Rusty nail in stem

Some 60 years algo, in Argentina, I saw a doctor to do that to a pine tree and he was succesfull .

I would let the tree establish on its own. Fertilizing go any kind should not be done until after one year of established growth

I don't recommend this since at some point in the future someone may run into it with a power saw.

I am assuming it is to give the tree iron from the rust. I would Google this if you don't get much info here.

Sounds like nonsense. Anything that penetrates a tree’s bark puts it at risk for disease or insect infestation. It can also damage the cambium, the area beneath the bark where cells rapidly divide and increase the girth of the tree.

Here are a couple links for you.

https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm

http://joyceshelpfulhints.blogspot.com/2008/01/nail-in-tree-trunk.html

I have seen the "rusty nail" trick, typically in countries where commercial fertilizers are hard to come by or are too expensive.

I would not try it. If you want to give your lemon tree a boost, try mixing a tablespoon of Epson Salt in a gallon of water. Sprinkle it on the soil at the base of the tree. Then water as usual. You can use this method on any plants or veggies. It puts enzymes in the soil and your plants will flourish. You can repeat once or twice a month.