celery problem

[ladybird]

I grew lots of celery this year, sowed in the greenhouse and planted out later in rather heavy soil which has compost added and though the stems wern`t particularly thick they look healthy. However, they taste quite bitter. The soup I made was bitter as well and I had to throw it out. I was so disappointing and just wondering would anyone have an idea of why they were so bitter and earthy tasting ?

[sweetpea]

Variety possibly or lack of blanching if it wasn't a self blanching variety.