My family owns tabacco plantations and crop rotation is something I was taught early on as fungal growth is very common in soil in the case of tobacco. 🌱
I feel you about the clay, that's one reason I started composting and got chickens, the only things that grow here is chaparral, cactus, sage, oak and manzanita and I really wanted an edible garden. We had to dig out the clay and replace it in some areas and in others I just mixed in compost and chicken waste. I think those parts are better though. I honestly think in regards to your peas it had to do with the planter as you said.