Generally speaking, we’re trying to “guide” the pH of the soil towards a desired target. For “most” plants, we’re trying to get somewhere around 5.8 to 6.5. (Or very slightly acidic.)

Some plants do prefer more alkaline soils, whereas some prefer even more acidity. So, it’s more about achieving what the plants want than it is an arbitrary target. In our gardens, we haven’t (yet) aimed to tailor our pH to any of these preferences, as doing so adds difficulty. For our purposes, our primary goal is to steer away from pH extremes (too much acidity) and back towards neutral. But, any given garden should be analyzed for contributing factors as the pH (and those contributing factors, discussed in the article) can be different. Hope that helps!

You certainly can do as you suggest. The reasoning for the change has to do with the compounds used to derive the fertilizer and their tendency to drift pH over time. While pH buffers do help resist change, it’s also about reducing the “fight” over the desired pH direction. If you’re correcting an “acidic tending” fertilizer to a higher pH, you’ll need to use more pH buffering to overcome that tendency than you would with an “alkaline tending” fertilizer.

An example of these compounds, using nitrogen? Common ways to derive it are using urea nitrogen or nitrate nitrogen. Both of these provide nitrogen your plants can use. Urea nitrogen will tend acidic, whereas nitrate nitrogen will tend more alkaline. If the fert is dominant in urea nitrogen, you have to buffer against a more acidic source. Whereas if it’s dominant in nitrate nitrogen, there will be less need for buffering. Hopefully that clears it up!