Add the phosphate - the Polk County extension agent says all the soils locally are severely deficient in it.

Unless you are amending the soil in a very wide area around the planting hole - at least 3' wide and 1-2' deep, don't do it. The latest studies of planting trees and shrubs find that amending the soil doesn't make matters any better - in fact, plants in holes back-filled solely with native soil do better than those with amended back-fill. If you are really worried about planting in heavy clay, and have drainage issues, you can leave the top of the root ball a couple of inches higher than ground level, making up the difference by piling native soil or mulch to make a smooth mound.

Since you say you are planting on a hill, you can cut into the hill on a wide-ish arc, making the uphill side not too steep (and depending on the degree of steepness to the hill), using the extra soil to create a dam/berm on the downhill side, so water stays in the hole and doesn't just run off down the hill. Water well, to soak the root ball, and water again when the root ball feels dry to a finger stuck down a few inches - don't let it get too dry, but don't drown it, either. You want to give the juniper the equivalent of an inch of water per week, but since conditions vary so much from day to day, do it when you check, not just on a set schedule, even if that's easier.

If you weren't going to do so, give thought to mulching the juniper - as widely as possible (at least 3', more is better), no deeper than 4", and no mulch in the 2-3" next to the trunk. Mulch will help keep the soil moist, will moderate temperature swings, will limit competition from grasses and weeds, will eventually break down to improve the soil, and will keep weed-eaters and lawn mowers away from vulnerable trunks!

Thanks dibbit! We are planning to mulch with pine needles. I went out and got some Nature's Helper today and thought I'd add a little bit of that to the holes. The area I'm planting in is full of hard clay balls and rocks. It's almost impossible to break it all up. I also dug up a huge boulder today! That left a great big hole right where I was going to plant.