If anyone is interested in super scaling up this method I made 3.3 MILLION, yes million, seed balls for a 35 acre restoration project in the back country of San Diego County. Our balls were made with a little bit of sand, peat moss (can substitute for manure I would think), red clay and seed. We used seeds from 7 native species, some grasses, herbaceous plants and shrubs, all part of a guild.
We got a regular old cement mixer, made sure there were no paddles in it so it was pretty smooth inside the mixing chamber, then added our dry mix with all the ingredients, started the mixer up and slowly added water with a hose nozzle. After a few minutes the spinning action of the mixer would start to produce perfectly round balls. Then we took a scoop, like a animal feed scoop, cut about 1 inch holes all around it, then used the scoop to get out the balls, letting the smaller ones fall through and the bigger ones we scooped out, put on a rack and let dry outside in the sun. We kept adding small amounts of water and scooping out the balls.
Later we went out and distributed the balls at our restoration site. Unfortunately the person that set up the monitoring of the project did not set up a good system to test the efficacy of the balls, like test plots, but we did monitor the balls and found them to properly dissolve after significant rain. We also found appropriately aged target species in the distribution area beginning to come up and it seemed to look better than other areas. As my own test to be sure the balls would sprout I took some of the balls, threw them out at my place and watered them by hand, they did sprout fantastically.