For pH 6 to 8 you can use phosphate buffers.

From pH 4 to 6 you can use acetate buffers.

The difference from 4 to 8 is about 10,000 based on the logarithmic scale of pH.

You might want to use cellulose pads, sand, vermiculite/perlite and soil as medium for the tests.

Use at least 4 seedloots of 50 to 100 seeds to get good mean separation.

Dear Paul Reed Hepperly ,

Thank you very much.

You might want to stay away from anything that has "nutrients" in it as that may impact your germination studies (enhancement effect). Maybe think about collecting rain water, and adjusting pH with sulfuric or HCl acid. It won't take much - and that way you are not using phosphate or ammonia based pH buffers.