Let’s face it, what we often take to be common horticultural knowledge is often a mixed bagged of misinformation, outdated concepts, and half-truths at the best of times. One that is persistent in horticulture and plant propagation is that pine acidifies soil and that the application of pine bark or pine needles promotes acidification and pH buffering against changes toward the alkaline. One of the most widely held beliefs is that conifers (division Pinophyta), such as pine trees, acidify soil and therefore the use of pine needless and pine bark are excellent sources of organic soil acidification. Trees, all tress, do change the pH of the soil they are planted in, but changes to soils take many years for even modest changes. While it is true that soils associated with conifers tend toward the acidic and those found under broadleaf trees tend toward the alkaline, the inference that this is due to the organic build-up of sloughed bark and needles and extrapolated belief that use of such plant material will have a similar benefit in the landscape is misplaced.
I can understand how this myth has been propagated as our ability to test broad markers in soil, such as pH, has been around much longer than our knowledge of soil, plant pathology, and the recent exponential growth in environmental sciences. If in testing the soil pH around conifer forest always came out more acidic and than those of broadleaf, then the trees must be responsible, and they largely are, but not for the reasons people believe. Turns out that the primary cause (any such change to chemical interactions outside of the lab will have multiple vectors leading to many various elements affecting soil chemistry) is the calcium content of the foliage. Understanding this allowed arboreal and soil scientist to run increasingly sophisticated experiments that found tree divisions has less to do with the acidification and instead it has almost everything to do with free ionized calcium content. This means that tree species see significant overlap in pH influence and that a coniferous species like the silver fir (Abies amablis) which has as much calcium in its foliage as the red oak (Quercus rubra) had soil pH profiles at almost the same levels despite one being a soft wooded”pine” tree and the other a hard wooded broad leaf. One of the regional trees that has the highest acidic foliage influence (and so one of the lowest free calcium contents intus folium) is the Cercis canadensis otherwise know as the Eastern Redbud testing at a pH of 4.3 (per the reference http://www.asecular.com/forests/phleaves.htm ). Not only does free calcium ion availability influence the pH but one must also account for buffering agents within the soil that will resist any meaningful change to the soils pH. To reliably change your soils pH, one must use chemical interdiction with known inputs and outputs as these tend to use methods that reliably affect pH and can be used to adequately overcome more natural buffers
So what to do when one needs to naturally and organically change soil pH? The top materials for affecting soil pH are sulfur powder (highly acidifying and long-lasting but takes time to begin soil pH change) and dolomitic limestone for raising soil pH. These natural materials slowly lower or raise your soils pH by providing a material, sulfur for example, that soil microbes can use to and as a waste product change your soil. We’ve all heard of sulfuric acid, well this is exactly what a healthy soil will produce when feed elemental sulfur and it is that acid which acidifies. There are other methods for lowering your soils pH, Ammonium Sulfate is a great product for quickly lowering pH (slightly) while adding nitrates as well as sulfur for long-term control though it is not considered an organic material for the purpose of labeling food grow with it. One should NEVER use aluminum sulfate (expect with hydrangeas in pots where the aluminum sulfate will not leach into the surrounding soil) as it is a poison in soil and will adversely affect your landscape and soil healthy for many years after application.
Hopefully this provided some insight into how complex soil pH is (we only roughly scratched the surface) and how the use of pine parts to lower pH is a soil myth in need of an end.