Not only that, but there are buffering effects. Weak acids and bases do not respond linearly to the addition of more acid or base. Most relevant to soil systems is the carbonate-bicarbonate equilibrium. Carbonate can exist in the form of insoluble carbonate salts like calcium carbonate, or dissolved carbonate and bicarbonate. And there's atmospheric CO2 as well. The ratio between carbonate and bicarbonate in solution (in soil pore water) will depend on the pH. But if you start adding acid or base, the pH of a carbonate system will not even change linearly. It has flat and steep places along the way.

Getting back to what you said, it should be SORT OF linear though. The mix you described might settle somewhere around 6. Only one way to find out. Do you have a pH meter of any sort?

@toxcrusadr The point is I want to be able to at least estimate the final pH of the soil when I design the soil mix. It is very inefficient to keep creating different mixes and testing their pH along the way. I would like the first mix I make to be at least close to the target.

@tapla Do you have any rules of thumb you use to predict the final pH of a soil, given the parameters in my question?

I don't. Doesn't mean there aren't any, but I haven't tried to do this. If you have consistent inputs - the same peat moss and the same other ingredients - you can make a mix and try it out and see. Then you'll know, and you only have to do it once.

Although I have never bothered to dissect the question - IMO just not worth the time and effort for a negligible difference - the relationship is more to do with volume than it does percentages. If you are creating something like the 5-1-1 mix that comes so frequently recommended for a multitude of container growing situations, peat plays a very small role....as it should :-) And the pH of peat is pretty variable, ranging from 3.0 to 4.5 so a strict formulaic application is not going to be all that helpful.

The inclusion of the lime initially is to buffer the immediate acidity created by the use of the bark and the peat in the mix so that it does not shock newly potted plants that may not be accustomed to extremes. But the lime leaches out over time anyway and the media will approximate a more moderate level of acidity. The need for frequent fertilization negates any nutrient issues that might be impacted by a pH is that is slightly too high or too low.

I am a big proponent of the 5-1-1, which I have used for many years as I find it ideal for just about any container planting situation. But instead of peat, I use a similar volume of a high quality potting mix (also heavy on the bark and very light on the peat) which is already pH buffered and I have never added any additional lime. I don't even own any lime :-) Granted, most of what I am growing are acid lovers or the vast majority of plants that prefer an acidic soil but I have never encountered any issues by omitting the lime altogether.

btw, the recipe for the 5-1-1 suggests 1 Tbsp of dolomitic lime for every gallon of blended mix.

westes, I have to admire your inquiring mind for wanting to nail down all the details involved with these soil issues. But I do think you tend to overwork them :-) Most plants have a high degree of flexibility and precision is just not necessary. Do you have a background in engineering?