Hello all! I'm compost joe, an organic grower/student. There seems to be a wealth of knowledge and numerous passionate people here so I decided to sign up and learn some more. My question involves the difference in cal/mag ratios between dolomitic lime and basalt rock dust. now I understand that these two rock dusts are used for different things but it is the cal/mag ratio of them that confuses me. I often see it written that one of the reasons dolomitic lime is no good is because the low cal/mag ratio. The dolomitic lime I have is about 20% calcium and 10% magnesium, or a 2:1 ratio. The basalt rock dust I have has 9.15% calcium oxide and 5.77% magnesium oxide, or a 1.58:1 ratio. So why is using 2 tbsp. per gallon of dolomitic lime not a good idea but using 4 cups of basalt per gallon a good one? Thanks for taking the time to read this, Compost Joe
Solubility/reactivity/atomic weight. Dolo' is a sedimentary rock. The cal'/mag' in dolo' is made readily available much more easily than that contained within basalt, which is an igneous rock. The minerals in dolo' are considered slow release, whereas those in basalt are REALLY slow release. Not to mention that when you consider the actual atomic weight of the cal/mag contained within basalt (you must subtract the "oxide" from the atomic weight), the ratios are even smaller, more like .65% calcium to .0347% magnesium. Compared to dolomite, the amount of cal'/mag' is almost negligible...
The *links* and quotes will be showing up soon enough with the technical stuff. I'll just offer some observations and opinions. First off, *I* think the 4cups of Basalt is in error and has been for some time. 1-2 cups is more realistic. The second is the Ca oxide. Apparently, just the Carbonate form of Ca has any effect on pH. That right there puts basalt out as a liming agent. IDK about the oxide form of Mg and will not offer any opinion. I also think that the 'problems' with dolomite are the result of soil studies and trying to transpose them to soiless container mixes. I've been using dolomite for years and have never had a problem with my container mixes. I DO take heed of these studies when it comes to dolomite and my (mostly red clay), soil garden, but not my peat based soiless container mixes. Trying to equate either one with the other, besides being derived from rocks, is silly. One is rich in minerals, with little, if any, pH buffering and the other is rich in pH buffering and lacking in minerals. Not quite apples and oranges, but close. Wet
Appreciate the kudos, wet'! But you've made an important distinction that I failed to include... Trying to equate either one with the other, besides being derived from rocks, is silly. One is rich in minerals, with little, if any, pH buffering and the other is rich in pH buffering and lacking in minerals. Not quite apples and oranges, but close. Dolomitic lime is used primarily for it's ability to neutralize acidity created by the microbial degradation of organic materials in soil. ph buffering that liberates cal'/mag' for nutrient uptake... Rock dusts are utilized to provide TRACE elements, and microbial attachment sites. The trace elements present and available in finely ground rock dusts serve to provide minerals needed for microbes to CREATE the enzymes that are responsible for the breakdown/buildup of compounds that are necessary for healthy plant growth.
thanks for the responses everyone! I understood that the two were used for completely different things, I was just confused because I thought they had a similar ratio yet one was recommended in high amounts (up to 4 cups) while the other was said to be bad in much smaller amounts (up to 2 tbsp) per gallon. waktoo: thanks for pointing out the difference between sedimentary and igneous rocks. i'll have to take some time to learn more about the differences between the two. I will also have to look into figuring out how to convert calcium oxide and magnesium oxide to calcium and magnesium. one of my favorite things about growing is that their is always something new to learn! CJ
Right on this thread goes in my bookmarks! Ive been kinda anti dolomite lime since i talked to coot about its ratio. But ive been asked how to treat calcium deficiency and ive been telling people gypsum for calcium and epsom salt or molasses for magnesium. Anyone care to chime in? Is dolomite safe if things get outta wack? This ok advice?
"First off, *I* think the 4cups of Basalt is in error and has been for some time" Wetdog, regarding the recommended 4-5 cups of rock dust per cubic foot - Before any rock dust is added to a soil mix it is basically ALL organic matter. I know it is in mine because I use buckwheat hulls for aeration (when I do add aeration). All organic matter - every bit of it. I'm just not seeing how on earth 4-5 cups could possibly be too much. There are 16 cups in a gallon and 120 cups in 7.5 gallons, or a cubic foot. This means that if we add 4 or 5 cups of rock powder to our all organic matter mix, that we are still only ending up with a total rock dust amount of around 3% of our total mix. 3%. How could 3% possibly be too much? "1-2 cups is more realistic" 1 cup of rock dust isn't even 1% if your soil mix of 7.5 gallons translates to 120 cups... I wonder how much of a percentage is out in our gardens and fields? Respectfully, J
Jerry et al, Isn't soil tilth just as important if not more so than "X" cups of rock/mineral dust? Wet's comment about "soil" vs container gardening is absolutely spot on. What happens chemically inside a container over 4 months has different results than open field agriculture. My only point is that the tilth of the soilless mixture must be properly constructed regardless of whether x or y number of cups of something is "better/best". Just speaking from personal experience that it's reletatively easy to mix up a batch of growing goodness that turns into a rock hard cement-like nature by adding "too much" of something and esp true with rock dusts. Respectfully. P.
Luebke composting suggests using 10-20# (160-320 oz.) of basalt rock dust per cubic yard of compost. I don't have finely ground basalt rock dust on hand, but I do have glacial rock dust. Assuming that they are of similar density, I will use GRD as an example. It weighs ~ 10.75 oz./cup. For ease of calculation, I will consider it to weigh 11 oz./cup. For FIELD application of compost (which is 100% organic material) at the maximum suggested amendment amounts, this converts to approximately 29 cups (320/11=29.09) per cubic YARD. There are 27 cubic feet in a cubic yard. 29/27=1.074. That's one cup of rock dust per cubic foot of compost. I'd really like to know how we ever arrived at an application rate of 4-5 cups/cubic foot for the "easy indoor potted soil mix", which is only 2/3 organic material. Happy Turkey Day all! Respectfully, of course...
That is just how I arrived at the conclusion that 4 cups/cf is too much. My usual fairly light mix turned into something, if not rock hard cement, more like the red clay in the garden. That's slowly getting lightened with lots of organic material, but it's a lengthy process. Wish I had read this before reaching the same conclusion the hard way. Happy Turkey day ya'll! MMmmmm .... cornbread dressing from scratch and giblet gravy. Yum-o Made the cornbread 4 days ago, 2 pans and 6 Qts of chicken stock 2 days ago. For me, the dressing and gravy is the main event and the turkey a nice afterthought. With respect. Wet
In my thread mr s wrote a response to my question on using counter cut leftovers What are your thoughts on this stuff guys? I actually heard about it from you guys! ~ poke
Hey Poke! I know Coot has said several times that granite is an excllent rock dust input. He said granite would be just a bit below basalt, and then GRD. I have also heard to be careful about a lubricant or polymer they put on the blade or something at granite cutting places? Maybe someone can clarify? I've been adding 4c of rock dust per cu ft for quite some time now based on Coot's recommendation. Seems to be working fine for me? View attachment A Rock Dust Primer.pdf Peace! P-
Mornin', poke'! This was what I did my first go around with using RD in my soil mix. There was definitely a noticeable difference in the health of my plants. It worked fine, with no noticeable ill effects. But... As time passed and I learned a more, I came to the realization that the polymers used to glue the granite to the cutting templates and the materials used to lubricate the cutting blades and then polish the stone might not be the kinds of things that I want in my soil, being "organic" and all. I think you'd be better off all around if you tried to find a rock quarry in your neck of the woods. The "fines" from around the "crusher" serve as a fine substitute for commercially available "rock dusts", and most closely resemble what would be sold as "glacial rock dust".
Thank you both for the replies I think ill skip the half assed approach due to glues/oils (eww) and just spend a little money on the good stuff Though the thought of shipping rock dust pisses me off to no end. :lol: Sounds funny! I will look local, but ive got a few online resources where its about 90 cents a pound, or 24 dollars for 50lbs. That a good price? ~ poke
Once you price the shipping on that 50# bag, you're going to be looking REAL hard locally... If you order and have it shipped, make darn certain that you're getting rock DUST, the fine stuff, 20 microns or what will pass through a 300 mesh screen or smaller. I got hosed on a bag of basalt rock dust that I ordered once. Definitely was NOT dust...
FWIW, I've used dolomite for years and have never had a problem with it, certainly no fear/hate. I still use dolomite lime, but have added calcitic lime for the garden. Have you ever used it, or, are just going on opinions? Very cheap to find out for yourself and form your own opinion, less than $5. Wet
Well, first, every potting mix is different and everyone's garden is different. Every recipe is different. Recipe amounts are recommended and not set in stone. The "4-5 cups/CF" amounts originally came from Gaia Green GRD research trials some time ago. If my memory serves me correctly the benefits ran out at around 5-6 cups/CF. BAS, in their Pioneer Valley Brix Blend basalt shows "May be incorporated into potting soil mixes up to 5% by volume depending on soils and plantings." (http://buildasoil.com/products/rock-dust-local-premium-basalt). I've never used the compost method you're talking about Waktoo so can't comment. My own potting mix IS 100% OM before I add rock flour. I've never seen anything remotely like it turning to concrete at those rate but hey, maybe others are having different results. Who knows. I do think there's something wrong happening tho if adding 3%-4% Rock dust to a largely organic matter mix is changing the structure of the soil that much. Then again, everybody has a different soil recipe so I don't know what others results might be like. J