Natural soil amendments used for fertilizer should be mixed into the soil at least a couple months before a plant is transplanted into it, and that soil kept damp so it can decompose.
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Once the plant is there and has established root growth, that's the point where you should leave the soil alone and top water or foliar feed with either liquid fertilizer or powder or granules that dissolve in water.
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At this point if I were you, I'd get together a pile of that clay ground soil and mix in some rabbit manure and a LOT of yard debris like leaves, dead weeds, grass clippings, tree bark, little twigs, etc. Leave it sit, water it every now and then. Stir it up if you feel ambitious. Next spring it'll be on its way to a decent soil but at that point you can add even more fine dry woody material like peat moss.
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As for planting directly into compacted clay soil, it can be done. Till up an area at least a half foot deep a foot circle. Make it an even bigger area if you have more replacement soil to replace what is taken out in the next step.
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Remove at least a quart of soil from the middle of that area leaving a hole. Transplant a small plant into that hole and fill the hole with potting soil mixed with a lot peat moss, or other fine dry material that is fluffy in consistency. This keeps the root ball from sitting in muck and once the plant gets a bit larger the roots will better penetrate the clay around it.Â
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With plants in this much clay, do try to plant where there isn't a lot of standing water after a rain, and if you need to water, water deeply but only as often as necessary. Mulch on top is a good idea and over time will degrade to form a nice layer of topsoil over the clay under it, which you can till in next year. It may take a few seasons but that clay soil can be made fit without a big investment, just mixing yard waste into it.