Examining the Effect of Established Conifer and Deciduous Trees On Soil pH

The idea that leaf litter from coniferous trees reduces soil pH more than non-gymnosperm deciduous trees is frequently touted in popular horticulture literature as the reason plants are difficult to grow beneath conifers.  Adding lime to elevate pH and growing acid tolerant plants are the commonly recommended solutions. We examined soil pH beneath ten tree species (seven conifers: Abies concolor, Larix decidua, Picea glauca, Pinus banksiana, P. ponderosa, P. strobus, P. sylvestris; three non-gymnosperm deciduous trees: Acer saccharum, Malus sp., and Quercus macrocarpa) with two trees represented per species (20 trees total) at the University of Minnesota Landscape Arboretum, Chaska, MN.  The trees were growing in their location for over 25 years and natural leaf liter was allowed to accumulate at their bases.  Soil was sampled three directions out from each tree at 5 foot intervals (up to 20’).  Soil samples at a depth of 0-3” and 3-6” were taken at each collection site. If leaf litter was lowering the pH during their tree’s lifetime, we would expect to detect a pH gradient with the lowest pH nearest the tree base. We found differences in soil pH across general locations at the arboretum (pH 5.9-7.6), but pH gradients by a specific tree or differences in average pH for the soil around coniferous and non-coniferous trees were not detected. Challenges to growing plants under conifers are likely due to  factors other than soil pH.