I’ve been spending my rainy winter days making a master plan for my garden, and I’m hoping to get some more fruit in there, since a small handful of berries lights up Brian’s face in a way that a whole wheelbarrow full of chard and onions never will. Because of this, blueberry bushes have been popping up all over the place in my garden plan. Advice abounds telling me to put blueberries in acidic soil, and how I can easily change the pH of my soil by mulching with pine needles. Since I have to rake pine needles off my deck anyhow, mulching with pine needles sounded great- but then I started reading about how the claims of pine needles changing soil pH were untrue.
Before I get to the shocking conclusion about the effects of mulching with pine needles, let me quickly remind you what the pH scale is all about. A pH less than 7 is acidic, and a pH more than 7 is more alkaline, which is also sometimes called basic. From a chemistry perspective, pH all adds up to which ions the soil has an excess of, especially the ions that add up to water, H+ and OH- (it’s not just a coincidence that the letters add up to H2O). Don’t remember what an ion is? Ions are charged particles, and when it comes to soil science, ions are usually the product of dissolving something in water. Some essential plant nutrients are present in damp soil as ions, including calcium, magnesium, and potassium.
A few of the things that effect native soil acidity are heavy rainfall, the parent material of the soil, and the amount of decaying organic matter. While those factors add up to generally acidic soil in Western Washington, who knows how much of the dirt I’m gardening with was naturally deposited on my little plot of land and how much was hauled in.
So do studies show that mulching with pine needles increase acidity? It depends on which study. Some folks have seen slight decreases in pH (which means increases in acidity), and others haven’t seen any pH effects. Does this mean that some of the studies must be wrong? Of course not- whether a mulch or soil additive will change the pH of a soil depends on lots of factors, including the amount of rainfall in the area and especially the composition of the soil. Some soils have more buffer capacity, which means that they are more able to resist changes in pH. Clay soils are known for their ability to resist pH changes, as opposed to sandy soils.
Am I going to put those pine needles that I raked off my deck around my blueberry bushes? You bet I am. My pine needle mulch may not make the soil acidic enough to keep my blueberry bushes happy, but at least it will help smother the weeds.
 M.L. Duryea, R.J. English, and L.A. Hermansen, “A comparison of landscape mulches: Chemical, allelopathic, and decomposition properties,” Journal of Arboriculture, vol. 25, 1999, p. 88–97.