A&L labs has a great rep for soil testing. I use them for my farm. The one
nearest you is probably Modesto, Ca
Call them at 800-624-2044. The will either deal with your directly or point
you to someone locally who works with them.
More on-point to you, perhaps, is New Era Farm Service in Tulare,CA
(209)686-3833, owned and operated by Ralph Jurgens. He makes some of the
finest finished compost and compost blends available using the dairy wastes
in his area. I believe he has his own lab. He uses lab results to balance
and blend his products.