Q: I’ve heard that pine needles and oak leaves cause soil to be acid. Is this true?
A: This is a gardening myth that should be put to rest. While needles and leaves from all trees may be acidic initially, they do no acidify the soil as they break down. Soil has a natural ability to buffer changes in pH. Soil acidity might increase slightly for a few weeks after leaves fall but the acidity will go away after a few months. It’s true that pines can grow in poor soil, but they didn’t make the soil poor to begin with.
Check the following references:
“Concern: Woody mulches will acidify soils.
Evidence: None. In field situations it is difficult to significantly alter soil pH without addition of chemicals. Transient changes in pH may be found in the decomposing mulch layer itself, but these have little effect on underlying soils.”
“In Tables 5 and 6 are soil test results from May- and October 1997, respectively. There was an initial
concern of oak leaves changing the pH of the soil when the study was initiated in 1991. However, no changes
in pH have occurred after six seasons of mulching tree leaves.”
“Oak leaves may be acid when fresh, but as decomposition progresses, the net result is an alkaline reaction.”
“Soils can buffer the slight acidity of freshly fallen needles and thus there is no negative impact to plants from the slight acidity of pine straw when used as mulch.”