup, which
is then run through window screening or a rice strainer, produces a safe
product. The screening filters out all the roots, worms and other
critters, and other "raw" pieces of organic matter that might cause a
large oxygen demand. About a quarter inch to a half inch of the soup is
placed in the bottom of the tank or planting tray and covered with about
a half to one inch of gravel. Water can be added with precautions to
prevent stirring up the gravel, and almost no cloudiness will develop.
The gravel can be put on top of the soup, and then everything can be
allowed to dry out. The soup turns quite hard, but when water is added,
it gets soft again, and the plants seem to grow just as well. If the
soup has been allowed to dry out, the chances of any cloudiness
developing when water is added are much less. The soup is relatively low
in organic matter, and I have found that, when a tray becomes packed
with roots, low-level iron deficiency develops, and additions of soluble
iron stimulate growth. Soil-peat mixtures or soil-manure mixtures
beneath gravel seem to be able to supply iron for a longer time. I have
not seen any tannins come from soil-soup, but I have seen tannins get
into the water from soil-manure and soil-peat mixtures. A water change
gets rid of them, and they don't seem to come back."
