5.8 to 6.0 isn't a significant difference-- you probably can't even measure that accurately. If you are looking for a target pH, I would use pH 6.5. The pH will usually drop steadily in pots because we overfertilize and the excess nitrate leaches with the heavy watering that is needed. The maximum amount of finely ground limestone to apply at a time would be 1 oz/sq ft, which I think is a little over a TB. It is a dense material. So if pH is below 5.8, you could just apply that amount and check the pH in six months.

I agree with Michaelg. A few points that may help:

1) Lime doesn't "break down" in the soil and then suddenly start to neutralize. Lime has a small, constant, solubility in the soil water and it is this small but constant amount of dissolved lime which reacts with the acid in the soil to raise the pH. It's a more or less steady process from the moment you add the lime.

2) Soil pH alone won't tell you how much lime to add, it depends on the soil type. Soil test laboratories determine a quantity called the "buffer index" or "buffer pH" which can be used to predict the precise amount of lime required to raise the soil pH a specific number of units.

3) A common rule of thumb is that 6 pounds of lime (lime is heavy, 6 pounds of lime is only 6 cups of lime)per 100 sq ft of average garden soil will raise the pH to a depth of 6 inches by one unit. Another common rule of thumb is that this is also the maximum amount of lime you should apply per season in any case. If you like, 6 cups of lime per 100 sq ft to a depth of 6 inches is the same as 1/4 tablespoon of lime per gallon of soil.

4) The enclosed link from Iowa State University gives the capacity of standard nursery pots in gallons (Table 1, in the link). You might note that a 12 inch pot holds 3 1/2 gallons of soil, so using the rule of thumb of 1/4 tbs per gallon leads to 0.875 tbs of lime per 12 inch container, essentially what Michaelg suggested.

Here is a link that might be useful: Soil Capacity of Nursery Pots

Mike, thanks for the calculation. I should have warned pkapeck that any numbers you see from me are sloppy ballpark estimates. The figure of 1/4 TB limestone/gallon of soil is a handy number to know.

I learned yesterday that a malfunctioning geranium has a shockingly low pH, though most of the medium is not old. I was thinking wood ash would make a faster correction than limestone, is that right? Sloppy ballpark estimate, ash (vs. limestone) has maybe 1/2 the weight per volume and 1/2 the alkalinity per weight, so I could try 1 TB ash (partially dissolved in water) per gallon of soil. What do you think?

Michael, I think 1/2 the alkalinity per weight is probably likely, but the density of wood ashes varies all over the place. I have a note from somewhere that 1 1/2 gallons of wood ashes weighs 5 pounds. That would mean you'd need 2 1/2 tbs of wood ashes to equal 1/4 tbs of lime. Very fresh wood ash contains some calcium oxide which would be fairly soluble and fast-acting but most of the neutralizing power of ashes comes from calcium carbonate - plain old limestone.

Thanks, Mike. (Actually I started to guess 1/4 the density of lime, which would have been right but sounded risky.)

Wouldn't the potassium oxide in ash also be a fast alkalizer?

Michael, yes, the potassium oxide (potash) in wood ashes is a strong base and is very water soluble and this makes it a very fast soil alkalizer - but it amounts to only about 10 to 20% of the total alkali in wood ashes. Potash is easily leached if wood ashes are exposed to rain before application in the garden.

I would expect that if your wood ashes are very fresh and almost entirely soluble in water, then they will be mostly calcium oxide or hydroxide and they will be fast-acting. If your wood ashes are mostly insoluble, then they are mostly calcium carbonate and they will be slow-acting.

pkapeck,

Did you get the information you need? I never know what to think when somebody asks a question and doesn't come back.

michaelg yes sorry I havent checked back in here since I posted that. The odd thing is I have roses I need to LOWER the PH and they are on drip irrigation with well water which is usually considered sulphur water which you would think would be very acidic over time. pH was right at 7 in the containers. I want to drop it a half a point.

On my own I did another experiment as I wanted to use pine bark (I read on here it is a great soil additive to bulk up soil and to provide air as well as a supply of organic nitrogen when it breaks down albeit slower than other additives.) and it made a nice blend. What I didn;t realize is how acidic the stuff was. I didn;t use any scientific means to blend it as I was looking for a nice aerated mix, but when I tested pH, my normally neutral soil blend was now at a pH of 4!!!!!! So I started adding normal soil (pH 7) to balance it out and it made no difference. I added a 3-4 tablespoons of hydrated lime (the stuff that acts quickly) to the 12 gallon pot and the Ph was still only like 4.5. Then I figured if pot ash was alkaline so would cigarette ash. I think it actually dropped it back down to ph 3.5 as I used maybe a few tablespoons of it. The I started adding vermiculit and perlite as they tend to be ph 7-9 but it didn't affect it much. So as it sits now the ph of that pot is about 5, but all the linme and other stuff I added I feel is going to make the soil unuseable, as it is taking on a different texture than the blend it originally was with all the powders, ash and granuals.

I think the 1/4 teaspoon / gallon of soil is a good rule of thumb. I wonder why references to ph don't make this standard notation as who would use 100 sq feet of lime whebn they have pots?

Another question is on some of the smaller 12 gallon pots I already planted the pepper plants I have and the ph Itested after they were planted were low like 5.0-5.25 and they need to be at least 6. Can I just add lime to top dress the soil and will it actually benefit the roots or do I need to take the plant out apply the lime and mix the lime thouroughly and re-test ph before re-planting?

PPPP-- it's 1/4 TB of limestone per gallon of soil, not TSP. One TB per 4 gallons. Hydrated lime is 25% stronger, so you would use less, maybe 2 TSP per 4 gallons. Hydrated lime is more soluble, so I guess you could just work it into the top layer.

I don't think the effect of lime on soil texture is harmful, but it sounds like you used too much and the soil pH will overshoot.

michaelg, I didn't overshoot, as the pH is still at 5. Now that I think about it the scoop I used was a tablespoon but actually larger as I used a heaping spoonful. I probaly used 3-5 of them even.

Any comment about using cigarette ashes? I dumped a bunch and it LOWERED the pH, which had the opposite effect I was expecting.

Also any reason why sulphur water would be alkaline?

Florida well water usually comes from limestone aquifers and that's why it's alkaline. I remember that sulphur water. It contains iron, which stains the plumbing fixtures, and smells like sulfide, but I don't know exactly what all is in it.

I said I think the pH "will" overshoot, since it takes weeks or months for even hydrated lime to react fully (or I guess so).

oh ok michaelg, I was worried it would take that long to figure out the pH changed. It is weird that the pH did change instantaneously when I added cigarette ashes mainly and it lowered the pH to such a low level. So how long does hydrated lime take to effectively alter the pH? Days? Weeks? Months?

Right now the 12 gallon pot filled with the soil with a 5 pH is basically unuseable to grow the plants I have, so I am just going to use a scoop of it here and there to mix in with containers filled with pH 7 soil to lower it down to 6.5, and use up the 12 gallons of 5 pH soil that way. I don't want to start playing yo-yo with my pH trying to get the 12 gallons of soil to balance at 6.5. By the time I'd be done I'd have more lime and pine bark than actual soil lol.

Pkapeck, the pH of cigarette ashes is about 11 or 12. If you added cigarette ashes and the pH dropped, you need a new pH meter.

If you mix very finely powdered lime with soil, add an excess of water and stir constantly, the pH will settle down to a constant, equilibrium, value in a few hours. Under typical garden conditions, especially if the lime is not worked into the soil, equilibrium might take the better part of a growing season. You should understand that if you take a soil sample just after you add lime, the pH you get will mostly reflect the presence of lime and be quite alkaline. The pH of such a sample will gradually fall, maybe over a period of months, until equilibrium is established at some pH value which will be higher than the initial soil pH but lower than the initial sample reading.

thanks mike, I was just going to post a thead over in the soil/composting forum here if I didn't find an answer on the cigarette ashes. It is true that the pH dropped quite a bit, but maybe it was still due to the pine bark which is finely chopped up pieces, not like pine nuggets you normally find.

I doubt the meter is bad as it is a brand new Kelway HB1 meter and I double checked it with 2 other less expensive meters and all were the same.
I did mix about 4 more TBSPs of lime and it is around 5-6 pH depending where I stick it in the soil and depth. I will probably leave it now and wait for it to equalize itself and use it to buffer other pots with neutral soil I need to just lower a little, or wait and after a few months it may hit the magic 6.5. Thanks.

Pkapeck, the term "soil pH" actually means the final, equilibrium, pH of a sample of water which is in contact with the soil. A soil laboratory determines soil pH by mixing a volume of soil with, typically, a double volume of pure water and stirring the mixture until a steady value for the water's pH is obtained. My comments in the post above assumed this situation. The Kelway meter, on the other hand, is used by sticking a probe directly into the soil. The pH is derived from a relationship between a voltage difference between two differents metals on the probe and the soil pH. I suspect that if you add either dry cigarette ashes or dry lime to the soil and then use the kelway, neither the ashes nor the lime will have any effect at all on the pH reading. I'm curious, why don't you stick the Kelway probe into pure powdered lime and see what reading you get?

I have some experience with pH meters, and my first rule is not to believe a screwy reading, but to repeat until I get the same plausible reading twice, or a whole string of the same screwy reading. In other words, I try to make the meter tell me what I expect to hear ;)

If I use a meter in situ, I remove the top inch of soil, which is going to be contaminated by fertilizer, lime, pesticides, and extra OM. I also add water until the area is saturated. (I would use distilled water except our water is so pure I can't measure a difference.)

Since you haven't mixed the lime and ashes into the soil, and the stuff hasn't had time to dissolve and react much, I don't think it is feasible to measure the effect of your actions at this point.

Mike RIvers and michaelg, actually I spent 20 minutes mixing the soil in the container. I probably removed the soil into another container at least 5 times to make sure it was blended well. My fingers turned blue even though I didn;t use my hands to mix I guess the soil got on it, and I am guessing that was due to the limestone? In any event I did get it almost perfect at 6.25-6.5. Close enough to use now. Thanks for the help.

Re: screwy readings on the meter, before I spent all the time mixing the soil I did mix it haphazardly and then I was getting wild swings with the pH pegging the acidic side then creeping up to about 5-5.5. I probably stuck it in 10 different areas within the pot and the readings were fairly consistent 4.75-5.25 depending how deep and where it was stuck.

Now that I added a healthy dose of lime, it is 6-6.5 which was just where I wanted it. Hope now that it won;t slowly start gettting alkaline in the coming months, but I will keep tabs on it.