Answer: Earthworm activity would certainly help, but importing them might not be the best approach to helping your landscape. There is something you can do to improve your soil: add organic matter.
On an established lawn, it takes a little longer to improve the soil, but it can be done. Or you can simply rototill the entire lawn, amend the soil with organic matter and then reseed or lay sod. It's labor-intensive, but you'll enjoy the results for many years to come. If you're not interested in total renovation of your lawn, you can rent a power dethatcher and or/aerator, then spread a thin layer of organic matter over the top and reseed. The organic matter will work its way down to root level. Adding organic matter in thin layers over a period of years will eventually improve your soil.
Beds and borders can be worked to improve the soil prior to planting trees and shrubs. Again, lots of organic matter (compost, aged manure, peat moss, shredded leaves, etc.) will help. Spread a 4-5" layer over the top of the entire bed and dig it in to a depth of 8-10". This is where most of the roots of your annuals, perennials and shrubs will be so they will benefit the most. After planting, spread an additional 2-3" of organic mulch over the bare soil. You can dig this into the soil at the end of the growing season, then add fresh mulch. After a few years you'll have wonderful garden loam instead of concrete dirt.
Good luck with your new home and your new landscape!
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