Hello. The slide in the lecture showed a colour change in the soil depending on NH4+ or NO3- being given to the plant as a nitrogen source. The colour change of the soil shown is because of the release of H+ or OH- as discussed above. To see the pH change in soil, a chemical dye was added to the soil. So the colour difference in the slide is something one will not see in soil normally. We have to add the pH colour dye to the soil. I hope this helps. Mario
