Elderberries love organic matter. I wonder if you have 1. selected a cultivar for best flavor or are harvesting from naturally occurring bushes here in Michigan? Thankfully they are all fairly pest free (knock on wood-with climate freaking out). so if the bushes are small yet you can take advantage of the space between and plant a crimson clover that is inoculated (with OMRI approved inoculum). See this list for dealers of cc seed in Michigan https://www.canr.msu.edu/resources/cover-crop-provider-directory.  note this is a draft as the cover crop team just started this in the winter this year. but it will give you possible sources of cover crop seed in your area.  Its also recommended to maximize your berry yield is to have 2 varieties to allow for cross pollination.  Have you had the soil tested where the bushes are growing? it would be good to do at least initially then you know where you are starting from. Then you can test every couple years if you are feeling your plants are not as vigorous or not as healthy as you expected. You can send your soil sample (about $10/sample plus mailing) to A&L soil Great Lakes as they are geared to work with Michigan soil (their standards they based results are similar to Michigan soils as they are focused on the Great lakes area).https://algreatlakes.com/pages/soil-analysis and the S1 is typically adequate for what you are growing. If you are not familiar with how to take a soil sample here is their guide.  https://cdn.shopify.com/s/files/1/0979/5626/files/Soil_Sampling_Analysis.pdf?<personal data hidden>32982961
Hope this answers your questions. Let me know if you need additional info.
Vicki Morrone

also here is a list, that really needs to be vetted (part of our list to do for the directory that i sent) that has many Mi, In, Il companies that sell cover crop seed. Of course if you need just a bit given your 12 beds you could check with an area O farmer for seed to buy 10 pounds off of them. https://midwestcovercrops.org/wp-content/uploads/2016/09/MI_OH_IN_2011_seed_dealer_factsheet_08-29-11.pdf