John's point is very valid. The value of pH (power of Hydrogen is what I was taught, although potential of hydrogen is also used) is based on the molar concentration of hydronium ions (H30+) to hydroxyls (OH-) in a liter of water at 25C (had to look that part up, kept thinking it was 20C). It is a logarithmic scale so varying concentrations of hydrogen in the water will occur due to impurities in the water. When you use a soil pH meter, you are dependent upon a certain amount of water to be available to get an accurate reading. Pure water is a pH of 7, and is made more acidic or alkaline based upon the -log10 ratio of hydrogen ions to hydroxyl cations. Lots of science there.
Basically the question is how the acidic products of pine (for a broad definition of pine) will affect a hugelkultur bed. The other observation to find out about then it what is the "normal" pH of hugelkultur beds that have been successfully demonstrated with other woods? Most of the
trees that have been spoken of for hugelkultur, I believe, are fairly acidic in nature. But I'm not totally sure on that since I haven't researched it thoroughly
enough. If they are alkaline in nature, then the solution would be to use lime in a correct ratio to raise the alkalinity level such that it is on par with beds that are successful with an alkaline character, if that is what they are. I don't know if any of that data is available.
Since the goal of the wood in a hugelkultur is to spongify it so it acts a sink for moisture, that is freely available for the
roots of the plants during drier times, and replenishes through natural water migration of surface water into the soil/bed during wetter ones. This is a function of the woods decomposition to its cellulose base as the non-cellulose material is decomposed away and is available for the nutritional use of the plant systems put on it. If a pine wood based hugel is constructed, I would think that the cover trees/bushes would be more productive if they loved acidic soil - which is the natural ecology of fruit bearing plants in Montana. Does pine rot to its cellulose base in the presence of moisture - yes it does, especially with insect activity to accelerate it. I've had many a "solid" pine log collapse when I stepped on it due to this.
Part of my intended experimentation this coming spring/summer/fall season is to see what this effect will have on a hugelbed and whether it will be productive with cover foliage I'm intending to use. The foliage being specific
native fruiting plants and "weeds" that the critters find so tasty. I know that pigweed and amaranthus grow the best under my pine trees and in the part of my garden that is fairly acidic. The raspberries, my apples and native plums all seem to like it as well. In fact, one of the outcomes of my neglect of my garden and mini-orchard the past couple of years has been the propagation of the native plums and the raspberries to the more acidic part of my garden. Potatoes seem to love it as well once I put some inert material (composted mint silage) into that soil to loosen it up.
A long winded post, hopefully it gave you something to think about and didn't make your eyes roll back in your head. If I have made any errors in assumption, please feel free to correct me.