Questions for Al: calculating soil, dolomitic lime, etc

kristimamaJanuary 26, 2012

Hi Al,

Could you please help me with a few questions and validate a few calculations? Thanks so much!

1) I am wondering about the correct amount of dolomitic lime for some very very large containers that I built out of 2x10s with plywood bottoms. The interior dimensions of these boxes are 21" x 45" x 19" deep.

If I calculate this into cubic yards, I get .38 cubic yards.

If I convert that into gallons, google gives me 76.7 gallons.

I have seen you say before that you use 1 TBSP/per gallon of soil, or 1/2 cup per cubic foot.

If I use the 76.7 gallons, I come up with about 4.8 cups. (OTOH, if I calculate it based on 10.26 square feet, I end up with 5 cups, so about the same.)

Does that sound right, about 4.8-5 cups of dolomitic lime for my large boxes, assuming they are filled to the top? (I suppose it jives with the ratios in the big batch you make of 33 gallons; my planters are about 2.5 times bigger than 33 gallons, and 5 cups is 2.5 times of 2.) But it just seems like SO much dolomitic lime. I'm worried about overliming.

2) How long does dolomitic lime live in the 5-1-1 mix or other soilless mixes? Will it continue to do its job for the life of the 5-1-1 mix? Or is it a relatively permanent addition to a container mix?

3) Is dolomitic lime something that needs to be mixed *into* the container mix, or can it work sprinkled onto the top? Here's why I'm asking: Some of my citrus are in a *modified* version of your soil. ;-) The peat component is from a potting mix that contained some lime, and so I didn't add additional lime last year when I mixed it up. After I have now read what feels like every post on here I could find about dolomitic lime, I realize how important it is in the pH of a bark-based potting mix, and I also realize in hindsight I probably *should* have either added more lime or *not* have used that bagged mix and used straight peat. Oh well... lesson learned for next time.

Anyway, what I'm wondering about now is whether I can put the dolomitic lime on top of those containers (if not in its total amount, then maybe a sprinkling)... either on top or worked into the top of the containers. And in what quantity or ratio.

And finally, a question about adding dolomitic lime to Mel's Mix (of SFG fame)... but feel free to disregard this part of the question if you just don't want to deal with the relative merits/structure of that mix. I would understand. :-)

4) My very rudimentary understanding of why one would add dolomitic lime is because the bark fines contain such a low pH? Do you know (or would you guess) at the possible pH of Mel's Mix and the proper dosing (if any) of dolomitic lime to such a mix? I'm just wondering, in a mix like that with more peat and Vermiculite (which are more ph Neutral) and compost, which is only slightly acidic, would you still use dolomitic lime? And if so, in what ratio? I do use Mel's mix for some of my containers and it works fine for me, and it gets refreshed each year with bark fines and composted plant matter. I'm just wondering if dolomitic lime might be appropriate to add at this point to help pH and micronutrient uptake.

Thanks so much for your advice and guidance, Al.


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tapla (mid-MI z5b-6a)

Grrrr! Lol - I had your whole list of questions answered and somehow lost the whole thing. I hate it when that happens! Sigh

1) Ok - the going rate for liming bark/peat-based container media is 5 lbs/ cu yd. The bulk density of prilled dolomitic lime is about 50 lbs/cu ft, so a pint weighs about .85 lbs. You need 1.9 lbs or 2.25 pints or 4.5 cups for .38 cu yd.

2) The Mg fraction in dolomite is about 125X more soluble than the Ca fraction, so it's not unusual to start seeing some sign of Mg deficiency in the second year in the same soil, but that depends on what's in the fertilizer and your water. If your fertilizer has Mg, I wouldn't worry about it unless you see something obvious on the nutritional front.

3) Lime is best incorporated where it can react with the soil and so it's available throughout the root zone. The pH of container media isn't as important as the pH of mineral soils, and fortunately, your citrus prefers a pH of somewhere around 5.0 (for container media) anyway, so I wouldn't get too nervous. If your fertilizer contains Ca & Mg (Foliage-Pro 9-3-6) ;-), you're prolly in fine shape.

4) As you can see, micronutrient uptake is actually improved at lower pH in container media (left)

as opposed to mineral soils (right). See the Sweet spot at around 5.2?

I would lime Mel's mix based only on the volume of peat (and bark if you add any) in it. If it was me - I'd add 1 tsp of lime for every gallon of peat - less than usual because of its low bulk density compared to bark.

Take care - good to see you posting again, K.


    Bookmark   January 26, 2012 at 10:20PM
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Al, I love ya. You made my night giving me these answers. When I get on one of these research kicks I head down a rabbit hole in search of new information.

1) Glad to know my math was about right. I failed chemistry in high school, and it scares me a bit. I will use your number.

2) Actually, my citrus are starting to show some signs of a Magnesium deficiency... it could be the winter, and then those freezing days we had last week, or it could be something else like my fertilization. I'm waiting for a few weeks of warm weather to see what happens. If my memory serves me, I think they looked like this last winter and our first weeks of sunshine they perked back up.

But, as you know, I *do* fertilize organically with the Dr. Earth products and my micros come from their Liquid 3-3-3 with Micros. (I'm *not*---at least not yet---using Foliage Pro. I know, bad bad student.)

The 3-3-3 apparently has: Micronized and Hydrolyzed Soy Protein, Micronized Mined and Micronutrient-dense Colloidal Soft Rock Phosphate, Naturally-mined Potassium Sulfate, Potassium-Magnesium Sulfate, Copper Sulfate, Iron Sulfate, Manganese Sulfate, Molybdic Oxide, and Zinc Sulfate. I use this weakly, if not weekly then at least once a month.

3) Re the citrus, since I'm not using FP... (quietly bows her head and cringes), I'll ask again whether somehow working a small amount of dolomitic lime around the top of the mix, or scratching it around the outside rim of the pot, would do anything at all to help the magnesium levels? Or are you saying that it won't work because it's not incorporated. I can live with either answer... I just wanted to ask again since you initially responded that "if I was using FP."

4) Thanks for answering. That's actually interesting, because I thought they needed a little higher pH to get those micros. The tsp per gallon of peat (and wood) is a great place to start. I've never actually had any problem with the soil for my tomatoes in this mix, but after a few years of growing in it, I'm just assuming it needs a little boost of something at this point.


    Bookmark   January 26, 2012 at 11:46PM
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