The pH of Dissapointment

DIY Test for Soil pH

plants chemistry DIY

It has occurred to me that I should have run this soil-suitability test before letting my seed-sowing-mania take full control. Despite the [spoilers] poor soil-pH exploration results I have 47 sprouts showing their heads above soil. There are still two whole bowls of seeds with a big fat 0 on their sprout-count. :|

I'm going to talk a little bit about the basics of acids and bases in laymen's terms and then go into how my soil turned out. By all means, skip ahead if chem-lite isn't your interest!

Anyway, on to some quick and clean chemistry! It won't get too hairy in here guys; I'll restrain myself. Anthropomorphism and doofy metaphors abound!

Citric acid, found in ...citrus fruits.

Acids and bases are the forces behind the ebb and flow of loose hydrogen ("H+ ions") concentration in solution. That H+ concentration is measured on the pH scale: 1 to 14. A lower pH means a higher H+ concentration: acidic. A higher pH means a lower concentration of H+ ions: basic. H+ ions are like the pennies of the chemistry world. Some molecules hold on to them very loosely, and some have 'holes' (lone electron pairs) where H+s would fit nicely. So these ions get traded back and forth. (This works best in water, because water is a good medium for H+ transfers. It loses and gains them easily.)

Acids always seemed so 'mean.' Turns out, they are the ones who so generously donate H+ in solution! Look at all those Hs that citric acid has to give- and they're not glued on too tightly. It's a good acid! When in water, it looses its Hs to the surrounding water molecules, and forms H3O+. If citric acid runs into other molecules that are capable of taking on one of it's H+, it will act on them as an acid as well.

Ammonia, a common base.

Bases, on the other hand, are greedy and use that lone electron pair to grab H+ ions. (For the record, its holding those other 3 Hs pretty darn tight. They're not going anywhere.) In a water solution, NH3 will grab H+ ions from the water, forming OH-. It can steal from anything that's not holding on to it's H+s very tightly. Stronger bases can take even the H+s that are held tightly by another molecule. (As long as their basic power is stronger than the will of the other molecule to keep its H+. This 'strength of H+ holding' measurement is called pKa.)

Extra Credit

Now, not to get too confusing, but let's think this through to it's logical conclusion: once an ammonia (NH3) has grabbed an H+, it becomes NH4+, and has occupied that lone pair... So, you might ask, couldn't this NH4 now act as an acid, and donate that sweet little H+? Technically YES. It would just take a bit of metaphorical prying to get that H+ off the ammonia, who has grabbed it with its base powers. Every acid has a corollary base, and vice versa. The trick is that some chemicals have one action that is stronger than the other. Whichever action is stronger dictates whether it is an acid or base. Stronger in one role, weaker in the other! We've been talking about water going to H3O+ and OH- easily. Water is a weak base and a weak acid.

Some chemicals have a hair-trigger regarding those frequent back-and-forth H+ transfers. When an H+ is removed or added, it changes the way the chemical absorbs and re-emits light. Aka, a color change based on pH! These chemicals are called indicators because they can be used to give a visual indication of a solution's pH.

Which brings us to the star of our show: Anthocyanin!

Anthocyanin is in the skin of blackberries, cherries, soybeans, eggplants... and of course red cabbage. This chemical has the useful property of changing color very easily-- with those small additions or removals of "loose" hydrogens. In a pure water solution (pH 7) anthocyanin constantly switches between both H+ rich and H+ depleted stages. As the solution turns basic or acidic this equilibrium is shifted, favoring one state or the other.

purple cabbage solution

Here I've leeched some shredded red cabbage with some boiling distilled water. Anthrocyanin is lending the purple hue! And here I've set up equal parts ammonia and lemon juice in separate, clean glass jars (Left). Then 1Tbs of filtered cabbage indicator to each... (Right)

before after

Just by taking H+ ions from anthocyanin (and therefore throwing the total anthocyanin H+ equilibrium towards H+ depletion) the ammonia has changed anthocyanin's net color to blue. Citric acid donated some H+ ions, and the anthocyanin solution has turned pink. I could make a point here, and show a nice gradient from basic(green) through purple(neutral) to acidic(pink), but all the other mason jars are in the fridge holding homemade salad dressing of various kinds. (Or homemade dulce de leche ...much tastier than cabbage.)

pH! It's neat!

So, why the "disappointment?"

Pictured below are:

  1. Neutral cabbage solution in distilled water.
  2. My soil with the cabbage solution.

OH MY F@)(#$. After all that work setting up my lovely mesembs who love acidity, my soil is more basic than I need it to be?

I also did this indicator run with the quartzite gravel that I have, since my sources were conflicting on if it would add slight acidic to a soil. It was about the same pH as the control cabbage solution, which is slightly acidic itself. So the mystery is solved! Next time I mix a soil that needs to be acidic, I should include the quartzite gravel.

Oh well, things seem to be doing alright as far as the sprouts go. Although I think I tamped down the surface soil a bit too much in the effort to prevent the seeds from washing down too far. When the seeds rocket out their roots, the sprout is glancing sideways; I'm assuming because they are having a hard time penetrating the soil, made dense by my efforts.


Time will tell!

(Read the next Mesmb update here)

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