Topic: Pine needles for mulching blueberries?

[ceres]

A large Scots Pine in front of my windows has shed a thick layer of needles on the grass.  Would these make a good mulch for blueberries in pots?

[Baccy Man]

Green pine needles will not change the PH of the soil despite popular belief as the acids leach out of the pine needles extremely slowly due to the protective coating on the needles preventing leaching. Those acids break down very rapidly so are not present long enough to mke a difference to the PH.
Green pine needles contain high levels of terpenes which are alleopathic so they inhibit germination of seedlings, beneficial to restrict annual weeds but they can also stunt the growth of existing plants by making water & nutrients unavailable to them. By the time the pine needles have turned brown the terpenes will of broken down & any problems they have caused with the soil chemistry will cease.
Pine needles are very slow to break down so they make a long lasting mulch with all the usual benefits, water retention, warming the soil, inhibiting weed growth etc... Pine needles curl slightly when used as a mulch which allows air to get to the roots of the mulched plant. Pine Needles produce a fungally dominated compost perfect for mulching with a PH of 6.5 (neutral). This is an ideal soil amendment for trees, shrubs, and most perennials.

Personally I would leave them to go brown before using them as a mulch.

[ceres]

They're brown already.  I think the tree has been stressed by the recent very hot weather and the lower level branches have gone brown - it's these that are shedding.  The tree surgeon has been round recently and doesn't think there's anything serious going on.

[Baccy Man]

As they are already brown the terpenes will of dissipated & they will be fine as a mulch.

[ceres]

Thanks, I'll get the rake out then!

[amphibian]

Aren't the pine needles also an important source of ericoid mycorrhizal funghi, without this funghi being present in the soil the blueberries suffer inhibited nutrient uptake.

[Robert_Brenchley]

I doubt whether it would be on freshly fallen needles, but it may well provide a good substrate for it.

[Deb P]

I have used shredded Christmas tree for mulching strawberries a few years ago, and that was very successful as a soil improver. When I discarded the strawberries last year, the soil was beautifully friable, and the sweetcorn I grew there did brilliantly too. So worth a go I think!

[Kepouros]

Pine Needles produce a fungally dominated compost perfect for mulching with a PH of 6.5 (neutral). This is an ideal soil amendment for trees, shrubs, and most perennials.

I find this somewhat puzzling.  I have an area of mature pine trees in my garden - they were large trees when I moved here 50 years ago and the ground below them has remained undisturbed since then.  This consists of gravel subsoil with a 6inch layer of rotted pine needles compacted to a hard crust.  The pH reading of this area is mainly 4.5, and never higher than 5.  By contrast the rest of my garden, including those areas with deciduous trees and a depth of leafmould, gives a reading of around 6.5.   The RHS also appears to think that decomposed pine foliage produces an acid compost.

I was also slightly surprised to note that pH6.5 is neutral, which is generally considered to be close to 7,  making 6.5 slightly, but very definitely acid.  Admittedly the precise level of neutrality can vary slightly according to temperature, but surely not to that extent. Certainly all my calcifuges thrive at this level and my hydrangeas turn blue unless I use lime.

[Robert_Brenchley]

They won't produce a neutral compost. They rot slowly, producing lots of humic acids, giving an acid soil which isn't a good environment for worms and similar soil fauna. Herbaceous plants and deciduous trees rot down to much more neutral material, giving the sort of results that Kepouros reports. If you plant conifers on a neutral lowland soil, you get acidification, and the development of a much less fertile type of soil known as a podsol.

[Kepouros]

Thank you Robert, although I was perfectly well aware of the processes and results involved and probably learned them before you were born. But that is precisely my point.  They won`t produce a neutral compost, they produce a highly acidic one.  In fact, the rotted pine debris from under pine trees makes a highly acidic compost which (when finely broken up) is excellent for mixing in planting soils, or top dressing, for calcifuges where the pH is bordering on doubtful. That is why I was puzzled at Baccy Man`s post which appears a direct contradiction of this.

[ceres]

Baccy Man's info appears to come from here, from someone called the Long Island Gardener:

http://en.allexperts.com/q/Fertilizer-717/pine-needles-mulch.htm

[Kepouros]

And mine comes from a good deal of study and 50 years experience of pine trees and their ecological effects.

[sambucus]

The idea that pine needles make soil acidic is an old hard to dispel myth that has been debunked by good solid research which unfortunately does little to change peoples minds, some people tend to be stubborn & unwilling to accept modern ideas no matter how much evidence there is to support them.

Kepourus during your 'good deal of study' did you happen to come across Dr Abigail Maynards study which clearly demonstrated that neither oak leaves nor pine needles both popularly believed to be acidic caused soil ph to decrease. In reality the pH of soil amended with 50 tons per acre of said materials for 3 years increased from 5.4 to 5.7.
Numerous other studies published over the last 40 years have demonstrated similar results.

Robert Brenchley You may want to read up on titratable acids and bases in tree and shrub leaf litters as your ideas seem a little outdated too.

[allaboutliverpool]

Well, there you are Ceres!

If you have several blueberries, measure the pH now, mulch half with pine needles and record the results, (crop size and pH) over the next 2-3 years and let us know.

Someone has to be right.

[Robert_Brenchley]

Thank you Robert, although I was perfectly well aware of the processes and results involved and probably learned them before you were born. But that is precisely my point. 

You probably were, but it's clear that not everyone is, and some explanation was needed! Sambucus, do you have any sources for this? You might, eventually, get a neutral compost once decay had ceased, and acids (and a lot of other things) had leached out. But I've seen podsolisation at work so I do have some idea what happens in practice.

[Baccy Man]

Baccy Man's info appears to come from here, from someone called the Long Island Gardener:

http://en.allexperts.com/q/Fertilizer-717/pine-needles-mulch.htm

I was under the impression my info came from numerous sources amongst other things there are multiple studies I have read on the subject & numerous magazine articles which have covered this topic (along with references to where they sourced the information which I may of looked up to ensure things were not being taken out of context) such as:

Excerpt from “Organic Gardening”

July-August: 1997. V44. n6 p.5

Complete Guide to Organic Mulch, by Scott Meyer


“Maybe you’ve avoided using pine needles in your garden because you’ve heard that they’ll acidify your soil. Well, when Clarence Johnson, Ph.d., State College in Georgia tested the pH of soil in plots that had been mulched for 2 years with 3 inches of pine needles, they found that the soil remained at about the same pH (a near neutral, garden friendly 6.8) that it was when they tested it before any of those pine needles were applied. An those needles were also a great mulch! Eggplants mulched with pine needles produced 20% more fruit than plants that were grown with no mulch at all.”

Obviously typing a few words from my post would reveal numerous unsubstantiated claims made on other gardening forums or other questionable sources such as wikipedia essentially saying the same thing. As there are an awful lot of things written about on the internet things tend to get repeated an awful lot. If you prefer to believe that i sourced the info from one of these unsubstantiated sources that's fine with me.

BTW the study sambucus appears to be referring to by Dr Abigail Maynard was published on the University of Connecticut website in 2001 if I recall correctly. I believe it should still be available in the public archives, I am sure there are numerous other sources too but as she works for the UCONN Agricultural Research Station & it is the site I originally read it on it is the first place I would start looking, I may still have a copy somewhere if you can't find it.

[ceres]

As there are an awful lot of things written about on the internet things tend to get repeated an awful lot.

Yes, it can be interesting though tracing back verbatim passages to see who originally wrote what and when.