I'd try to answer that to help Geri, if I might?

Our soil is Vertisol, a rather heavy black clay that's high in nutrients. The valley we're situated is an ancient lake bed. I'm thinking that the siberian peas would like a poorer soil and maybe better drainage or lighter soil. I planted some Elaeagnus angustifolia from seed and while this plant grows around the countryside in rocky outcrops or in recent excavations with very poor soil, it struggles in our soil. Most are less than 1 meter in size.... and they say it's invasive :D Well not in our garden it's not!

If time permits come autumn, I'll uproot one or two of those and plant them in a hole with a bucket of sand added and see what happens.