The tiny, scale-like needles from junipers make an acceptable soil amendment without even composting; the same is true of cypress needles. Spruce needles can be composted (as can juniper and cypress needles), but they require frequent turning in order to bring necessary air into the center of the mass. That's because they tend to pack down when piled up.
With my recipe for potting mix (see , you really need actual compost. This supplies the beneficial soil bacteria that convert nutrients (e.g., blood meal, bone meal) into forms that plant roots can use. The recipe only requires a small amount of compost, perhaps just one handful in 5 gallons of mix (assuming that everything is thoroughly blended together). Once the mix is blended and watered, the soil bacteria will propagate throughout the mix.