Help mature lemon tree has never fruit

[sapote]

"If you prune it back this resets the node count "

Wow, I didn't know lemon can count and has a reset button -- a computer indeed

[Tropicdude]

Just graft a few other citrus onto it, keeping many of the original branches,  then you can have a citrus cocktail tree.  you might end up getting fruit from the grafts before the original seedling decides to wake up.

[Mike T]

If it is a Lisbon or classic lemon it may never fruit regardless of soil or nutrition.

[Lory]

It will fruit Mike.
All lemons fruit soon or later (given that the growing conditions are right).
The problem is that overcoming its juvenile period can take many years.
And until that so long awaited moment has come you don't know what kind of fruit to expect 

[knlim000]

try repotting and cutting the roots.  I noticed when I dug my citrus tree up, and trim the roots. they all flowers like crazy.

[Lory]

That tree is in the soil and it's a big one!

[knlim000]

even in the ground, i think you should still be able to trim some roots off.  For some reason, cutting the roots trigger the plant to want to flower.  Same thing they say about chestnut tree, it can take 15yrs to fruit, but if you trim it and street it out, it will want to fruit.

I bought a grapefruit tree from home depot and had it for 7yrs. Last year, i dug it out of the ground and planted into a pot, trimming lots of roots off.  This year, i see several fruits forming.  1st year it is fruiting after 7-8yrs.  It wasn't even a seedling either.

[Lory]

It's amazing, maybe the stress will induce it to flower and reproduce

[Peekaboo]

I've been hitting them massive doses of 5 1 3 npk via foliar trying to correct those nutritional deficiencies

[Lory]

I think higher potassium would be better to induce flowerng.
I usually use a 20-5-30 i.e.  a NPK ratio of  4-1-6