Measuring Bonsai Soil pH

Does anyone have a reliable method for measuring the pH of their inorganic substrate?

I use the Mantec soil pH test kit. It's as accurate as anything else on the market.

that should be manUtec

I am curious as to why you would want to go to the effort of measuring the PH. Seems to me it would me more important to measure the PH of the water. Just Curious!

So right now I am fertilizing with 2:1 cottonseed meal/blood meal cakes, and weekly fish emulsion. My substrate is 100% diatomacous earth. I'm down in New Orleans where it's quite hot, and with such fast draining soil I water every day. I've got a couple of trees (a pear and an apple) that are sort of chronically chlorotic. Last year I treated with iron but it was a soil acidifier that contained iron and sulfur and some other things. That seemed to help quite a bit, but I was worried about altering the pH without knowing what the pH WAS. So this spring when they started showing symptoms again I treated instead with chelated iron (sprayed directly on the leaves) and that has really helped a lot, but haven't gotten the pH bug out of my head and thought it would be interesting to see.

Measuring water pH is important, of course, but in the long run it is only a weak player in the game of soil pH. More important for water is total alkalinity, which has a much stronger influence in raising soil pH.

Fertilizer is also a strong player in changing soil pH as trees will raise or lower soil pH depending on type of nitrogen present. So, over feeding with nitrate can raise soil pH. Many trees are susceptible to iron chlorosis with increasing soil pH, so its important to check the soil pH if you think plants are suffering with this problem.

Additionally, some plants are susceptible to nitrate induced chlorosis, where high nitrate levels in fertilizer impact on iron uptake when soil pH is acceptable. This can also occur without actually using a nitrate based fertilizer. In porous media during warm temperatures, microbial conversion of ammonium to nitrate is strongly favored, which can be very fast. Using a lower N fertilizers would be recommended.

I have had issues with using zeolite in my potting mixes and ammonium based liquid feeds. Zeolite has a high adsorption capacity for ammonium, and as weather warmed up many of my J. maple trees became chlorotic and sickly. pH of soils kept rising even though water was acidified, and soil treatment with acidifying agents had little long term effect at keeping pH low. It wasn't until I reduced ammonium content in the liquid feed in warm weather and repoted using a lower % of zeolite in the mix that I got on top of the problem.

Good luck with your soil tests.

Paul

I'd get a bottle of distiller water (which I think is ph neutral), soak some of your mix in it for a few days and then take it to a pool company and asked to test it. Dunno if it'd work, but worth a shot.

Your local county extension may provide soil testing which would include pH, CEC, (cation exchange coefficient) and lots of other interesting stuff. Gardeners can test their soil and specify what grasses or plants they intend to grow for customized recommendations.

You know LSU (Louisana State University) Agriculture Center offers great soil testing that is very affordable. My hope, however, was to find a method that was quick and easy (so I could test every tree on my bench in the course of an afternoon if need be) and would require minimal disruption of the soil. The rootball of a bonsai, especially one who's health your not 100% satisfied with, always seems like a very sensitive area, and I'm not too inclined to start digging around to remove soil samples, either to test my self or to send off to be tested. I'm looking now at these digital pH meters with the long metal probes. I may pick one up and do some experiments: measure water pH as a baseline, measure pH after watering, after fish emulsion, after adding the soil acidifier, etc. It might be very straightforward after all. There may be something I can add to my fertilizer cakes to titrate the pH on the trees that seem most sensitive.

I'm retired from a sometimes miserable day job in the chemical industries. had to do pH determinations many time a day, though not soil pH. I can offer this, unless you are lab trained, none of the less than $1000 usd meters will give you a test result you can trust. And, and Paul said, pH is trivial. Roots of plants can buffer the water film around them to the ideal range easy enough. Critical is total alkalinity. And the CEC of your media. Paul talked about the problems using Zeolite as a potting media. Diatomaceous earth behaves somewhat similarly to Zeolite Your problem with chlorosis is your potting mix, or the fact that it is not a mix. I never had success with any mix that was more than 25% diatomaceous earth. It holds too much in the way of salts. I suggest switching to a mix that has less diatomaceous earth.

I suggest you find a source of pumice, or lava, and maybe some crushed granite. Also, there is a reason many mix at least some Akadama into their potting mixes, it works. You could also use composted pine bark (the size used for seedling orchids). Home Depot carries a recycled glass product that behaves exactly like crushed lava. Growstone Soil Aerator - if your HD doesn't have it in stock you can order it on line for pick up at your local HD.

My mix is pumice, roughly 50%, and roughly equal parts pine bark, horticultural charcoal, akadama, diatomaceous earth, and crushed granite, the granite is also used as a top dressing, because I am able to get the ''cherrystone'' brand of granite, and it has a nice gray-purple-slightly reddish color to it.

but you can grow bonsai well in just about any mix, but it should be a mix, not a single component media. If you have to use only one component, I would use either pumice or perlite as my first choices. Turface has same issues as diatomaceous earth, too much in a mix and it causes problems.

You can grow in just about anything, if you understand how it affects your watering and fertilizing. You did discover that foliar sprays of iron chelate are needed with diatomaceous earth. You might just have to continue that until you get around to correct potting season for the trees you have. I would work on coming up with a better mix.

Leo Schordje wrote:I'm retired from a sometimes miserable day job in the chemical industries. had to do pH determinations many time a day, though not soil pH. I can offer this, unless you are lab trained, none of the less than $1000 usd meters will give you a test result you can trust.

I see so many people being told to use those extremely cheap pH meters from Lowes/Home Depot to test their soil or water...lately there's been a rash of this on some of the facebook groups. Whenever I warn about using such "equipment" (as well as the importance of alkalinity versus pH) I'm shouted down. I've decided it's not worth it. Glad to see you are still at it!

Chris

I do lose a little sleep over my substrate choices. I love the idea of using a more sophisticated mix but at the end of the day the diatomaceous earth is so cheap and easy (I use Napa Auto Parts oil absorbant) it's a hard bandaid to rip off. After reading Walter Pall's short treatice on watering feeding and substrate I decided I didn't need to change. Maybe it's time though. Thanks for all your insight.

Several years ago I submitted several 'soil' samples to my Clemson Extension office. I had a couple of variations of my basic mix (1/3 each of Turface, sifted Nature's Helper [pine bark mix from Home Depot] and gravel [obtained in various sizes from a local well drilling supply place]) tested out of curiosity and on preparation for a talk on soils. Interestingly, the akadama sample had a lower pH than did the kanuma. Not what I expected but confirmed after a call to the testing folks.

Leo, I was hoping to pick your brain a little more since you seem to be familiar with this subject. You seem to suggest that pH of the soil isn't worth testing, because the roots titrate the pH naturally at their surface to facilitate nutrient absorption. I'm just trying to wrap my head all the way around that, because it makes perfect sense: of course all organisms have mechanisms to buffer their environment to maintain physiologic conditions. So WHY then do horticulturalists make such a to-do about soil pH in a traditional garden environment?

By definition, pure water has a pH of 7. Water can pick-up things from the air and pipes that change the pH.

I've got hard water here in Iowa with a pH >7. I don't know if the trees care much, but I have to scrub the carbonates off the pot with a brush every so often.

Soil has no pH. Only water has. You measure the concentration of protonated water molecules in your watery solution.

So you have to add soil to a watery solution and measure the pH of said watery solution. You can then make a measurement of the pH of your test solution, and that pH can be a property of the soil. Using that number you can make inferences.

It all depends on what watery solution you add the soil to as this solution will already have a pH. Soil may change it. It will change a lot if the water is very pure and has no buffering substances like carbonate in it. It will change more if you add more soil.

There is a standard test in Europe that is done to put a number on bags of potting soil. They use H2O and follow a certain protocol.

If you want to measure the effect the soil has on pH more accurately, it is better to use a 1 M KCl solution.


You have to realize what you are actually measuring when you take such a test kit and it comes up with some number.
You can argue that the pH, and especially the buffering capacity, of the water that drains from the pot just after you are watering is a lot more meaningful, but even that can be debated.

I sometimes see people add vinegar or whatever, but unless their alkalinity is very low all they are doing is lowering the alkalinity while the pH stays constant.

Glaucus wrote:

I sometimes see people add vinegar or whatever, but unless their alkalinity is very low all they are doing is lowering the alkalinity while the pH stays constant.

Right, this is the titration process...as you add the acid, it has little to no effect on the pH until most or all of the alkalinity is neutralized, then the pH drops very rapidly. I've tried adding sulfuric acid to my water (standard municipal pH 8 water with a modest amount of alkalinity) and have seen this exact process. It takes a fair amount of acid to bring the pH down to around 6, but you can then easily overshoot the mark.

Here's a question for those of you with more chemistry background than me. I've titrated a gallon of water down to pH 5.5-6.0 using sulfuric acid. If I let it sit for a few days and measure it again, I find that the pH has risen back up to 7 or higher. Can someone explain this? My guess would be that the sulfur is outgassing but I'd like to know for sure.

Chris

pH is the negative logarithm of the percent free Hydrogen (plus 7). Water is H20. Some part of it is always H and OH (hydroxyls), but they are in equal numbers (-log is zero). There is a dearth of H when the solution is basic (alkaline). There is an excess of H when acidic. The extra H disappears as hydrogen gas, so overtime acidic solutions tend toward neutral.

... though I am not a chemist.

coh wrote:

Glaucus wrote:

I sometimes see people add vinegar or whatever, but unless their alkalinity is very low all they are doing is lowering the alkalinity while the pH stays constant.

Right, this is the titration process...as you add the acid, it has little to no effect on the pH until most or all of the alkalinity is neutralized, then the pH drops very rapidly. I've tried adding sulfuric acid to my water (standard municipal pH 8 water with a modest amount of alkalinity) and have seen this exact process. It takes a fair amount of acid to bring the pH down to around 6, but you can then easily overshoot the mark.

Here's a question for those of you with more chemistry background than me. I've titrated a gallon of water down to pH 5.5-6.0 using sulfuric acid. If I let it sit for a few days and measure it again, I find that the pH has risen back up to 7 or higher. Can someone explain this? My guess would be that the sulfur is outgassing but I'd like to know for sure.

Chris

Sulfuric acid is  not volatile.  What is probably happening is that your water is picking-up carbon dioxide  and maybe other things like ammonia from the air and changing the pH.  I deal with this every day at work.   We toss our buffers every 7 days or so.

Carbon dioxide + water --> carbonic acid, right? So I don't think that would do it. Ammonia...is there enough ammonia around to have an impact?

I was just reading a website about reefs and there was a discussion of this issue. The claim was made that the subsequent rise in pH hours or days after addition of acid occurs because of the following:

1) initial addition of acid neutralizes the alkalinity (bicarbonate) and lowers pH
2) some carbonic acid is formed
3) carbonic acid disassociates into carbon dioxide and water, leading to an excess of carbon dioxide in the water relative to the atmosphere
4) carbon dioxide diffuses out of the water into the air to reestablish equilibrium, resulting in a rise in pH

I don't remember my chemistry well enough to know if this is a reasonable explanation or just more hand waving.

hi chris.

The reason pH rises is becuse not all off the bicarblnate has reacted with the acid. The reaction takes a little longer when the concentration of bicarbonate gets small.

You need to wait awhile before measuring the pH. I think the rate limiting step is the dissociation of HCO3 to CO3.

The manutec product really is the best for testing potting mixes cheaply and easily and dosn't use much soil. just a teaspoon full will do.

Paul

63pmp wrote:hi chris.

The reason pH rises is becuse not all off the bicarblnate has reacted with the acid.  The reaction takes a little longer when the concentration of bicarbonate gets small.

You need to wait awhile before measuring the pH.  I think the rate limiting step is the dissociation of HCO3 to CO3.

The manutec product really is the best for testing potting mixes cheaply and easily and dosn't use much soil. just a teaspoon full will do.

Paul

OK, so if that's true...then it sounds like I could eventually titrate the water down to whatever pH I want, and it should be relatively stable...as opposed to rising back up to 7 or higher?

Though...if the goal is more to neutralize alkalinity (since that is a big impact on soil pH) than to lower the water pH to some level, I've probably accomplished that with the first pass, even if the pH several days later has risen...right?

Chris

Hi Chris,

I'm hoping this link to a table will work.






The issue with your pH rising after acidification is that bicarbonate can coexist with H2CO3 in solution at pH 5.0.

From the table, at pH >6.35 the dominate carbonate species in solution will be bicarbonate with diminishing CO2 till you reach a pH of about 9.  Between 4.4 and 6.35 there will be a mix of  species with no bicarbonate at about 3.8 to majority at 6.35.

When you add acid to bring pH down to 5.0, say.  A lot of bicarbonate will be converted to CO2, and some will remain. There will be a mix of acid, H2CO3 and HCO3, the pH will fluctuate, until the system reaches some kind of equilibrium.

The only way to reduce this is to add a lot of acid and neutralize all of the alkalinity, then bring pH back up to 6.0 with hydroxide.

Or simply push pH down to 5.5 with acid and use plant ammonium uptake to balance the last of the alkalinity in the water.


Paul

Thanks Paul! Interesting stuff. Any chance that diagram came from an on-line article that you can provide a link to?

I'm going to guess that "real world" applications might be more complicated because most water sources have other compounds in addition to the carbonates. Though maybe in small enough amounts that it's not so important?

Any way, thanks again - hope things are going well over there!

Chris

Hi Chris,

I got it from here:

http://chemwiki.ucdavis.edu/Textbook_Maps/General_Chemistry_Textbook_Maps/Map%3A_Lower%27s_Chem1/07b._Solution_Chemistry/7b.6%3A_Solubility_of_Salts%2F%2FSolubility_Equilibria

Section 4

The graph is for an idealized solution of sodium bicarbonate, mixed solutions will behave slightly differently. Different salts affect dissociation of bicarbonate, but not enough to matter in a bonsai, backyard bucket chemistry approach to life.

Regards

Paul