Answer: Whether or not you need to add specific nutrients (and in what quantity), such as nitrogen, depends on the condition of your soil and what you are trying to grow. The first step in answering your question is to run some basic soil tests and then amend the soil based on the results. Your county extension office offers a testing service, and will offer recommendations for soil improvment.
Soil pH has a tremendous effect on plant health. Juniper, rhododendrons, and azaleas require an acid soil (pH 5.0-5.5), but most of your plants will do well with a pH closer to neutral (around 6.5).
Most plants will do best in a soil that provide roots with a good balance of air and moisture. In many cases it is necessary to add lots of organic matter (such as compost, old rotted leaves, or aged stable manure and bedding) to the soil. If your soil is mostly clay, your plants should do better in raised beds, as this allows more effective drainage. A year round mulch of bark or other organic material will help to improve the soil structure and feed the soil so it can support healthier growth overall. Best of luck to you!
You may find that a combination of steps is needed to bring your planting area to its full potential.
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