Plants often require certain levels of acidity or alkalinity in the soil to enable them to access the various nutrients it holds. The degree of acidity/alkalinity in a soil is expressed in terms of its pH. A numerical measure, pH is indicated by a numerical scale from an extremely acid 1.0 to an extremely alkaline 9.0, with 7.0 indicating neutral. Rhododendrons, azaleas, and hollies prefer a fairly acidic soil with a pH around 4.5 to 6.5. On the other end of the pH scale rose of sharon, butterfly bush and sedums will accept a more alkaline soil. Most common plants in the home landscape prefer a more neutral environment with a pH ranging from 6.0 to 7.2 or so.
Compost definitely influences soil pH in many circumstances. What is not entirely understood is how it does this. It has been determined that a plant growing in soil with a high percentage of compost (2 to 5%) seems to be able to modify the soil in the area of its root system to conform to its particular pH preference. Earlier in this book we mentioned the study that revealed that rhododendrons planted in highly alkaline soil had created acidic conditions around their rootballs and all that had been done for 20 years was to add 1 inch layer of compost around those plants. While this phenomenon is not likely to occur in just one season with annual flowers and vegetables, it may occur with trees and shrubs after many years of compost applications.
Generally, compost produced from various organic materials found in a typical residential landscape measures roughly neutral on the pH scale. Moreover, research has not yet provided information on how, if it is possible at all, to specifically create composts which have inherently acidic or more neutral pH ratings. Oak leaves are usually very acidic, but compost made from oak leaves may have a neutral pH or, sometimes, an acidic pH. The variables are not yet understood, so it is currently impossible to fine tune the composting process in order to produce a product that is predictably and reliably within a certain pH range. Also, it is not clear at this time if a compost that is acidic, will remain acidic once it is introduced into soil. Some studies have indicated that an acidic compost will become neutral in just a few weeks after it is introduced into soil. It is clear that more work is necessary around the questions of pH and compost.
In the meantime, what is known is that most plants in the home landscape prefer a fairly neutral pH and that most home composts have a fairly neutral pH. So the obvious conclusion is that if there is lots of compost in your soil, most of your plants will find their preferred pH levels without any other intervention required by you.