Blankets, ratchet straps, open windows and slow speed.
I had a 3pt cement mixer for my tractor. I would have everything figured out on the weight/volume all the items and have everything fully measured out and separated. I would mix all my smaller volume/weight amendments together in the drum, and if needed more to fill the mixer up, I would add what ever was the largest bulk item. It's easier to get a nice even mix of all your smaller nutes if you mix them together in this manor and just evenly spread that on top of your bulkier part of your mix laying out on the tarp.
If you don't have a mixer, just do the same thing with a tarp. Spread your smaller nutes out over the tarp and just fold it in on itself, spread it back out an fold it in on itself. Then just evenly spread that on your bulk like the other method and fold and spread until its well mixed.