I think there's more to it. Last year it was all black lemon this year it's 50-50 black lemon and proper lemons as the Picture Show. How should I fertilize? It's in a pot

Can we see your whole tree.

Of course but give me a few hours. Thanks

Here is the whole tree. It's on the left. The pot is about 4 foot high with a diameter of 5 foot. The entire tire is about foot tall with a canopy width of 10 feet. It has about 10-20 nicely developed lemons about 2-4 inches in length and 10 or so tiny black lemons no more than 1/2 an inch in length.

CA Kate is correct. The problem your tree is having (black lemons) is common on very young trees. The problem will reduce every year as the tree matures.

Thanks @Silica. Can you see the tiny black lemon in the picture next to the regular lemon? So you're saying it's normal for some lemons to develop and other to stay.tiny and up and turn black?

Citrus fruits compete against each other for the trees available energy (Food). The fruitlets that loose out are discarded by the tree. A few will stick to the branch die and shrivel turning black. Of all the fruit producing flowers on a citrus tree, only about 1 to 2 percent will produce fruit to maturity.

Ian, I use a liquid fertilizer formulated for Citrus from Urban Farms... Mail order out of Texas... or, Amazon. I give them a dose about once a month.