Yes, rainfall can have a very significant effect on soil pH.......it is often one of the primary indicators of an acidic versus a more alkaline soil. But the effect will be negligible during the course of a year. Acidification from rainfall does not occur in one growing season but instead over hundreds of years of high rainfall amounts. Rain can change the pH more quickly in sand, partially because sand does not bind basic elements very well, but also because water drains quickly through sand.

I'm in OC (CA). My ph is said to be around 7.9, however if rain and irrig. affect ph, how do they come up with a definite number since it would obviously always be in a state of flux? Ex: During the Summer w/ no rain and irrigation water with ph - 7.8 only wouldn't the ph be higher than winter w/o irrig. and only rainfall?

It is only affected by rainfall over an extended period of time.....decades, if not longer. The existing pH of any soil is pretty stable so it will not increase or decrease seasonly unless your irrigation water is highly skewed in one direction or another And even that will be a very temporary effect.

And pH is a very complex set of chemical interactions. For example, I live in the Great Tennessee Valley. Some crops do amazingly well in this region. There are micro regions within the whole, but generally speaking, the soil of the Great Tennessee Valley is based on decomposed limestone. When we think of limestone, the first thing that comes to mind is an alkaline pH. However, because of the eons of decomposition and chemical reactions, the pH of the soil is largely neutral to slightly acidic. As mentioned, that will greatly fluctuate depending on the micro-region, but most of the lowland makes for fairly good soil.

So, the pH of a soil is most determined, not just on mineral composition, but - as others have pointed out - factors as diverse as the pH of irrigation water, level of organic matter, the number of chemical interactions happening over a period of time, and even the chemical nature of introduced amendments.

If, however, the pH of the base mineral content is extreme one way or another (like our regional limestone here in the Valley was eons ago,) it can take a VERY long time, and A LOT of interactions before it changes to something more moderate.

pH of a soil that is below 7 can drop as much as 1 unit from spring to fall, but that is the extreme. Soil with a pH close to 8 just don’t fluctuate due to the massive amount of carbonate (lime) in the soil.

The difference between salts and pH is that it is theoretically possible to leach all the soluble salts out of something, but ANY water based liquid will have an inherent pH. It is impossible to 'leach it away'. It can be changed depending on what is in (or is NOT in) the water that goes on it. But it will always have a pH that is dependent on a large number of interacting factors.