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A local deli packs frozen foods in 5"x9"x2" aluminum trays with a plastic top. They are a convenient size for starting seeds (built in moisture cover!). I used a couple that way last year with fairly good results and this year I have a whole stack of them. However I dimly remember someone on DG saying aluminum was not good for seed germination. I also dimly remember from high school chemistry that aluminum quickly forms an oxide coating, meaning the soil would probably not contact the Al directly, so I am not sure if the warning is valid.
So, is Aluminum a no-no? If it is, would spraying the inside of the tray with plastic alleviate the problem?
>> would spraying the inside of the tray with plastic alleviate the problem?
May I suggest using plastic film instead of a spray? Chemicals seem very likely to leach out of a spray (though perhaps not a waterproof spray paint.
When I run low on "1020 trays (11"x21" flimsy plastic trays) I use cardboard "flats" free from Home Depot, or cardboard lids from cases of copy paper (corrugated cardboard, not chipboars). Then I line those with plastic bags to keep the cardboard dry. You could do the same thing with Al trays - line with plastic bags.
5"x9" sounds like you could line them wirth 12" Saran Wrap. If not, restaurant supply places carry 18" rolls, one of which will last a lifetime.
I like smaller, stiffer trays than 11x21" commerical flimsy "1020" trays - - easier to carry without cracking ! It seems to me that "flimsy" should be part of the name. They really need something under them if you need to move them aorund, like very thin plywood or maybe wood paneling.
I don't actually use 1020 trays or cardboard lids as "flats", which I think means one big tray where all the roots tangle together. I put trays of cells ("six-packs", "inserts", or plugs from propagation trays) into the water-holding trays. i like the way the individual cells protect seedling root balls from each other, and let me "pop" a root ball out intact forplanting out or potting up.
However, I believe that not very much Al will corrode off a surface in the short time it takes to start seeds, especially if your pH is close to 7. I forget which corrodes Al faster: acid or caustic. Probably both! That adhereing oxide layer protects better in dry environemnts, but not against wet acid or base.
I have used those plastic trays that hold coldcuts like ham and or turkey, etc. It seems that much that we buy now come in plastic container. They are easy to poke drainage holes into. Likewise meat at the grocery also comes in trays. Even foam trays if deep enough are great for sowing seeds. Most important is drainage holes.
As far as aluminum trays for seeds, just line them with saran wrap if you are concerned.
Aluminum is a definite no-no, since its cation is phytotoxic. You may, however, use these trays for Buckwheat, Briars, Gorse and suchlike which do grow in Al3+-addled, acidic soils.
Buckwheat used to be the only possible food crop in French Brittany, that is, before the advent of King Potato...
I don't know about seeds but I've used aluminum pans for years for cuttings without any adverse effects. Some pans have held the same cuttings for at least 3 years and the plants are thriving. Eventually they corrode to the point of being ready for the trash heap so I just go buy a new stack of them at Sam's Club (15 full size for less than $10 or 30 half size for about the same price). They also make effective drip pans for a few years.
Thanks for that information. I realize that the Aluminum cation is phytotoxic, but it must not be extremely so, because Alum is included as an ingredient in some acid plant nutrient products. And I understand that aluminum can form an ion with sulfate that is reasonably non-phytotoxic.
Thanks to all for sharing your thoughts, and especially hcmcdole and Rick for your insight. Some friends on another forum had also reported using these with no ill effects. Sounds likely that not much metal should transfer in the few weeks the seedlings would be in these trays before transplanting to plastic cells.
I think I'll continue to use these this year. I'll spray some and use some as is and see if there is any difference.