Not sure on the soil type, the dude that did my back yard said it was topsoil. When I planted it I used a mix of miracle grow tree soil and the soil that was there.
generally you do not want to mic much more than 5% organic matter into the soil.
You most likely have clay soil, if you get some wet and it feels like clay, its clay soil. Super highly compacted, fine particles have very little pore space.
This is what causes the water to runnoff rather than infiltrate then percolate down. Clay soils need slow low flow watering, or water applied in short intervals over fairly close time periods (water for 10 min, then water an hour later for 10 min, then an hour later).
By mixing in too much planter mix, you are changing the soil structure around the plan, but not in the soil outside the hole you dug. If water infiltrates clay soil very slowly, and you fill a hole with nice soft loose soil with a high infiltration rate...what happens when the water that soaked into the amended soil all fast leaches the edge of the hole? It cannot infiltrate the clay soils at nearly the same rate as you applied it, and sits in the hole, creating a perched water table and effectively drowning the plant.