SOIL 3600 Question

[Meaghan Bartley]

Hi there,
I have a question regarding something discussed right at the end of today’s lecture (slide 37). You were discussing the following processes:
NH4+ -> plant -> H+ (increases pH) and
NO3- -> plant -> OH- (drops pH).
I was wondering, would the protons released from the first equation not decrease the pH of the soil (make it more acidic)? And for the second equation, the pH of the soil would increase (make it more basic)? I may have this mixed up though! I was wondering you could please clarify what equations yield a higher or lower soil pH?

[Jennifer Lansang]

Thanks for asking this! I’d also think the same thing for both those equations…

[Mario Tenuta]

Hello Meaghan.

Release of protons (H+) to soil from roots will decrease soil pH, especially very close to the root. So when plants take up predominantly NH4+ has a nitrogen source, the volume around roots is acidified.

When plants predominantly take up NO3-, they release hydroxyl (OH-) to soil. This increase the pH around roots.

Mario

[Ayrton Darukhanawala]

Hi, I’m sorry if I’m posting this question as a reply or in any wrong way but how does the pH level affect the colour of the soil?

[Mario Tenuta]

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

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