It is the lack of nutrients in the soil. Add some.

Dan

Yes, topsoil is a very broad term, and doesn't usually mean "great dirt with lots of nutrients". Like Dan says, you'll need to add a little somethin somethin to it.

Joe

It could be something in the soil. I would recommend a soil test before just adding this and that to it, without knowing what's already in it.

I would recommend a soil test before just adding this and that to it,

Right. And not one of those cheapies from the BigBox either. County Extension or private lab.

Dan