Because you used garden soil, it has probably compressed. Even with large containers, it's really best to use potting mix in containers because it is light, and does not compress. With no air in the soil, there is a risk that roots can starve of oxygen and rot.

Peat moss by itself tends to dry out very easily so I think I would add some potting mix to the top of the container.

Linda

It hasn't eroded....it compressed. That's what garden soil will do in a container and one of the many reasons it is not recommended for container gardening. But since that is what you started with, it doesn't make much difference now what sort of soil you add to top it off. No peat, though. That will acidify unnecessarily and can too easily dry out and become hydrophobic.

If you plan on growing in containers next season, you might want to investigate and study some of the many posts in the Container Gardening forum. As growing in containers presents very different conditions from growing in the ground, you cannot approach in a similar manner or with similar materials.

it's probably a bit too late if there are already established plants in the pots.. in my opinion, a mix of coco, peat and maybe a little perlite/vermiculite works great. 100% coco coir would be even better (it's better than peat for root growth) but it's typically more expensive. one of the other benefits to coco is that it's ph neutral so you don't have to fuss with lime if using peat since it's naturally acidic. just remember that you use a water soluble fertilizer with growing in pots and fertilize about once a week at 1/4-1/2 strength.