I don't really know, but I do this as well. I keep old eggshells and a rusty nail in my watering can at all times. Just don't sniff the eggshell water...

Rust is iron oxide, and it is basically insoluable. That means plants do not use or cannot uptake this form of iron. Most soils are not iron deficient as it is an abundant element of earth. What usually makes iron inavailable to plants are other factors like the pH of the soil, and whether the soil is waterlogged or suffering from aeration problems.

If you want to make sure iron is available to your plants the most important thing you can probably do is to correct poor drainage, and do a soil test to see if your soil pH is too high (lime soils).

Of course a nail in your water bucket won't hurt anything, either.

Thank you both for your responses. :)

Our soil is always very alkaline, about 8.5, and after years of trying to get it closer to ideal pH of 7 or so, a representative from the U. of Arizona pointed out that our water has a high pH, so it won't work very well, or for long.
Now I apply a chelated iron preparation from Fertilome, and the plants are getting greener.
According to Organic Gardening, in an article a year or so ago, Ironite, a product widely used for the same purpose, has very high lead levels, and shouldn't be used on fruits or vegetables. I hope the chelated iron stuff doesn't have the same problem.

I just saw this post. I got the tip from a English lady who has beautiful gardens. I know the English are very into gardening and usually know what they are talking about. As far as the iron being insoluble, I don't really know, but it doesn't cost anything and doesn't hurt anything either - so why not.

'Ironite ... has very high lead levels, and shouldn't be used on fruits or vegetables'

I looked for another link but could not find it. This one tells about lead and arsenic in Ironite though. From what I understand, the Liquid Ironite is okay to use and doesn't contain heavy metals.

www.envirolaw.org/poison.html

Here is a link that might be useful: www.envirolaw.org/poison.html

Chelated iron is a good "greener" and is used as a foliar applicatton in the nursery business when plants go chlorotic and need to be greened up for appearance and sales. It is a fix however, and the underlying problem is what should be addressed. Iron deficiency can be present in sandy soils, but isn't in most soils. Just in a form the plant cannot use. Usually adjusting the pH of the soil and using proper watering techniques will correct the deficiency in the plant, without tinkering with the soil's iron levels at all.

I found the link below. I quote in part ".... iron [that is] not water-soluble ... cannot be absorbed by plant roots. Such iron will be tied up indefinitely unless soil conditions change. This ... explains why rusty nails or iron shavings do not correct iron deficiency....

"Several methods are available for treating iron deficiency. These are: 1) soil application of elemental sulfur combined with ferrous (iron) sulfate; 2) soil application of iron chelates; 3) foliarsprays containing ferrous sulfate or chelated iron; or 4) trunk injection of ferric ammonium citrate or iron sulfate (trees only)....

The link is to an extension.usu.edu file and has much more information and explainations. (The link is to .pdf file. If you don't want to bother with that you may google for "control of iron chlorosis" and get a link to display in HTTP.)

Here is a link that might be useful: CONTROL OF IRON CHLOROSIS IN ORNAMENTAL AND CROP PLANTS

Thank you all for your responses. :)

There's an old myth that you should bury some nails in the ground whenever you plant a tree.

my Mother who lives in Central Florida has a grapefruit tree that was not producing anything. The soil is alkaline as far as I know. She had a friend who recommended she drive a rusty nail into the trunk of the tree and see if it helped the tree produce.
Lo and behold, every year since the tree has exponentially produced grapefruit!! Now I hear explanations regarding iron deficiancy, but would like to hear a scientific explanation for how this helped?
I have another theory, but it is not tested and I would love to have any experts shed some light on this. Could the production of fruit be related to stress from the nail, in turn triggering a "survival mechanism" which in turn creates fruit, which of course have seeds to perpetuate the species??? Far stretch, but I do not like old wives tales as an explanation, and I must understand this phenomenon!! Please advise

Summers, your theory could be the answer.

I'm in central Florida and my soil is PH neutral. I kept testing in different parts of the yard, I couldn't believe it.

Summers_end, if you can find another non-producing tree and get permission, just drive some nails into it that are not iron. Someone makes aluminum nails for something and a galvanized roofing nail should function as non-iron even thought it has an iron core.

BTW, I have hear of stressing lone apple trees to produce fruit but I had a semester of citrus studies in Arizona and never heard of stressing grapefruit.

Another BTW, at the time your mother drove nails into her grapefruit did a neighbor just happen to plant grapefruit? Citrus sometimes shows this goofy behavior called "grove physiology" where lone trees are loath to produce but the same tree in a grove produces just fine.

People don't know what they are talking about when they say that Rust isn't a good source of iron. People are absolutely correct when they say that plants can't absorb rust, but they completely forget that rust is turned into organic chelates or free ions over time.

If you want a faster process for making free iron ions, just dissolve nails or steel wool in vinegar. If you have already alkaline soils, you can just dilute as needed. If you suffer from slightly acidic soils, you might want to buffer the solution with ashes. You most likely wont have to as the process of rust dissolving in acid neutralizes it. Plus, excess acetic acid most likely will evaporate.

Robubba,
How long it takes for rust to turn into chelates?
I want to use metal coffee cans for drip irrigation for tomatoes. They will rust possible in a month. So, how it will affect my tomatoes?