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[Alan Haigh <alandhaigh@gmail.com>]

Softwood tree mulch tends to have less calcium in it than hardwood so I assume it is more acidic.  I think Ginda was referring to measurements I took on a site with 80 year old pine trees where needles had never been removed under the trees.  The pH readings were identical under the needles as they were quite a distance away where there was mowed turf and no needle influence. 

 

I was not surprised by the readings because whatever calcium was being drawn out of the soil by grass or trees was cycling and the readings merely suggested that calcium loss was probably occurring at about the same rate in turf as under trees. 

 

I have observed over the years very little acidifying affect from the use of pine needles as a mulch.  However, I once got very low readings from soil that had been mulched with pine sawdust for several years at a site where surrounding soil was much sweeter.  My guess is that it was the result of the sawdust.  I wonder if most of the calcium being held by conifers isn't in the needles themselves.  I really don't have enough experience with mulching with sawdust to draw any conclusions.

 

The very act of composting helps create acidic conditions at the interface between mulch and soil where feeder roots of blueberries presumably can pick up adequate iron from fine roots that tend to thrive there.  That and the blueberries poor tolerance of drought and love of generous organic matter may explain why under a good mulch blueberries can thrive where pH is close to neutral. 

 

Carl Whitcomb has proven that acid loving plants (pin oaks) can obtain adequate iron from a very thin surface layer of acidified soil in otherwise excessively alkaline soils.  He used surface applications of pelletized sulfur to do this.