my very un-scientific opinion FWIW: I once read in aa magazine dedicated to organic gardening that pine needles will not make your soil more acidic. So, that year I tried pine needle mulch around my tomatoes. It did a great job of preventing weeds. A soil test at the end of the year showed that my soil was more acidic than a year earlier despite the lime that I added in the spring. I'm assuming that it was the pine needles that caused the problem. Tom