My understanding is that healthy soil microbes will move the pH toward 7.0 from wherever it is. Of course you have underlying soil minerals that govern how far pH can be moved. I've not heard of plants moving pH but I'm anxious to read what others have seen/heard.

Soil pH is primarily determined by the mineral content of the soil and the amount of annual rainfall. Areas of high rainfall tend towards more acidic soils, more arid areas tend towards alkaline soils. The rainfall works to leach away basic ions and tends to contribute an acidic component in varying degrees as well.

pH will also be affected by the amount of undigested, undecomposed organic matter - a number of organic and inorganic acids are formed, at least temporarily, during the decomposition process. The use of inorganic fertilizers will create a similar situation. Finished compost is pretty much neutral with regards to pH, but has a relatively minor effect on soil pH overall due to the buffering abilities of most soils.

Most suburban residential soils will evidence some variability in soil pH. Concrete - from foundations, driveways or walkways, retaining walls - will leach lime for many years, causing the soils immediately adjacent to them to be a higher pH than those further away. And there could be pockets of lower pH, depending on the amount of undecomposed organic matter or the use of manufactured fertilizers.

Plants do not alter pH to any significant degree. It is commonly thought that conifers and their needle drop, etc. as well as the debris from certain other types of plants will lower soil pH. This is not valid - it's confusing causation with association. Plants that are known to prefer acidic conditions or generate acidic debris tend to grow in naturally acidic soil conditions but they do not create or substantially add to those conditions.

Assuming that you are talking only about a vegetable garden with moderate rainfall - yes, the soil will gradually become more acidic. I don't know of any particular vegetable that is more acidifying than others. It is a very gradual process though.

When plant roots take up nutrient cations ( Ca++, K+ ), the plant gives up H+ in exchange. This increase in H+ increases the soil acidity.

Watering will leach out basic nutrients ( Ca++, K+ ), increasing acidity.

High rainfall and acid rain ( pH Urea and ammonium fertilizers increases acidity.

Microbial decomposition of organic matter increases acidity.

So depending on the initial pH of the soil and the number of factors that affect your soil, liming the soil to counteract this acidification may be necessary. I would think that annual applications of lime or dolomite or gypsum would prevent acidification.

Darn it! You 2 gals took up every point I had but one, irrigation water quality and it's effect on soil chemistry.
I shall defer to the experts who can do it justice better than I except to say that it can have a profound effect especially if managed improperly.

Arid areas have high pH. My understanding was that moisture in the ground transports CA and K up to the surface as soil dries out. High pH and caliche can result.

mattjjd24, are you just interested or really curious? If the latter you will need to study buffer capacity and cation exchange capacity.

Soil pH is determined by the amount of free radical Hydrogen ions floating around in your soil, and where there is plentiful annual rainfall, such as in the New York area, that rain will move oxygen ions out of the soil solution leaving more free Hydrogen ions which gives a low pH reading, or an acidic soil.

Here is a link that might be useful: About soil pH

Uh, didn't gardengal and cowgirl disagree with each other? How can you agree with both of them?
I'm with gardengal on this one.

Here's a link to some info on caliche. It starts out like this,
Caliche is a sedimentary rock, a hardened deposit of calcium carbonate. This calcium carbonate cements together other materials, including gravel, sand, clay, and silt.

Caliche is enormously misunderstood in gardening, unless you live in New Mexico where they seem to have a perfect grasp of it. True caliche is nature's concrete. That link is interesting reading if you think you have caliche soil. Larger amounts of clay in pulverized caliche can keep it from hardening but it sure does make a mess (often called gumbo).

Whilst digging a hole to plant a tree, at a depth of about 18" I hit a whitish layer of soil that was so hard I could only chip small bits of it off with a forged steel irrigation shovel. The bits chipped off in small shards. The soil above that layer was a medium clay.

The hard layer was about 6" thick and below that was a gravelly, rocky layer. It is my clear understanding that this hard layer is formed by calcium and other minerals leaching toward the surface in hot weather. The pH of the clay soil above this hard layer is about 8.

I guess you'd have to appreciate how hot and dry it gets in SW Idaho to believe that this is the mechanism that formed this concrete-like layer. The average rainfall is 11" and almost all of that falls in winter months. Summertime temps reach over 100 degrees, sometimes for weeks.

In California near Redding/Red Bluff I have family that built a house up on the bluff. Again, it gets well over 100 in the summertime. The dominant vegetation is a scrubby kind of oak, mesquite, and sparse grasses. Temperatures sometimes 115 degrees.

To install a septic leach field required bringing in a tractor with a huge auger. The auger was used to bust through the hardpan and make a drywell that gets filled up with gravel.

This hardpan, found in arid parts of the country, is caliche, a layer of calcium carbonate.

Quoting Wikipedia; "caliche can also form in other ways. It can form when water rises through capillary action. In an arid region, rainwater will sink into the ground very quickly. Later, as the surface dries out, the water below the surface will rise, carrying dissolved minerals from lower layers upward with it. ... Plants can contribute to the formation of caliche as well. The plant roots take up water through transpiration, leaving behind the dissolved calcium carbonate, which precipitates to form caliche."

idaho_gardener: Do you find this caliche everywhere on your property? I think I live within a few miles of you and I have not seen it where I live. I've dug down 2 to 3 feet and I have not found a rocky caliche. There is a clay hardpan that is a real pain to dig through, but it is brown/black clay, not caliche. I guess I should consider myself lucky. Actually, I think the soil at my site is very good once some organic matter is incorporated and the PH is slightly lowered.

backyardener,

No, not everywhere on my acre. Part of my yard is treed and I have gone down 6 feet without encountering caliche under the trees.

As you probably know, our subsoil is alluvial deposits and varies in depth. In the front yard, the soil was not so rocky when I went down 6 feet to repair an irrigation shutoff valve. But 100 yards diagonally away, I found caliche over rocky, sandy subsoil when I dug a 3 foot deep hole for a tree.

When I double-dug a garden bed 100 feet from that spot, I found a pale clay, probably caliche in progress, at a depth of 2 feet. I amended that with sulfur and humic acid in an attempt to lower its pH. The pH of the topsoil was 7.8 and it had previously been treated with Zamzows Huma Green humic acid.