On Sun, 23 Mar 2003 09:05:01 -0600, "Greg Peterson"
wrote:

Hi all,
I have what may sound like a silly question. I am putting together a couple
raised beds for a veggie garden and I would like to construct the bed to
conserve water as much as possible. I know that you can buy the super
absorbent polymers too add to soil that will soak up the water and then
release it again when the soil dries out. Granted, these are for smaller
flower pots, but I don't see any reason why the concept wouldn't work on a
larger scale. Unfortunately, given the size of the raised beds, using these
advertised materials would be cost prohibitive. So, I was thinking that
disposable diapers also contain super absorbent polymers that are not as
expensive as the garden variety. I was thinking that I could rip the
batting out of the diapers and mix it into the soil.

My concern is that 1) the diaper polymers may release undesired chemicals
into the soil that would either harm the plants or get into the food, and 2)
the type of polymer in the diaper is such that it would not release the
moisture readily.

Does anyone know of any data on this subject? I can't believe that I am the
first to have thought about it. Any other alternatives to consider?
Thanks.

Greg


The absorbent polymer in diapers is a very fine powder called sodium
polyacrylate. It is unsuitable for use in planting media because it is
too fine, it decomposes too quickly, and it releases sodium into the
soil when it breaks down. It does hold more moisture (about 500 times
its weight) than polyacrylamide (about 400 times its weight), but
polyacrylamide lasts much longer (five to ten years) and releases
nitrogen when it completely breaks down. (Both polymers also release
carbon dioxide and water when they decompose.)

Polyacrylamide certainly seems expensive, but it may seem more
reasonably priced when you consider how long it lasts and how
beneficial it is, and also the fact that as little as ten pounds will
treat 500 cubic feet at a cost of less than $50. It seems unlikely
that you could get that much of the unsuitable sodium polyacrylate
from $50 dollars of disposable diapers, but I might be wrong on this
point since I've never bought disposable diapers and I don't know how
much polymer they contain.

If you do decide to use polyacrylamide you ought to buy it in the
medium (1 to 2 mm) or large (2 to 4 mm) particle size. It is also
available as powder and fine particles. Powder is suitable only for
coating the bare roots of plants prior to shipping or planting and
lasts only a year or so. Fine particles are best for plants in
relatively small pots (under 12 inches in diameter). Also, don't mix
the polymer particles into the top one or two inches of your planting
medium. When the polymer hydrates it will swell considerably, and if
it rises out of the soil it will decompose very quickly because of
exposure to sunlight. Of course, you could always mix the polymer
throughout the medium and use a thick mulch to protect emergent
particles from the sun, and the mulch would also help to conserve soil
moisture.

Two other points worth considering - some people believe that
polyacrylamide is toxic because acrylamide is (they must assume that
the polymer bond is less stable than the acrylamide molecule, but I've
never seen any evidence that this is true), and of course
polyacrylamide doesn't last forever.

BTW, no, you are not the first person to think that disposable diapers
might be usable. This has been a topic of (frequently heated)
discussion for several years in various gardening forums. Some people
refuse to believe that sodium acrylate and polyacrylamide are
different, or that the former is actually harmful. I've even seen one
person insisting that the very best thing to do is to cut disposable
diapers into pieces and line the bottom of pots with them!

If you have any questions or need a supplier of polyacrylamide please
let me know, either here in this group or by email. (Remove UCETRAP
from my return address to reach me by email).

HTH,
Harold