Relax Mary, 7.5 is alkaline, but you can grow great stuff in it. My soil is 7.8 and there are very few plants I cannot grow. How much is too much organic material? If you are trying to grow water wise xeric plants, yes that can be an issue. But many plants love humus rich soil as do veggies. Humus in organic rich soil will help to buffer the pH. Unless you are using all kinds of chemical pesticides herbicides and fertilizers, you should have lots of soil microbes. If you are having trouble growing things you my have other issues. Hard to tell not knowing the history of the soil. Best And Happy Yardening, Nancy.