1. Is Clean Rain Already a Little Acidic?
Yes. Carbon dioxide from the air dissolves in rainwater and makes a weak acid, so rain with nothing but carbon dioxide dissolved in it is expected to have a pH of about 5.6. A pH of 7 is neutral, and lower numbers are more acidic. Japan’s Ministry of the Environment uses 5.6 as the line below which rain is generally called acid rain [1].
The acid rain that causes environmental trouble is a different matter. The US Environmental Protection Agency (EPA) explains that it forms when sulfur dioxide and nitrogen oxides, mostly from burning fossil fuels, react in the air to make sulfuric and nitric acids [2]. Pollution is a real extra source of acid. The question here is narrower: what does a forest do to its own soil, with or without that extra push?
2. How Does Ordinary Rain Slowly Wash Nutrients Out of Forest Soil?
A 1983 review looked at several well-studied forest sites in North America, some that received acid precipitation and some that did not. At the tropical and the temperate conifer sites, natural leaching by carbonic acid was the main way cations (positively charged nutrient ions such as calcium) were washed out of the soil [3].
A simple way to picture it is this: acid in the soil water can push nutrient ions, such as calcium, off soil particles, and the water can then carry those nutrients away. As this goes on, the soil can become more acidic over time. The review also reached a careful conclusion: at only the temperate deciduous sites did acid from the air play a major role, and in no case were the yearly nutrient losses regarded as alarming compared with the nutrients the soil holds in reserve [3]. In other words, this process is real but slow.
3. What Do Fallen Leaves and Soil Microbes Add to the Acid?
Two more sources of acid come from inside the forest itself. In the 1983 review, organic acids, which form as plant material breaks down, were the main cause of leaching in the surface soil at a high-mountain (subalpine) site, and they contributed to leaching in several other sites [3].
The other source is nitrogen. When soil microbes convert nitrogen compounds in a process called nitrification, acid is produced. At one temperate deciduous site with nitrogen-fixing trees, which add nitrogen to the soil, nitric acid produced by nitrification dominated the leaching [3]. The authors point out that these patterns come from a handful of sites, and that they need to be checked at more places before anyone treats them as general rules [3]. So the honest answer to “what acidifies a forest?” is: it depends on the climate, the soil, and the plants.
4. Does the Kind of Tree Change How Acidic the Soil Becomes?
It can. Researchers grew 14 temperate tree species, each in its own plots, in the same common-garden experiment. Because the trees all grew in the same experimental site, the researchers could compare the soils under them. Soil pH and acidity differed by species, and they were linked to parts of the nitrogen cycle that produce acid, such as nitrification. Those, in turn, were linked to how much nitrogen the species’ fine roots contained [4].
The same study found that soils with more total extractable acidity also held more carbon [4]. The authors explain this link with the way trees acidify soil and speed up the weathering of minerals, which releases ions that help hold organic matter in place. That is the authors’ interpretation, and the experiment shows that these things go together rather than proving a single cause. It also suggests a caution for anyone tempted to say “pine forests make soil sour”: what matters is the kind of tree and the kind of soil, not a simple label like conifer or broadleaf.
5. Why Doesn’t Forest Soil Just Keep Getting More Acidic?
Soil resists change. In a Japanese national monitoring program, researchers measured soil at 25 sites (50 plots) in 19 regions. Most of the topsoils (0–10 cm, or about 4 inches) were at pH 5.5 or lower, and a few sites were below pH 4.0. Their report explains that between pH 4 and 5, aluminum in the soil takes part in soaking up acid, so even a large load of acid does not easily push the pH lower [5].
The same report found that the top layer was more acidic than the layer just below it (10–20 cm). The authors think the likely reasons are that minerals, which buffer acid, are scarce in the surface layer, and that organic acids from plant material add to the effect [5]. Buffering also differs among soil types, so two forests getting the same rain can end up with different soils.
So the picture is a tug of war. Rain, leaves, microbes, and roots keep adding acid, and the soil’s chemistry slows the change. Adding polluted rain on top shifts the balance, which is why scientists measure forest soils year after year.
6. How Can You Look at Forest Soil Yourself?
If you have permission to visit a small woodland, compare it with a nearby lawn or flower bed. In the woodland, gently push aside some fallen leaves and compare how thick the leaf layer is, how much has already crumbled into dark material, and what color the soil underneath is. Then compare with the lawn. Put everything back where you found it, leave the soil there, and wash your hands afterward. A thick layer of leaves gives you something to compare, but looking cannot tell you whether organic acids have built up or what the pH is, so treat it as a way to notice differences, not to measure acidity.
If you want to read more, the 1983 review [3] and the tree-species paper [4] are good starting points. They are written for scientists, so ask an adult to help with the harder parts.