Acidic rain effect
You wonder, what happens if acidic rain or acidic solutions are dripping onto the soil? We built artificial soil columns by mixing two soil samples with quartz sand (just for better drainage). One soil contains carbonate, one does not. Instead of acidic rain (which would take too long) we take diluted HCl (hydrochloric acid) that is passing through the artificial soil column. Below, we have a water with color indicator, which is indicating a change in the pH in the solution by color change. In other words, if acids are dripping into the solution, the pH of the solution changes, which also changes the color. Note that one column is frequently dripping and it takes very long to pass some acid through, while already the first droplet changes the color in the other column. So, what is happening? In the carbonate-free sample the acid is not buffered in the system and is directly passing through, leading to the rapid color change. In contrary, the soil sample with the carbonate is reacting with the acid, leading to the neutralization of the acid and release of CO2 bubbles. This means that instead of acid, only water is dripping into the solution with the color indicator as long as there is some carbonate remaining in the soil column. Only if the carbonate is being consumed, the pH of the soil drops and the acid can pass through, resulting in a time-lag of the color change of the solution. In natural soils, our rain that is slightly acidic as well as other acids in soil like organic acids released during decomposition of organic matter or root exsudates can lead to the same effect. However, this does not take minutes and some milliliters of acid as in our experiment, but it takes centuries to millennia and thousands liters of rain water or other percolating solutions.