What is wrong with my Basil?

It's yellowing and losing leaves.

How old is the plant?

One plant per pot.

The potting mix looks too much like mulch which is usually nitrogen poor. If the top leaves are green and the bottom ones are yellow that indicates inadequate nitrogen and/or over watering. Try a nitrogen fertilizer and allow the pot to slightly dry out between waterings. Also, one plant well grown can make a small bush. Over planting a small container makes for competition in re: space and nutrients.

Odin, we cross posted regarding plant crowding though I think that's only part of the problem.

It's almost brand new; soil is a mix of garden soil and compost. Too rich? I also had it in a pot that wasn't draining well so I just moved it to a pot with better drainage.

How you define garden soil is not clear. Compost, depending on age, can be very short on nitrogen. Whatever your garden fertilizer philosophy, add nitrogen and thin to one best plant, have good drainage but some water retention ability as basil will not tolerate dry spells. I don't do much with container growing but, considering the time a container food crop spends in a medium, it's probably best to use a reliable container medium or research how to make an all purpose mix.

Sorry I wasn't more specific. Miracle Gro garden soil in a bag, and mushroom compost, ratio 4:1

if it specifically says 'garden soil' it may be too heavy. It should say 'potting mix' and feel fluffy. Garden soil is usually dirt cheap.

Yes, garden soil is not an appropriate medium for potted plants. It's not fully composted and, because it is still in the process of decomposition, it is nitrogen robbing. Many brand name mixes contain little more than partially decomposed pine bark and fertilizer the pH is also questionable. As Sally suggested, something labeled potting mix or container mix will be formulated for container growing.

I see another problem- basil likes LOTS of deep root room...

Ok, thanks everyone!! I planted in better soil, deeper pot, and separated it, and it's greening up well. Thank you so much.

My basils are outdoors, african blue, a cutting wintered over and rooted, stuck in ground mid Feb. Its still fighting with the fern leaf dill, but the raised bed drains well, there is too much shade now in this bed- the surrounding trees have grown.