It’s a common misconception that pine needles can change soil pH. It is also thought that pine trees themselves change soil pH. Pines generally like slightly acidic soil conditions for healthy growth. It is known that they host various soil micro organisms (fungus, bacteria, etc.) that aid them in many ways and can even buffer the soil pH for preferred pH levels, but the needles themselves have minimal to no affect on changing soil pH. It is true that green needles are acidic (3.2 – 3.8 range) but once they turn brown and fall off the tree, these brown needles are pH neutral (7) or even slightly alkaline. I will discuss what’s going on with the needles, why, and a multitude of benefits of using this under-rated resource as a mulch and its ability to aid in building the soil food web and nutrient cycling in your soils as well as benefits to water conservation and weed suppression.
Green pine needles are acidic and have a number of various acids (shikimic, phenolic, etc.), polyphenols (flavonoids), and aromatic compounds (terpene, limonene, etc.) but these components are not responsible for soil acidity. Some of these compounds either get leached out or reabsorbed into the tree before the needles brown up and fall off. The brown needles on the soil surface under the trees are neutral in pH. By the time they hit the ground, the acids have left leaving a source of flavonoids, a bunch of minerals and nutrients, and most importantly, a host of beneficial micro biota such as fungus and bacteria gathered from the healthy forest system.
Brown pine needles, also known as ‘pine straw’ in the South, are a wonderful, underutilized resource for your garden or planting beds. When taken from a healthy forest and used in your garden or planting beds, they offer a host of micro biota (fungus, bacteria, etc.) which will help with rebuilding the soil food web. They are high in flavonoids, vitamin compounds (A, B, C) and micro nutrients such as Calcium (Ca), Magnesium (Mg), Phosphorus (P), Potassium (K), Iron (Fe), Manganese (Mn), Zinc (Zn) and other trace elements of copper, aluminum and silicon. These add a wonderfully diverse mix to your soils. The content of the flavonoids (and hosting the micro biota) are the most interesting parts.
Flavonoids are extremely beneficial in soils; there are many. They help promote nitrogen-fixing bacteria, promoting nodule formation in legumes. They help encourage the formation of arbuscular mycorrhizal (AMF or endomycorrhizal) associations with plants. They help plants defend against pathogenic fungi and bacteria. They can inhibit weed growth. They help the processes/stimulation of root growth, nutrient uptake and how the roots interact with the soil. Additionally, they can send chemical messages to the plants and microbes impacting the interactions between plants and soil biome for better nutrient availability. There are some methods to increase the flavonoids in pine needles which mostly revolve around a heat or thermal treatment; 120-130F. A warm tea at that temp is effective at increasing flavonoids, then the cooled liquid can be added to the soil. However, the heat treatment will likely kill off any micro biota that existed on the needles. It’s best to just lay it down and let nature do its thing.
In addition to the chemical and biological components of pine needles, it is used as a mulch. It is going help protect the soil and reduce soil temperatures in those hot dry months, while increasing soil water holding capacity, aiding in water conservation and preventing weed growth. Additionally, it is relatively slow to break down so an application will last quite awhile. There aren’t very many reasons why one should not to use pine needles as a mulch.
Now that we have established that pine needles don’t change pH, but do add nutrients, host microbes promoting soil food web, increase soil organic matter, water conservation… How Do You Use Them? Typically, 2 – 4 inches of needles is good. One can go about it two (2) ways, maybe more. If you’re not using a cover crop, lay it down thick in the fall, then come planting season, move it aside and plant your transplants. If using a covercrop (preferred), chop and drop the cover or turn it in, then plant your transplants and mulch around them. Pine needles also work great in a more permanent planting system such as berry bushes, cane plants or ornamental landscaping. I will note, it does not work well for small seeds (such as carrots, parsley, etc.) that you are trying to germinate in-situ, but that’s a situation easily avoided. Pine needles are going to have all those benefits of a mulch with some added bonuses helping to build that soil and soil food web and making your life a lot easier.
Take home message: pine needles/pine straw is awesome! Plant ON!
_____________________________________________________________
. Patrick Rahilly, founder of Pinecrest Permaculture LLC, began with his roots in a 5th generation farming and ranching family coupled with a love of the outdoors and spending time in the wilderness. Observations of nature and natural ecological systems came naturally with the writings of John Muir having a large impact in life path. Formal studies in undergraduate, graduate and post graduate work were focused in wildland soils, ecology, botany, wetlands, water resources and agroforestry. Patrick received Permaculture Design Certification in 2021 with instructor Alan Booker. In 2022, Pinecrest Permaculture was formed. Primary focus of design work revolves around design of small homesteads coupled with habitat restoration. Patrick’s diverse background from youth, to formal training and love of the land has aided in a unique perspective of permaculture design. He strives to create self regenerative systems that mimic natural ecological systems.