Lewis Hill (see below for examples) wrote an excellent reference of the pH requirements for most vegetables.  I have not found a reference on the internet that gives the pH requirements for plants.  If you know of one, please contact me and let me know!

My vegetable garden soil initially tested at 8.3 pH which is too high for plants to successfully take up the nutrients in the soil.  I do the following to lower the pH of my soil:

1.  Add organic matter.  While this will not dramatically lower the pH of any soil, organic matter does help the plants take up nutrients as well as helping the soil retain moisture.  I mostly add aged horse manure which I can get free in abundance here in rural Pine.  The usual sources of organic matter such as compost or peat moss work well also.

Every few years, we have a major thunder storm which washes pine needles, cones, twigs, and other organic matter (as well as rocks) down into one of our meadows.  Rather then let this organic matter stand as large, unsightly piles throughout the meadow, we rake and pick up the material, pile it into our old pickup truck, and place it into my vegetable garden.  Pine needles are naturally acidic and the material, worked into the soil with my tiller, quickly decomposes.  A gardening windfall from a seemingly unsightly disaster.

2.  Never add wood ashes.  I added the ashes from our wood stove to the garden soil for the first few years that I gardened.  Wood ashes are a good source of potash and I did not want to waste the ashes.  However, over the years I noticed that the garden soil was ever more alkaline and I had to add more and more sulphur to attain a pH where vegetables would grow.  Wood ashes are very alkaline.  Now, we scatter the wood ashes around in the forest (making sure there are no hot coals in the ashes) and never place the ashes in the garden.  The forest plants seem to benefit from the potash and, since the wood ashes are spread over a wider area, the soil does not seem to be adversely affected.

3.  Add sulphur or ammonium sulfate.  The best method for lowering soil pH that I have found is to add sulphur directly to the soil prior to planting the garden.  4 lb. sulphur per 100 square feet will lower the pH one point.  Ammonium sulfate is also effective but is 23% sulphur and takes four time as much to lower the pH one point.  However, ammonium sulfate also contains 21% nitrogen which is an added benefit.  I have also used Miracle Grow Acid Fertilizer during the growing season whenever I notice any plants turing yellowish.  I apply the Miracle Grow directly to the leaves using a hose sprayer attachment.

The only source that I know for garden sulphur is The Rocky Mountain Seed Company (P.O. Box 5204, Denver, CO 80217-5204, telephone 303.623.6223, no web address).  Do you know of any other sources?  I use the granular soil sulphur which is easy to measure and apply.  It costs $12.50 per 50 lb. bag.

Since the soil pH is so important to maintain properly, I invested in a Hanna Instruments Checker 1 pH meter.  I have used soil test kits but I found the meter to be easier to use, more accurate, and, since I test each garden bed separately, cheaper in the long run as there are no chemicals to use.  I listed some sources for pH testers below.

Links to pH Information Sites:

USGS Water Properties. A basic tutorial on pH mainly relating to water properties but is informative about pH also. Directed to science students.

Soil pH  Wikipedia - the webpage has an excellent table on the effect of nutrient availability in relation to soil pH.

Changing the pH of Your Soil   Prepared by Marjan Kluepfel, HGIC Information Specialist, and Bob Lippert, Extension Soil Fertility Specialist, Clemson University. Towards the bottom, there is a good section on lowering soil pH.

The pH Factor  Created by the Miami Museum of Science, this is a very basic, low level but informative site directed to those teaching science.

Links to Sources for pH Testers (I do not endorse any of these sites but list them for your information only):

Hanna Instruments  Use their search engine with "ph and tester" and you will get a list of their instruments.  I use the Checker 1 (MI 98103) but they list other pH meters that also look interesting.

Gempler's  Search on "pH" and you will get a list of their pH meters.

Johnny's  While you can find soil test kits through many catelogs/sites, I added Johnny's because of their reliability and the fact that they test all the products they sell.