Answer: Most perennials do best on a rich, moist but well drained (meaning not soggy soil) soil well enriched with organic matter. As you have realized, the time to improve the soil really is before planting. To be honest the best way to improve your soil is to add copious amounts of organic matter such as compost, rotted leaves, or aged manure and bedding, and also, guided by the results of soil tests, add any additional amendments as indicated by the test results. The only way to do this effectively and fast is to lift the plants, fix the soil and replant. In addition to enriching the soil, this process improves the soil structure and that is just as important to the plants as the nutrients are. At this point, if you are unwilling to do that you can try topdressing with a few inches of compost and be sure to use an organic mulch. These will feed the soil as the break down over time and will encourage earthworm activity which is good for the soil.
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