Al is AWOL so I will answer.

pH is a measurement of a property of the water. As it relates to plants, pH of the water affects nutrient availability.

Alkalinity has to do with the amount of carbonates/bi-carbonates in the water. More to the point, it measures how capable the solution is of neutralizing acids.

We could have 2 jars of water. Both have a pH of 8.0. One has little alkalinity and the other a high degree. We then pour them into 2 separate containers with a potting mix consisting of primarily bark or peat (acidic media).

The solution with high alkalinity will permanently change the pH of the mix faster than the one with low alkalinity. It's kind of like adding lime to the mix.

The high alkalinity solution is more able to neutralize acids (the bark's acidity) than the low one.

As it relates to plants, add an ounce or two of vinegar to each gallon of tap water and it's a non issue.

Hi gardenerme,
I use 0.4 ml of white vinegar (5% acidity)/ 1 liter water/ each degree of KH (carbonate hardness) in my well water.
KH testing strips are sold at aquarium pet stores.
For small scale projects I aerate the water for 48 hours first; it vaporizes a lot of the CO2 from the bicarbonate ions (Ca & Mg). Then tweak with the vinegar & resultant acetates of Ca & Mg allow pH stability.
At end add fertilizer formula & check pH; which you can give final tweak.

I can't believe Al is Banned! What is going on? I already wrote to them and got no reply yet. Hope it helps, we really need him ... GW really needs him!

If you're still interested, here is a copy/paste job from one of my other posts:

"When we measure pH, we are measuring the concentration of hydrogen (H+) ions in the irrigation water or soil (nutrient) solution. As a guideline, water for irrigation purposes is usually best if its pH lies between 5.0 - 7.0. The measure of alkalinity gives us the water's ability to neutralize acidity. The level of bicarbonates, carbonates, and hydroxides in water determine its alkalinity. The desirable alkalinity range for irrigation water is 0 to 100 ppm carbonates, with 30 - 60 ppm optimum for most plants.

Whenever you test the water you'll be using for irrigation, you should always include the measure of both pH and alkalinity. A pH test on its own, does not give any indication of alkalinity. Water with high levels of bicarbonates or carbonates (high alkalinity) always has a pH value >7, but water with a high pH doesn't necessarily have high alkalinity. This is a very important concept because high alkalinity exerts more significant effect on plant nutrition and the fertility of growing media than pH.

High pH irrigation water generally causes no problems as long as the alkalinity is low. Since high pH water with low alkalinity has little ability to neutralize acidity, it's effect on media pH will be minimal. Cause for more concern are situations where you must utilize water having both high pH and high alkalinity for irrigation, which will cause the pH of the growing medium to increase substantially as media ages. If your water analysis is known to be alkaline, you may need to significantly reduce the addition of compounds containing Ca or Mg because of the dilute solution of limestone in the water. The smaller the container - the more serious the issue because small volumes of soil offer less buffering to pH change. In high alkaline situations, you can often expect/experience Ca and Mg deficiencies along with micronutrient deficiencies (both real and antagonistic).

Acid forming fertilizer will not actually lower media pH as long as there is limestone in the soil - unless the soil is acidic already, but vinegar (and other acid) applications will help to neutralize alkalinity in the irrigation water, which will be helpful."

Al