From my understanding of soil science the amount of organic matter and biology present has a large influence on how the pH of the soil affects the growth of plants. The more biology (micro and macro) present the more the organic matter is being cycled to humus (the lowest decomposition state of organic matter). The more organic matter and biology present the higher the buffering capacity of the soil. Typically areas with large populations of conifers are areas of low organic matter (i.e. sandy etc) and relatively low concentrations of biology when compared to productive agricultural soil or soil with mixed deciduous and conifers present. Now there are large parts of the world where the coniferous ecosystems have developed naturally and there are also large parts where humans played a large role of perpetuating and or creating these ecosystems. I live in Maine and the evidence of this is all around me. Due to the intentional planting and cultivation of primarily coniferous trees over large areas, the overall productivity of the soil and the surrounding ecosystem has been greatly diminished. Unfortunately this is true all over the world. Sepp points it out over and over again, as do many others. These coniferous trees have shallow root systems that are very limited in their reach to subsoil minerals and negatively effect the water table by doing little to raise it. The intentional mono cropping of these conifers creates less fertile soil with less biology and a shallower water table. The high concentration of shallow root systems causes more runoff and less sinking of water creating less and less fertility over time. Not to mention the dense shade created by the conifers which negatively effects the growth of the under story further disrupting the deposition of mixed organic matter. Most of the time these systems favor the regrowth of more conifers continuing the trend of less fertility and disrupting the nutrient and water cycling abilities of the diminished biology.
That being said it seems to me that hugelkulture by definition has a large amount of organic matter present in many different stages of decomposition and this high amount of organic matter greatly affects the Cation Exchange Capacity, or CEC, of the soil. It is the CEC that has the greatest effect on the productivity of the soil in regards to plant growth, not necessarily the pH. Typically the higher the CEC the more nutrients and organic matter is being cycled which typically creates soil that becomes less acidic over time. the higher the CEC the more buffering capacity the soil has in regards to pH, water quality etc. This buffering capacity is why many people (including myself) who have used coniferous material in the construction of their hugelmounds can often get away with growing plants that typically require soil closer to neutral or alkaline. Of coarse the size and amount of the material affects the speed of the breakdown of the material and thus the cycling of nutrients and retention and release of water. The source of material used in the construction of the mound does affect the composition of biology present and its the biology that is producing humus, or perfect plant food. Each plant has evolved with specific biology and its this biology that proves most vital to the success and productivity of the plants growing on that soil. My suggestion is typically to brew aerated
compost tea with a handful (or more for larger batches) of soil from a local natural area where the plants (or closest relatives) are thriving as a main source of the beneficial biology in the tea. Tailor your tea to be more fungal for woody perennials, balanced for grasses, biennials, and long season annuals, and more bacterial for short season annuals and brassicas, by what type of inputs you use in your tea. In my experience, adding the correct biology, or making sure it already exists has a profound effect of the success of your plants. I also usually suggest adding as much diverse biology to your system as possible, typically the organisms you want or are appropriate will reproduce and the rest will become food for the organisms you want. Also to ensure greater success of your hugelkulture, source materials from an environment similar to the ideal native environment of the intended species, i.e. white pine with highbush blueberries etc, this will enhance the propagation of the correct soil biology for the intended plant. Over time pH becomes less of issue or even a non issue because the soil biology corrects and tailors it the specific species. Also by choosing the correct pioneer species to help create the right conditions of succession (i.e. biology, structure, nutrients, etc) for your intended cultivated species will also greatly affect the productivity of the particular intended species you would like to promote.
I know its a rather long winded for my first post and feel free to correct or comment on any of it. I will work on fine tuning my posts to be more concise in the future. Thanks