a layer of pine needles or pine cones will not make your soil acidic.
Some people will sprinkle on some pine needles & cones as mulch to their blueberry bushes thinking that is how you get acidic soil. But if the soil is not acidic in the first place the blueberries will likely die. You need acidic soil in the first place.
Once
carbon material is completely composted even if it comes from pine and confifers, the
compost is basicially neutral.
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If it was my raised garden bed, I would determine which plants I wanted to plant based upon the soil acidic/alkaline type.
One
should know the soil in their garden if it is acidic, neutral or alkaline. Then plant the plants that like the type of soil you have. It takes a lot of compost to change the alkaline or acidic nature. Ex. my soil is acidic. So if I want to grow annual vegetables, I need a high concentation of compose mixed in with my acidic soil for my neutral loving plants to thrive.
A layer of pine needles as mulch or some pine cones will not make neutral or alkaline soil acidic. I would not mix the fresh pine needles & cones that was freshly chopped up and mix it with a 50% ratio to your soil. But I do add pine needles & cones as a layer of mulch on top.
If I had large quantities of pine needles & cones, I would compost it in a pile and once composed, I would the add the now almost neutral compost to my veggie annual garden.
So as you see, it depends....