Answer: Low growing junipers are often used as groundcover on sunny slopes and are able to survive in less than ideal conditions. Junipers will not grow well in an extremely heavy clay soil that is poorly drained, however in many cases the slope itself promotes drainage so that the junipers do fine.
Since ultimately the goal is to encourage roots to spread beyond the original planting hole, your soil preparation would be best directed at loosening the soil over a wide area so the roots can grow through it more easily. Any additional amendments would be based on the results of soil tests.
Your county extension should be able to help you with the tests and interpreting the results. They, and local knowledgeable garden center professionals, may also have suggestions as to whether or not junipers would be the best choice in that location.
When it comes time to plant you will need to take into account erosion and soil stabilization measures which can be critical if the slope of the hill is steep. For this reason you may want to stage the project or possibly look into tiering the hill if it is a large one or a steep one.
Additionally, you will need to mulch between the plants to keep down weeds while the junipers grow and fill in the area. (Mulch will also help maintain soil moisture and feed the soil as it breaks down over time.)
You will also need to make provision for watering the plants during the establishment phase, they will require watering as needed to keep the soil evenly moist but not sopping wet for the first year and during the next few years will need supplemental water in times of drought and during the very hot dry spells we tend to have in late summer.
Good luck with your hillside.
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