How to keep birdseed from making a mess on ground?

If you feed the birds that is going to happen. Shovel up the mound and dispose.

Have you tried a fly through feeder?

https://www.wildbeaks.com/platform-fly-thru-feeders/?gclid=Cj0KEQjwmcTJBRCYirao6oWPyMsBEiQA9hQPbuBwzSXeP3_A649aSM6EFWGs14WoFg5K9rgryfM4Mq4aAjQs8P8HAQ

They make a no mess bird seed-

https://www.chewy.com/wagners-shell-free-premium-wild-bird/dp/124705?utm_source=google-product&utm_medium=cpc&utm_campaign=f&utm_content=Wagner%27s&utm_term=&gclid=Cj0KEQjwmcTJBRCYirao6oWPyMsBEiQA9hQPbgb7FwPzvP9QZZuc6gKMfJd3cl9MK2U7CvvKqFKEgNgaAgWx8P8HAQ&gclsrc=aw.ds

Put something under there, that you can pull out almost daily, to keep it clean, otherwise you will attract mice. Happened to us, under a very similar situation. Not fun.

Is it possible to pour concrete below the deck so that you could sweep up the seed excess?

There are attachments for underneath the feeder that will catch the falling seed or you can make one from a pie tin.

Birds are filthy, nasty, messy creatures and I love them. I've tried all kinds of different feeders and contraptions to catch the seeds they fling around and none have worked well enough to be worth the trouble and expense. I have a separate dust pan and broom that I use to clean up. I've also used a small shop vac to tidy up after them. Enjoy them in spite of it all!!

I used a string attached to the post holding the feeder and made a circle about 4' in diamete. Then I got some small retaining wall stones. (Cost about $1.50 each) after I made the circle with the stone I filled the circle with stone dust ( about $3.00 a bag) and I packed it down. It makes a hard surfice That looks good and the seed doesn't seem to sprout. Hope that will help!