No, in your lifetime, normal amounts of pine needles will not measurably acidify your soil.
They are somewhat acidic, and acidify soil over long periods of time, unless the soil base is extremely alkaline. They don't acidify soil more than other deciduous tree leaves, and oak leaves in particular (they have a pH of 4.5 to 4.7). Rain does leach the acid out, but the amount that is contained in the leaves isn't that much in comparison to the amount of acid produced during the decomposition process. Basically, if the only source of OM is pine needles, yes, they will build an acidic soil (the topsoil in most old coniferous forests), but not more acidic than that which was created by a hardwood forest.
It usually takes a very concentrated acid to usefully change the pH of soil in garden beds. Pine needles just aren't that effective, and while they build the soil and make a great mulch, even for acid loving plants, they don't change the pH fast enough to be of any use to that end.
Also of note: My neighbor mulches his blueberries with lawn clippings, has been doing so for years, and has some of the healthiest blueberry plants in the area.
See this article (no scientific backup, but or the most part accurate)
Are pine needles acidic?
It turns out that fresh pine needles taken from a tree are “acidic”. By the time a pine needle gets old and is ready to drop off the tree it is barely acidic. After a few days on the ground, it loses its acidity completely. The brown pine needles you will likely collect from the forest are not acidic(1).
But, but , but, you say – surely over many years, the acidity must build up. This seems very reasonable and so some scientists tested this theory. They collected soil samples from underneath 50 year old pines. They also collected nearby soil samples where no pines had been growing during the same time period. They found that the pH of both soil samples were the same. The growing pines did NOT acidify the soil even after 50 years(2).
Why does acid rain not acidify the soil?
Southern Ontario can be considered to be a large limestone rock. Our soil has been created over millions of years from this limestone. Limestone is alkaline and so our soil is also alkaline. Mine has pH of about 7.4.
Consider this. Rain that has no pollution in it has a pH of 5.6. You might expect it to have a pH of 7.0 since that is the pH of pure water. However, as rain falls, it absorbs CO2 from the air. When you add CO2 to water you create a weak acid (carbonic acid) and that acid has a pH of about 5.6. Keep in mind that this is taking place without pollution. Add in the pollution and we get acid rain. The rain falling in central Ontario is about 4.5.
For millions of years, Ontario has had rain fall with a pH of at 5.6. In all that time this amount of acid has not been enough to neutralize the alkalinity of our limestone rock. As the acidic rain hits the ground, it neutralizes (dissolves) a bit of limestone, but the amount is extremely small. It will take another billion or so years before it changes the soil pH.
I have used Ontario as an example, because I know it best. The same principle applies to most soils. It takes huge amounts of acid to change the pH of soil.
Before I close, let me say that adding pine needles to your garden is a good thing. They are organic and will help enrich your soil. They just won’t make it acidic.
(1) Pine needles actually retain their acidity, but the misinformation does not detract from the message.
(2) This is why almost any land that has been turned over to cultivation was initially heavily limed. It mostly started out acidic. There are exceptions, though.
From here:
Some gardeners shy away from using pine needles as a mulch or in their compost. The needles have a relatively acidic pH and we worry that they will alter the pH of the soil. According to soil scientist Lee Reich, pine needles will indeed probably lower the pH, but only temporarily. Soil pH may trend slightly acidic no matter what organic material is decomposing, but it eventually moves back into the neutral range, as the decomposition process completes.
I entirely agree with the part 'soil pH may trend slightly acidic no matter what organic material is decomposing'. It's very true. Decomposition causes acidity, and most plant matter starts as acidic. It just takes far too much acid to cause a big change, that pine needles don't need to be treated as more acidic than other leaves.
From here:
Although pine needles are very acidic, they do not necessarily lower soil pH. As noted in the Columbus Telegram, pine needles actually have very little effect on the pH levels of surrounding soil.
From here:
Q: I've heard that pine needles and oak leaves cause soil to be acid. Is this true?
A: This is a gardening myth that should be put to rest. While needles and leaves from all trees may be acidic initially, they do no acidify the soil as they break down. Soil has a natural ability to buffer changes in pH. Soil acidity might increase slightly for a few weeks after leaves fall but the acidity will go away after a few months. It’s true that pines can grow in poor soil, but they didn't make the soil poor to begin with.
References:
Plus years of personal experience.
