Bitter tasting greens

Our garden soil is inherently decomposed granite. I have amended it over the years with mulch of organic compost, peat, potting soil and just about anything I can get my hands on. No matter what I do, any lettuce of any variety or spinach and this year, many varieties of mint, come out tasting bitter.

We live in the mountains of Colorado at 9,400 ft. Can you help me???