What's this chelation thing that I'm doing with urine? And why? The "why" part is that I'm adding plant nutrients to the soil that otherwise wouldn't be there. For example : adding rusty nails doesn't provide plant available iron. Adding plain urine, while adding immediate nitrogen, doesn't supply slow release nitrogen.
The "what" part is that I'm doing is creating a chemical reaction between a metal (rusted iron) and an acidic solution (urine) where the nitrogen in the urine bonds with the metal. The chelate compound thus formed is slightly soluble, therefore the nitrogen doesn't simply go off into the air or get leached out with the rain. That's good, because nitrogen is typically rapidly lost from soils. Plus the iron itself is a plant nutrient. In chelated form, it's available to plants. Otherwise it is insoluble and will just sit there in the soil unused, unavailable. I'm no chemist, so I don't know exactly what is going on with this chelation process, at a chemical level. I've talked to several chemists over the years and they've tried to explain it to me. Much of their in depth discussion flies way over my head, so I just take their word on it. Combining urine and rusted iron = a solution of iron that is available to plants + nitrogen that is also available to plants but doesn't rapidly disappear.
I am aware that most iron in the soil is not bio-available. The plants cannot use it. The micro organisms cannot utilize it. I have seen gardeners poke old nails into their soil, but from what I've been told, it won't help the plants any. But if you take those rusty old nails and soak them in acid, then there is a chemical reaction. The iron gets converted into a compound that plants can eventually use.
Since soaking old rusty nails in urine creates better plant nutrients, that is what I'll do. And experience seems to confirm that it works. The garden rows where I sprayed iron/urine chelate as a foliar spray, the plants were visually darker green and more lush appearing. I now use the chelate mix as a soil or compost additive rather than a foliar spray. Foliar sprays are more difficult for me to work with, so I put my effort into creating good soil instead. Some day, when I'm bored, I'll experiment in greater detail with foliar sprays.
So, how exactly is urine chelation done? Well, I'm kind of lazy about it. I just put a bunch of old rusted nails into a milk jug or other plastic jug. Most of my jugs I keep 1/4 filled with nails, adding more when needed.
Now I just let it sit there until I need the urine to add to compost. That means anywhere from several weeks to a couple months. Every once in a while I'll give the jugs a shake to stir things up. I've been told that doing the process this way takes weeks to work. Setting the jugs in the warm sun makes the process go faster. But be aware that sun degrades the plastic that milk jugs are made out of. If you plan on setting the jug in the sun for weeks, perhaps a heavier duty plastic container would be wise. Either way of doing it is ok by me. I'm in no rush. But if you needed the process faster yet, you would soak the metal in HCl (swimming pool acid) overnight then add the solution and any leftover metal to the urine. Either way, the urine solution gets cloudy and a layer will settle to the bottom. I shake the jug before pouring off the chelated urine for using it. Any nails not dissolved get used for the next batch.
If you don't have old nails, you could use any kind of scrap iron. In the past I've used bits of leftover angle iron and also rebar scraps. It just has to be able to fit into a jug of some sort. I guess you could use a 5 gallon bucket with a lid, but so far I've been doing ok using jugs.