To my knowledge, acidic soils are found either near volcanoes or evergreen forests. It takes 1000 years or more for decaying pine needles to make an acidic soil, but eventually it happens. Some volcanic ash contains sulfur, and this is metabolized by bacteria in the soil, turning it into sulfuric acid, which makes the soil acidic.

Alkaline soils are associated with limestone or chalk, which is formed over time from seashells on the floor of an ocean. In the midwest of the USA there are alkaline clay deposits that were formed when the area was covered by water.

Soils are at the pH they are because of the number of free radical hydrogen ions in the soil, the more of those free redical ions the lower the soils pH and the nore acidic it is. Some plants grow in soils with a lower pH (acidic) and some grow in soils with a higher pH (alkaline). Adding lime to acidic soils provides some thing for those free radical ions to latch onto which then reduces the number of them and changes the soils pH. Some soils that do not get enough rainfall accumulate elements (the lack of rainfall does not wash them out of the soil) that tie up those hydrogen ions resulting in an alkaline soil while in areas that get a lot of rainfall those elements are washed out of the soil and the soils tend to be acidic.

I've never checked this but wonder how the pH of soil varies with depth. In particular one rose I have lingered for 6 years or so before finally shooting up to it's climber status. (Illusion) Pondered whether it had something to do with soil depth and differences between the layers. Yes there is that old saw about the first year they.....

Now that I've finally figured that oxidation or whatever developed on the probe of my pH meter it is now working again so I'll check when digging holes.

Maybe I should have phrased this diffently.
What cause the soil to change PH in a natual setting, a garden. If it don't change, why would people add lime each year?

Here in Madison, Wisconsin, we have soil that is high in clay, and also high in pH, around 7.6. I have to add sulfur to our soil in order to lower pH for growing blueberries. Our garden gets an application of compost, made from shredded tree leaves, every spring. But we don't put lime down, on either the lawn or the garden. Both the lawn and garden have soil that tests pH = 7.6, so I see no reason to add lime.

"What cause the soil to change PH in a natual setting, a garden. If it don't change, why would people add lime each year?"

There are many things that factor into it. One is the amount of rainfall. Areas with higher rainfall tend to have lower pH. Of course, if you're on an ancient lake bed and the soil has a lot of calcium, abundant rainfall won't make the soil acidic.

If conditions are such that your soil is acidic, adding lime will raise the pH temporarily, but the core conditions didn't change, so the soil will tend to revert back to normal. I can add sulfur every year, but my soil will revert back to alkaline over time.

People lime their soils every year because someone told them they needed to. The only way to know if your soil really needs lime is to have a good, reliable soil test done and the people doing the test should tell your how much lime needs to be added to correct what the test results show and that, if done with the correct amount of and the right type of lime at the right time of year, should correct the soils pH for several years. The best time to apply lime to soils is in the fall, if the soil also has adequate levels of organic matter in the soil.
The base soil pH of my sand is 5.2 and today, after many years of adding compost and organic matter (no lime) is is in the 7.0 to 7.2 range, with a good balance of Calcium and Magnesium.

Eric in Wisconsin. I have also planted blueberries and I used sulfur to try to make the soil more acidic. I don't think it's working.

Sulfur is hard on fungi, and blueberries need fungi on their roots. I replaced a dead blueberry this spring and I didn't add sulfur this time.

I dumped some sulfur around a spruce tree in an effort to lower the soil pH to make it more amenable for the tree. The tree died. I think that spruces and pines also need fungi in order to survive.

I am now sticking to using organic matter to try to buffer the pH of the soil to allow the blueberry, raspberry and pines to thrive in my 7.8 pH clay soil.

I don't recommend ever using sulfur to try to alter the soil pH. Most plants form mycorrhizal relationships with fungi, everything from grass to trees. If you soil needs sulfur, add some for that reason. Otherwise, use organic material - compost, peat moss, mulch - to increase the buffer capacity of the soil.

For quite some time, people like kimmsr have been asserting that attempts to alter soil pH are folly. I am now a convert to that point of view.

Acid rain lowers soil pH. Irrigating with hard water causes a rise in soil pH.

Adding sulfur to soil will only reduce the soil pH if certain conditions are met. It may take years to see significant results since it is dependent on bacterial growth in the soil.

By adding sulfur to the soil you are attempting to provide a food source for a few specific types of aerobic soil bacteria. A byproduct of aerobic bacterial digestion of sulfur is sulphuric acid, which lowers soil pH. If temperature, moisture, and oxygen are not present in the correct proportions, the bacteria will not grow, the sulfur will not be digested, sulphuric acid will not be produced, and there will be no change in soil pH. If an anaerobic condition exists (poor drainage/over-watering), the sulfur may be digested by anaerobic bacteria, producing hydrogen sulfide, a poisonous gas with the odor of rotten eggs which will kill your plants.

To effect an immediate reduction in soil pH, an acid must be directly applied to the soil. This is being done successfully by commercial growers in California. The process normally involves injecting sulfuric acid into the irrigation system. Because of the dangers involved in the handling of acid, this is probably not practical for most home gardeners.

Just wanted to add that all kinds of ammonium fertilizers have a pH-lowering effect, even when the fertilizer itself is not that acidic. Bacteria metabolize ammonia to nitric acid, in the same way sulfur is oxidized to sulfuric acid.

I am having some success growing blueberries here in Madison, Wisconsin, even though our native soil has a pH around 7.6. In the past, I was adding white vinegar to our tap water, and using this on the blueberry shrubs, which lowered pH, and kept the leaves green. However, every spring, when the shrubs leafed out, the leaves would turn yellow, and the pH would test to something around 7, too high for blueberries. It seems that vinegar(acetic acid) is decomposed by soil microbes, and disappears over time. Last spring, 2009, I found a local source of agricultural sulfur, and since then I have been using this material to lower soil pH. It works, but I am doing considerable pH testing, and also applying tap water, to get the pH where I want it to be. Right now, most of our shrubs are in the range of 4.0 to 4.8, and I want them to be in the range 4.5 to 5.5. I don't see how anyone could do this, unless they had the ability to quickly test soil pH, on site.

The origin of the soil has the greatest effect on its natural pH as-found (that is, before amending with tons of compost, for example). It has to do with the rocks the soil came from. Clay soils derived from limestone (calcium carbonate) tend to be naturally alkaline. It seems sandy soils are often acidic, although I'm not sure exactly why that is because I live in limestone country. But again, it's the rocks.

That's why any attempt to change the pH using degradable things like vinegar, compost etc. will only work temporarily. Even lime will be eaten away eventually. I expect that as long as the original soil is still there, it will try to return to its natural pH.

Aluminum sulfate is an alternative to sulfur for acidifying soils. We've used it around our azaleas and hydrangeas and it doesn't seem to have the issues discussed above about sulfur.