Soil does not have fertilizer in it - unless you were going to buy that stuff from Miracle Gro that has fertilizer in it. You don't need fertilizer in the soil unless you have roots in the soil, so that would be a waste of money for the spaces where there is no grass. I'd use sand, but anyone who follows me would have predicted that. Sand is cheap, easy to spread, and makes a good base. I would use a coarse sand with sharp edges - the kind used for sandblasting.

it will be interesting to see how this works after a few years. The questions is will it be bumpy?

Thanks for the recommendation. You have a point about the soil not having anything in it to fertilize.

You can mix them but straight sand is better & easier. There is a third option, calcined clay such as "turface" or profile field & fairway. Very expensive synthetic dirt basically. Golf courses & sports fields use it by the pallet

Thanks Reeljake. Considering that checker boarded the sod to save money, I'm probably not going to get some calcined clay.

Sand is pretty cheap by the bag & available at any box store. Bags are convenient but it takes a lot of them to cover a given area. You get more grain options & more bang for your buck with a bulk delivery of sand from a mulch yard, but adding delivery fees can sometimes price this option out. I like low grade paver sand or all purpose sand, not play sand. Too many pebbles & soft edges in that stuff

Thanks Reeljake.

Here's a link to some sand varieties from a San Antonio based materials supplier. I like the coarse ones in these pictures.