The book by E.J. Hewitt (1966) has lots of details on
acid washing sand. If I remember correctly, he had
special machines for acid washing batches of sand and
used some very concentrated acids. I believe Hewitt's
acid treatments were aimed at purifying sand for
micronutrient deficiency studies, however, acid
washing cannot remove all the micronutrient
contamination if the sand minerals themselves contain
micronutrients. Micronutrient deficiency studies are
probably more efficiently conducted in solution
culture unless the crop does not lend itself to
solution culture.
A coarse, white silica sand is desirable for sand
culture where you want to obtain deficiency symptoms.
The whiter the sand, the less mineral nutrients it
contains. You also want a sand that has adequate
aeration. Sands that have a wide particle size
distribution, such as mortar sands, may have very low
aeration in shallow containers such as pots. One of
the drawbacks of sand cultures is that they may
require several times per day irrigation because sand
does not have a high water holding capacity. Other
sand culture disadvantages are that the roots are not
visible or easily recovered at harvest as in solution
culture. Also, if plants get too big for the
sand-filled container, you cannot easily transplant
them to a larger container as possible with solution
culture.
For some deficiencies, such as nitrogen, acid washing
might not even be necessary. If the sand contains
calcium carbonate and you want calcium deficiency then
an acid leaching with nitric acid or hydrochloric acid
would be desirable. Acid washing would also remove
nutrient cations from the sand cation exchange sites.
I did some sand culture for a teaching demonstration
of nutrient deficiencies many years ago using the
ceramic crocks designed for sand culture and just
plugged the drainage hole with a stopper. Then I added
acid, let it sit overnight, and then removed the
stopper to drain the acid away. The sand was then
leached thoroughly with distilled water. I think I
used a one molar acid. I didn't autoclave or steam the
sand but that might be necessary depending on the sand
source, and especially if the sand is being reused.
References
Hewiit, E.J. 1966. Sand and Water Culture Methods Used
in the Study of Plant Nutrition, rev. 2nd edition.
Commonwealth Bureau of Horticulture and Plantation
Crops, East Malling. Tech. Communication Number 22.
Color Pictures of Mineral Deficiencies in Potato Grown
in Sand Culture
http://www.luminet.net/~wenonah/min-def/potato.htm
David R. Hershey
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