Pine needles make a great mulch, but do they make soil acidic, or is this a gardening myth? Patrick Dolan and I share our thoughts on the topic.
Question 1: I’ve always heard that I could use a pine needle mulch to acidify the soil for blueberries and other acid loving plants, but a viewer recently sent me a link to research from xyz that indicates pine needles do not significantly increase soil acidity. What are your thoughts?
Response 1: Although it is thought to be a common conception that pine needles acidify soil it is in fact likely not the case. Even universities such as Cornell make reference to pine needles acidifying soils when used as a mulch. Pine needles are typically acidic in nature. During the very slow decomposition process pine needles lower from a pH of around 3.5 to nearly neutral 7.0. The mulch itself may be acidic however with minimal transfer to the surrounding soils. Generally soils have a neutral pH of 7.0 and are able through their own chemical to buffer any pH transfer from the needles. I have attached an information sheet from Washington State University as a reference.
Question 2: But if pine needles don’t make the soil more acid, why are the soils in pine forests often more acidic?
Response 2: Typically soils in pine forests are slightly acidic due to a number of features. The Boreal Forest spans Northern Latitudes from Canada, Russia United States and the Scandinavian Countries. A major feature in the Boreal forest of Canada is the presence of bogs and fens covered in a dense canopy of pine trees.
Soils in the boreal forest decompose slowly due to the low temperatures and limited microorganism activity. The dense canopy and annual precipitation rates result in low water evaporation is limited. This soil is often left water logged allowing limited nutrient cycling compared to southern forest types.
The longer the water is in contact with the mineral base the easier it is for elements to leach into solution and move into different layers of the soil. Elements like iron and aluminum leach from the A horizon (the top) to the B horizon (below A) followed by clay. That leaves the A horizon with a sandy neutral mineral content. Once this migration happens the elements and cations that buffer the addition of acidic material move lower allowing for the acidification of the A horizon.
The low decomposition rate, influence of the water regime and the aerobic situation, tannins and other acids materials decrease the pH of the boreal forest soil
Question 3: You said that the pine needle mulch is acidic and does not transfer. If I mix it into the soil would that lower the pH?
Response 3: unfortunately no. Pine needles decompose very slowly and even in situ will not transfer acidity to the soil.
Question 4: What can I do to lower the pH of the soil.
Response 4: I am glad you asked. There is a number of ways you can lower the pH of your soil.
Peat is a great way to lower the pH. If you grow in a peat mix of use peat to top dress the soil it will help lower it.
The second and the most effective is sulfur amendments. Elemental sulfur does require time to be converted to sulfuric acid with the help of the soils microbes.
The third and final method I use is cold black coffee. Cold coffee has a pH of around 5 and acts as a weak organic fertilizer. Much like an extraction compost tea the organic material that makes coffee so fantastic are also available to both the plant and the resident microorganisms. Just as an aside although used coffee grounds do not have the acidity that the coffee does they do add some fantastic N P K and can typically be picked up for free at local coffee shops.
There are other organic ways to lower the pH. From the most acidic to the least Lemon, vinegar, orange juice, tomato juice, black coffee. I do recommend if you are going to use anything else do the research and test out your application methods on some soil before applying it to your garden.
It is important to note it is recommended to avoid trying to drop the pH of your soil more then one point per year. If your soils are alkaline or have a lot of clay in them the pH will recover over time as the basic elements interact with the acidic ones to neutralize each other.
Please see the links to sources on this topic below:
Links provided by MrChipGardener:
Forest Industry Council – http://www.forestindustrycouncil.com….
Washington State University – http://spokane-county.wsu.edu/spokane…
Colorado State University Extension http://csuturf.colostate.edu/Powerpoi…
“Decoding Gardening Advice”, p. 49
Wood Chip Mulch: Landscape Boon or Bane?
Acid Rain Effects – Soils http://www.elmhurst.edu/~chm/vchemboo…