The soil mixes the pros use

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I thought it might be interesting to gather some information about what the pros use:

Tobias Spanner, the owner of Palme Per Paket, published the book "Das Palmen-Kulturhandbuch" 1999. He recommends these soil mixes:

  • 30% {60%} Peat based soil mix or limed peat (pH 6) or coconut substrate (e g "Kokohum")
  • 20% {10%} fine lime-free clay soil or loam
  • 25% {15%} quartz sand 0-4mm
  • 25% {15%} other mineral for drainage: pumice, perlite, vermiculite, LECA, Lavalit (crushed lava stone), Seramis (porous clay granulate from Germany), stone wool (such as rockwool)
  • Up to 15% {20%} of pine barch humus or loose compost may be mixed in

The figures within braces are for palms from moist areas (which like humus rich and a bit sour soil), the other figures are for palms in dry areas. For bigger, older palms the organic part can be lowered and clay part increased. The result should be loose and not sticky even when wet. Especially for palms from dry areas, deep pots should be used. He also writes that many palms do excellent in ready-made soil mixes but that the main problem would be not enough draining material. And today the recipy is published also on their site [PPP], with these changes: "Peat/Coconut" part increased to 50% {70%}, quarz sand diminished to 10% {10%} and "other minerals" changed to 20% {15%}.

I like this recipe for several reasons: It has obviously been tried out a lot, it has been written by someone who takes into account easy handling and low cost, it doesn't say "use exactly this" but gives a wide range of freedom, and there is only one recipe (with 2 variants) for all palms.

In my country (which is rich on peat) the soil mixes contain both sphagnum peat moss and "peat humus" (hard to translate, we call it light and dark sphagnum peat respectively; "peat humus" is the closest English term for the dark, decomposed peat, although usually used for reed-sedge or hypnum), for that reason I think I can probably skip the loam part. The reason for mixing in "peat humus" is probably because it is cheaper but it has also been proven that a mix of 50% peat moss and 50% "peat humus" has qualities comparable to pure peat moss (as long as moisture level is kept >25%)[Verdure][Groni][Bik][Bunt].

Regarding the draining material, even though following the recipe should be fine, I have some ideas of changes. First of all, why both sand and other types of material, and why quartz sand exactly? Some say the draining material should be "edgy" (non-round) [ArkUniv1][Boodley] such as quartz sand, I suppose they mean it creates bigger "holes" for drainage. Mainly I think they limit sand to ~10% due to weight. Recommended size usually is > 1 mm [Bunt][Ministr1] but to avoid > 8 mm because it lowers available oxygen probably because roots get more twisted. I have usually used all sand, size 2-3mm, bought in an aquarium shop, 20 to 40 vol%. Note that coarse organic material (e g peat) also helps with drainage.

Sand, perlite, LECA, LESA, pumice, vermiculite, perlite, Seramis, plastic (e g styrofoam), stone wool etc do the same job of creating drainage. Besides difference in weight there are two parameters of some importance: Ability to bind water, and ability to bind nutritients (so-called CEC value), making this accessible to the roots. Humus has the highest CEC value (200), followed by vermiculite, (150); aged coconut fibres and a mix of 50% peat moss and 50% vermiculite has similar CEC, pure peat has 100-120, pumice about 75, bark humus and fine clay about 60, sand and perlite close to 0. Vermiculite and Seramis has excellent water holding capability, similar to that of peat moss, pumice not so bad either; the other ones don't hold any water that I know of.

I find that mixing just 70% potting mix with 30% sand size 2-3 mm you have to be careful not to pack it too densely (just like you have to with pure peat moss). With big palm trees I don't want to pack it too loosely for stability so it might be good to add even more sand, but it would become rather heavy and have to be watered very often. Therefore I am at the moment considering 50% peat based potting mix with 50% vermiculite or Seramis. Since I can find vermiculite in size 1-4 mm I think I'll go for that. Seramis should be cheaper in Europe and vermiculite in e g the US but I can't see big price differences, and "Seramis für Zimmerpflanzen" seems a bit big. (Note: 100% vermiculite does not work well as growing medium.[IowaAgr])

Another recipe worth contemplating by a well known palm nursery is this by Jungle Music:

  • 10% amended topsoil (has about 1/3 high quality topsoil and 2/3 humus)
  • 15% pumice #2
  • 15% 0 - 1/8 inch pine bark
  • 15% nitrolized redwood shavings
  • 20% perlite #2
  • 10% coarse washed sand, #12
  • 15% coarse peat moss

For us outside the US it is hard to get some ingredients and even understand what all means though.

In countries where peat is expensive or exploits nature, one of course would like options. Many growers in the US use fresh bark but then add nitrogen (typically 0.6g/L) since the bark consumes nitrogen when decomposing. Composted bark is better but more expensive. An in-between option is aged bark. Bark from most deciduous trees is poisonous especially to young plants so use pine park, most of which is not poisonous. One drawback with bark though is big variation in properties. Comparing peat moss to bark, peat moss holds more water while bark is better aerated. Bark is a bit acidic (like peat moss) but bark humus is more neutral, but it can vary rather much. Composting bark has the advantage of CEC becoming considerably higher.[Bunt][MassUniv1][ArkUniv1][FloridUniv1][Raviv][Yapcoll]

An advantage of bark is that it takes several years to break down, but then it breaks down quickly needing immediate repotting, so that should work well provided you plan to change soil every 4 years or so. Adding even 10% peat moss adds a lot to water retaining capability without taking up much volume. As soils age, they tend to break down, reducing the particle size and retaining more water. This process can actually match the growth rate and water needs of the plant, if carefully balanced. Once soil is thoroughly root colonized it is not subject to collapse for most species unless the plant is subjected to poor treatment such as massive overwatering, not enough direct light, etc. The root network will tend to keep the soil aerated by creating a woody framework. In fact, fine particles will often be washed out of a healthy root network. [Walston] 

My comment though, is that even though Walson's text is highly interesting and believable, I don't feel sure if I should aim at organic part being all bark or 10% peat moss or all peat moss, and in any case it sounds like I should be prepared to exchange all soil after a few years, which sounds tricky for a root-bound big palm, and goes against the advice of Palmbob to never mess with roots but just add soil around old root ball. On the other hand I can imagine soil being full of residue from tap water and fert after a few years no matter how much I leach.

Note: There are several threads on Palmtalk about soil mix preferences of individual growers that you might also want to read: [P1][P2][P3][P4].

Sources not available online:
[Boodley] The commercial greenhouse handbook, James W Boodley 1981
[Bunt] "Media and Mixes for Container-Grown Plants", A C Bunt, UK 1988
[Raviv] "Substrates and their analysis" 2002, ch 2, Agricultural Research Org, Israel

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I thought about it for a bit, and I think I know the answer to whether peat moss, bark or coconut substrate (coir) is suitable for soil mixes regarding breaking down: It seems all of these, if good quality, don't break down for several years, at least a couple of years.[1][2][3][4][5] And since palms in general need repotting at least every 2 years, it is ok to choose either one of peat moss, bark or coir. What Walston was aiming at (above) is that for bonsais it is convenient if you can wait 4 years instead of 2 with repotting. I can't imagine a palm that doesn't need repotting every 1-2 years in an apartment except maybe when it is getting really big and you try to slow it down a bit.

One major question remains in my mind though: No matter what soil mix you choose, the organic part will break down within a few years. So should one just add soil around the root ball when repotting or try to remove all old soil? I believe very much in Palmbob and so I guess I'll have as baseline not to mess with the roots. Howevere I will probably check each palm individually in this forum since I know some are known to accept even root pruning. And when I look at my Lipstick Palm and Miniature Coconut Palm, who have both been in the same soil in the same kind of pot for the same time but the Lipstick having really filled the pot with roots up&down (without peeking out the holes much and still requiring watering "just" every 3 days) and the Coconut filled maybe half the soil with roots, I think (based on Walstons words) that for the Lipstick I can probably safely add soil around the rootball, while for the Coconut I will probably peel off whatever soil that is easy to remove. And if I started out with 50% vermiculite (or even 30% sand), much of the organic part is probably gone, and the roots add to the texture just like the mineral does, so the crowded Lipstick soil maybe has 80% draining material, so maybe even good to add a bit of potting mix on top (so-called "top dressing") and leach a bit, to get some humus or clay or such in there.

When I asked a retired associate professor focused on soil about these thoughts, he said (A) that one obvious way to check if drainage is good enough is to see how fast water level goes down after watering - I'm not sure what is normal but it sounded like a few secs is ideal but even a minute is fine. (B) If peat moss (and other organic material) breaks down, he said probably not all becomes "humus" that is leached out but that some threads (that can be part of soggyness) also might remain, and that also roots naturally die all the time and then break down. (C) He said he as a rule exchanges soil every few years, mainly because he sees a lot of chemical residue on it. However he is not into palms (but pretty much everything else).

But regarding chemical residue and leaching I have some input myself: One is that I used to know nothing at all about plants & soil until 3 years ago when I made a major effort to learn stuff (even wrote a book about potting soil, which anyone is welcome to have as PDF if you know Swedish, it is 49 pages based on 159 references, of which 95% are academic or professional growers, my text above mainly stems from that). I had a Kentia since several years which I was very proud I managed to keep alive and thought I had a major green thumb, only afterwards did I realize Kentia is the most forgiving palm and the reason it never gained height in those years was because of my poor treatment, mainly a combination of over- and underwatering and no leaching. Since then I've really been spoiling this and my other plants and of all actions I believe the leaching is the number one thing that made a major difference. Two weeks ago I had to give it away to a local public science center (Universeum) since it has outgrown my apartment:

5969d7da9f98f_KentiaGrowth3.thumb.JPG.92

I have been trying out different things since 2014 and the autumn of 2016 I thought I was just about finished trying and that I could kick back and relax, but then I decided to try the two things I've been contemplating that are on the verge of "too much effort", out of curiosity to see if the leaves would be perfectly green with no brown tips at all etc, and that was watering with reverse osmosis filtered water and regulating the humidity to between 45 and 55% plus a boost around 60-65% from morning to afternoon. And I'm thinking right now that the RO water maybe had very little effect, since my plants start to show signs I should have leached them already. So I suggest leaching is very important, and RO water not having much impact. I used to leach 2-3 times per year and use water volume 2-3 times the soil volume depending on how sensitive the plant is, maybe I should even increase to 3-4 times per year.

Anyway, those are my thoughts. It would be nice to hear about your experiences.

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Even a bad mix in a small pot could be better to the palm than the best mix in a pot that is too large, esp. in our latitudes. A small pot size allows a better control of watering and helps avoiding the reduction of NO3 to NO2. 

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I determined drainage for a number of mixes by measuring time for water to drop by 1 cm after filling pot (which is saturated at start of test):

TableEng.png.12f9eb57280f66f2e458e889e1e

Components:
• "Rölunda" is  Rölunda såjord, a mix of 45% light sphagnum peat moss, 45% sphagnum peat humus, 10% fine sand, inteded for seedlings
• "P-jord" is Hasselfors P-jord, a mix of 65% light sphagnum peat moss (H2-H4), 30% sphagnum peat humus (H6-H8) and 5% sand, fraction 0-35mm
• "Solmull" is 100% light sphagnum peat moss, 0-35mm fraction
• Sand is 2-4mm.
• Vermiculite: "S" means sifted >1mm, "NS" not sifted but I avoided dust.

Other notes:
• Loose means hardly compressed at all (like I might do with a tender indoors seedling).
• Dense means compacted as much as I would see fit for an indoors 1 meter palm to be stable.
• Tests A & B were with 6 months old soil mix. Test M through Q same soil type but new.
• Hard to see water level when vermiculite.
• Note 1: I also tried "very high compression" meaning compactness more than I would ever choose, result: 20s. 
• Test "R" is with the soil mix I now intend to use (2 grades of vermiculite since that's what I happened to have..)

Tried to keep it pretty "scientific" as far as kitchen experiments go.. Pot h=10, D=7-11cm. Pics of setup and pot used:

IMG_0021rr.thumb.jpg.46420e1a9e228b25e7d597cb53b8e645_Krukaitest-rc.jpg.3bba1c70

A few interesting observations:

  1. If one uses a pure peat mix (and 99% of what is sold in Sweden are peat mixes similar to Rölunda and P-jord) then one should keep it as loose as possible! This could be fine for small, short-lived plants but not for big palms really. Still, I have never seen anyone point out how important it is to not compact the soil (unless the plant doesn't mind poor drainage). Spanner does mention to keep it loose above, one of few. I can point at many who even say one should press firmly at the soil (1 2 3 4), although usually in the US where they might have less peat moss or at least lighter.
  2. I've often seen recommendations to add more mineral the more drainage one wants, e g 5%, 10%, 20%, 30%. As can be seen, sand (or perlite or vermiculite or any other mineral) up to 30% does usually not improve drainage, especially when compacted. They key is to make sure the amount of the finest material (such as clay and humus) is not so much that it filles the pores between the coarse grains. I've seen this pointed out clearly only in the pic below. Only way to be sure is to test, I think.
  3. Seems like a good idea to avoid all soil mixes in Sweden and go for the only available one with just light sphagnum peat moss, the one called "Solmull" which existed since 1961 and seems to be very impopular.

I'm not sure how important drainage is to palms, since in the best book I know about soil from an academic point of view, "Media and Mixes for Container-Grown Plants" by A C Bunt from 1988, he says rule-of-thumb is 10-20% of the volume should be air after watering but that among the meast demanding are palms and roses who only require 2-5%. Well even though the book is stuffed with highly interesting, scientifically verified information, maybe this is on the outskirts of what the book covers. In this and other forums, many advocate a highly draining mix. I think it might make sense, so I will now test to swap soil (more or less, depending on present individual situation). I also don't know what is to be considered "sufficient drainage". But I've noted that some soils drain in a matter of seconds. An associate professor focused on soil seemed to think 1 minute to drain is not too much in general. He knows about most plant types but is not focused on palms though. He also quoted one of his experiments where Sun flower was grown in 2 mixes: One cheap (low drainage, little structure in soil) and one with "more expensive" soil, and they both did well (I think actually the one in crappy soil did a bit better). This indicates roots were doing fine.

So far I have been using "Rölunda" adding 20 to 40% sand size 2-3mm. As can bee seen above, the addition of sand has little effect, waste of time, and explains why drainage is about a minute for most of my palms. With some exceptions! One is the Lipstick Palm, even when soil is saturated it drains at one second per centimeter! It has completely filled the pot with roots (I can feel it) which I believe is the explanation for this. Only 2 mm sticking out the bottom though. Another example is my Jatropha seedlings, or in fact all seedlings, reason being I kept the soil very loose.

BTW reason for using 10% sand (not just 50% peat moss 50% vermiculite) is that I am a bit suspiscious about how well vermiculite will remain undamaged, I hope that some sand will help it from being too much compressed in time, and also that by the time a big amount of roots cause increased pressure, the roots themselves will also add to the structure and cause better drainage, as with my Lipstick Palm.

And I found this great tip on how to check if your vermiculite is from one of those mines with alkaline material. Really easy: Just throw some in acetic acid or vinegar (or any stronger acid). If bubbles, it is alkaline (in which case I would buy from somewhere else).

Pic from Iowa Agriculture and Home Economics Experiment Station Special Report No. 99, 1997, Plant Growth Chamber Handbook, Chapter 7 - Plant Culture in Solid Media, by Prof L A Spomer et al:

Iowa1.png.1de201ab74457b860411bf3a9ffac8

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On the other hand, my favorite professionals, Floribunda Palms, use simply black lava rock and Nutricote!

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Interesting. The nutricote would take care of the CEC problem I suppose. But what grain size do they use? Size 1 to 4 (even 8) mm would give perfect drainage but nothing to retain water so you would have to water all the time it seems. Maybe they have a large part of fine grain then? Has to be pore size < about 0.3mm to hold much water. Seems equivalent to using perlite or LECA etc, or sand (though heavy), or seramis or hydroton, which might even let you omit the nutricote due to high CEC (don't know CEC for lava rocks). I also like how my cheap moisture meter works with my mix, I would feel insecure when to water with all-mineral or other very well draining mixes, so I'd end up watering every day and then not dare leave my home.. :-/

Most recommendations on soil mixes contain a big part of organic material, and usually a big part inorganic (e g this paper from Univ of California which recommends about 50% of each, as does the World Bank Technical Paper No. 264 "Organic and compost-based growing media for tree seedling nurseries" by J.H. Miller & N. Jones 1995), and there is a list of common potting mixes around the world of 1988 in "Media and Mixes for Container-Grown Plants" by A C Bunt, they all contain organic material, mostly peat moss, some even 100% organic.

It should be possible to grow in all-mineral mix with slow-release fert but one downside is that microorganisms of organic soil have a very active part, like extending reach of root system, counteracting disease, break down fertilizer, create hormones etc. I guess there are reasons why people (not the least palm growers) use organic part in soil mix, but it would be interesting to hear more about how it goes long-term with 100% mineral "soil".

BTW I just got some info from Seramis on the qualities for their "Seramis für Zimmerpflanzen", especially compared to vermiculite: It consists of approx. 70% 2.0 - 4,0 mm, 20% > 4.0 mm and 10% from 0.5-2,0 mm large clay granules. It does not compress (like vermiculite does a bit) and vermiculite is significantly lighter. They have comparable CEC, but the water absorption of vermiculite is much higher than of Seramis. Sounds to me I could have used e g 50% light peat moss, 25% vermiculite and 25% Seramis instead of what I did, or even exchange all vermiculite and sand for Seramis but I like the water holding capacity of vermiculite, which is comparable to peat moss, i e nothing holds more water. The way I understand it, with peat moss + vermiculite as I use it, it will hold as much water as if pure peat, but still very good drainage and aeration. I used to have coarse sand, and vermiculite gives the same drainage (due to grain size) but inside every grain is stored accessible water. So even though it gets really wet, it should be virtually impossible to overwater, I think. It will also be the base for my "self-watering pot" tests, I don't think all-mineral soil has enough "capillary suction" to work for that, but I haven't tried, depends on grain size.

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On 8/1/2017, 3:13:36, David_Sweden said:

Interesting. The nutricote would take care of the CEC problem I suppose. But what grain size do they use? Size 1 to 4 (even 8) mm would give perfect drainage but nothing to retain water so you would have to water all the time it seems. Maybe they have a large part of fine grain then? Has to be pore size < about 0.3mm to hold much water. Seems equivalent to using perlite or LECA etc, or sand (though heavy), or seramis or hydroton, which might even let you omit the nutricote due to high CEC (don't know CEC for lava rocks). I also like how my cheap moisture meter works with my mix, I would feel insecure when to water with all-mineral or other very well draining mixes, so I'd end up watering every day and then not dare leave my home.. :-/

Most recommendations on soil mixes contain a big part of organic material, and usually a big part inorganic (e g this paper from Univ of California which recommends about 50% of each, as does the World Bank Technical Paper No. 264 "Organic and compost-based growing media for tree seedling nurseries" by J.H. Miller & N. Jones 1995), and there is a list of common potting mixes around the world of 1988 in "Media and Mixes for Container-Grown Plants" by A C Bunt, they all contain organic material, mostly peat moss, some even 100% organic.

It should be possible to grow in all-mineral mix with slow-release fert but one downside is that microorganisms of organic soil have a very active part, like extending reach of root system, counteracting disease, break down fertilizer, create hormones etc. I guess there are reasons why people (not the least palm growers) use organic part in soil mix, but it would be interesting to hear more about how it goes long-term with 100% mineral "soil".

BTW I just got some info from Seramis on the qualities for their "Seramis für Zimmerpflanzen", especially compared to vermiculite: It consists of approx. 70% 2.0 - 4,0 mm, 20% > 4.0 mm and 10% from 0.5-2,0 mm large clay granules. It does not compress (like vermiculite does a bit) and vermiculite is significantly lighter. They have comparable CEC, but the water absorption of vermiculite is much higher than of Seramis. Sounds to me I could have used e g 50% light peat moss, 25% vermiculite and 25% Seramis instead of what I did, or even exchange all vermiculite and sand for Seramis but I like the water holding capacity of vermiculite, which is comparable to peat moss, i e nothing holds more water. The way I understand it, with peat moss + vermiculite as I use it, it will hold as much water as if pure peat, but still very good drainage and aeration. I used to have coarse sand, and vermiculite gives the same drainage (due to grain size) but inside every grain is stored accessible water. So even though it gets really wet, it should be virtually impossible to overwater, I think. It will also be the base for my "self-watering pot" tests, I don't think all-mineral soil has enough "capillary suction" to work for that, but I haven't tried, depends on grain size.

I would say the size of the black lava stone Floribunda uses is about 1 cm. However, I'll double-check, when I return home tonight, to be certain.

I purchased my Johannesteijmannia altifrons from Steve of Exotic Palms, another highly respected palm grower and distributor in the U.S., and he had it potted up in simply hydroton and said that is the way to go with that palm, with the suggestion of keeping its pot in a dish of shallow water. Skeptically I went ahead and did that, and it's thriving to this day! *knock on wood*  I've since done the same thing with my Johannesteijsmannia magnifica. I try to remember to change the water in the dishes a couple times a week.

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On 8/1/2017, 3:13:36, David_Sweden said:

It should be possible to grow in all-mineral mix with slow-release fert but one downside is that microorganisms of organic soil have a very active part, like extending reach of root system, counteracting disease, break down fertilizer, create hormones etc. I guess there are reasons why people (not the least palm growers) use organic part in soil mix, but it would be interesting to hear more about how it goes long-term with 100% mineral "soil".

Oooo I didn't even think about that. Okay, well I'll admit that I'm kind of still a skeptic of all-mineral, and I do add a pinch of potting soil here and there to my Floribunda plants, and even to my hydroton-grown plants. This must help in the long-term???

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Well I'm not an expert, just interested in the subject and read a bunch of books (perhaps enough to get confused on a higher level..), but I'd say adding an even small amount of any soil that contains very fine grains (like most potting soil does) to any coarse soil (like the black lava stone probably is?) could clog the pores to that drainage becomes very bad (see pic by Spomer above). If this happens, you will note that it takes forever for water level to drop after watering. As I said I'm not sure how important drainage actually is, and if e g water going down by 1cm/sek is sufficient or not, the general books I've read suggest it is fine, but after having swapped most of my soil I agree Pal Meir and other who recommend highly draining mixes might have a point because it looked rather dense, roots seemed to have a hard time getting through it (for most plants, not all, and only if compressed), maybe due to low oxygen level as well as high physical resistance to penetration. I have a Jatropha Podagrica in a small pot that's been in the same peat soil mix for probably 10 years and doing fine, I decided to fill the gaps in the soil at the top by ordinary potting mix, after that drainage is hopelessly slow.. So perhaps one should be careful to add fine grained soil, maybe do it a bit if drainage is very quick, then check drainage, and stop if it is getting slow. But it should increase water holding capacity a bit. I read people often do "top dressing" of soil for old palms which they no longer want to repot, i e replacing the top centimeters with (fine) soil mix. That's what I did with my Jatropha basically.. Could be a good idea if drainage is great, could be a bad idea if drainage already a bit slow.

One centimeter sized rocks? Are you sure that is not just the top layer? I think lava rocks are popular mainly to cover the top layer, what you English speaking call "mulching" I think (people also use coarse bark, leaves, peat, sand, pieces of wood, old newspapers..), for protection and decoration mainly I think.

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Im not an expert on this matter at all but I think this might all depend on your growing conditions, rainfall etc. Growing in nothing but lava stone is probably fantastic where you have heavy reliable constant rain. I noticed in some countries like very wet parts of Thailand and China they use nothing but chopped up coconut husks and fert, or dried pond clay chopped into inch cubes and fert. On Bali some use nothing but rice husk charcoal to grow everything. There is a heavy reliance on constant and correct feeding, with ferts so complete that they would probably also work in hydroponic systems. There is huge wastage of course as it drains straight out too.

If  you took the same palms and seedlings to another country you would have to amend that according to your conditions, right? 

Im not so fond of those minimal potting mixes like plain coconut husk cubes because it seems to me anyway that root initiation and development is also minimal. The plants need to move quickly or be planted out soon because too long and the new roots have nowhere to go, often ending up burnt by ferts and still ending up a soggy mess. It does reduce rot in those climates where there is also no cover. However for me I like a mix of real earth in there, though then again for transport there is nothing nicer than the almost weightlessness of coconut husk in a pot, with the added headache off course of worrying about drought at the roots!

I wish someone would invent the perfect non degradable (but recyclable), porous, none glog-upable cubes of something that could mix into soils that would keep them aerated, well drained and irrigated for the life of the plant in the pot. Or maybe more realistically one part that hydrates and the other that aerates. Maybe like those plastic things they use in Koi filter reservoirs except not plastic.

I once bough a few sacks of Hydrocell in Australia......this apparently aerates and saves water etc etc. This fact worried me. When I took it out the bag and soaked it I was even more worried. Its the strangest stuff, half powder half chunks and feels like a very very fine polystyrene gone off. Decided not to use it, just looked like trouble but apparently it's excellent if you live in dry climates.;) Though how much aerating it does Im not so sure cause if its dried out your plants are thirsty if its saturated they are drowning. Im sure it works in very specific but vast areas of Australia, though.

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1 hour ago, Rose said:

I wish someone would invent the perfect non degradable (but recyclable), porous, none glog-upable cubes of something that could mix into soils that would keep them aerated, well drained and irrigated for the life of the plant in the pot. Or maybe more realistically one part that hydrates and the other that aerates. Maybe like those plastic things they use in Koi filter reservoirs except not plastic.

I think peat moss, vermiculite and Seramis are pretty amazing for improving soil in pots. Imagine coming up with the idea to take some stone and heat it like popcorn and then use it in soil.. :mrlooney: Perhaps don't fulfill all your wishes (like maybe need e g sand to make it last longer and compress less).

You're probably right about rain etc but my pots are indoors :winkie:

Regarding downsides with very coarse media, especially (sharp) rocks, my understanding is that for seedlings you always want very fine soil that the roots can easily penetrate, so maybe not lava rocks for seedlings, I think they might both break (they can be delicate) and grow slow. And another thing might be that they might harm roots if you move the pot (not only seedlings). But sure, if it rains all the time, then maybe a good choice.

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On 8/2/2017, 10:36:48, David_Sweden said:

Well I'm not an expert, just interested in the subject and read a bunch of books (perhaps enough to get confused on a higher level..), but I'd say adding an even small amount of any soil that contains very fine grains (like most potting soil does) to any coarse soil (like the black lava stone probably is?) could clog the pores to that drainage becomes very bad (see pic by Spomer above). If this happens, you will note that it takes forever for water level to drop after watering. As I said I'm not sure how important drainage actually is, and if e g water going down by 1cm/sek is sufficient or not, the general books I've read suggest it is fine, but after having swapped most of my soil I agree Pal Meir and other who recommend highly draining mixes might have a point because it looked rather dense, roots seemed to have a hard time getting through it (for most plants, not all, and only if compressed), maybe due to low oxygen level as well as high physical resistance to penetration. I have a Jatropha Podagrica in a small pot that's been in the same peat soil mix for probably 10 years and doing fine, I decided to fill the gaps in the soil at the top by ordinary potting mix, after that drainage is hopelessly slow.. So perhaps one should be careful to add fine grained soil, maybe do it a bit if drainage is very quick, then check drainage, and stop if it is getting slow. But it should increase water holding capacity a bit. I read people often do "top dressing" of soil for old palms which they no longer want to repot, i e replacing the top centimeters with (fine) soil mix. That's what I did with my Jatropha basically.. Could be a good idea if drainage is great, could be a bad idea if drainage already a bit slow.

One centimeter sized rocks? Are you sure that is not just the top layer? I think lava rocks are popular mainly to cover the top layer, what you English speaking call "mulching" I think (people also use coarse bark, leaves, peat, sand, pieces of wood, old newspapers..), for protection and decoration mainly I think.

Correct, 1 cm sized black lava rock. That's all it is. I was surprised when I saw it as well! Not just a top dressing to the pot. 

@sashaeffer Is my buddy and a fellow-Floribunda junkie! Scott, can you chime in on Floribunda's use of the 1 cm (am I correct on the sizing?) black lava rock-only as their potting media? Maybe @PalmatierMeg would also be able to share some insight on this?

This is such an interesting discussion. 

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So many different palms require different types of drainage it can get mind numbing. I like buying palms in their pots with soil they have been used to but still a challenge to try and match proper amount of watering along with environment.

With the pumice that comes with floribunda palms can still be over watered.....I've done it. Moisture meter is useless in pumice, or sandy soils as it's not dense enough to get a proper reading.

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Yep that's one reason I like peat moss and also peat moss with vermiculite - the cheap moisture meters work then. I wouldn't want to be without it, just sticking your finger in says nothing compared to that. Even does some aerating like a mechanical worm ;-) I usually reuse the old holes though and don't use it more than I feel I have to, maybe 1-2 times per week.

A note regarding time for drainage: It seems one can get slow drainage when the soil is saturated with water even with well draining soil, if it is root bound enough at the bottom to obstruct the drainage holes. I just noted this. If to check drainage time for plant in soil then maybe best to check time for water level to go down for almost dry soil, before water starts to come out.

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On 7/31/2017, 2:43:50, Missi said:

On the other hand, my favorite professionals, Floribunda Palms, use simply black lava rock and Nutricote!

Hey all, just following up here. I spoke with Jeff at Floribunda last night, and the reason they use just the black lava rock is because they can use only one ingredient in their potting media to keep things sterile - they're certified to ship to CA, international, etc. Makes sense now!

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Warning for nerdy post! I got strangely long drainage times for some plants even though I checked thoroughly what to expect, see table above. First I thought my test above differed because the pot is so small so I tried a bigger pot. Similar result, i e does not depend much on pot size. But I noticed the time increased a bit for each watering. So I decided to test repeated watering for a number of mixes:

599838299e3ba_Drainageexperiment.jpg.ffa

I packed soil "densely" as above, and watered calmly when not dripping, and counted time from when soil fully saturated.

Obviously the best choice would be 100% plain "Solmull" (100% light peat moss)! Drainage gets worse when adding sand size 2-4mm to Solmull, which is very interesting sine everyone always say to add sand, vermiculite etc for improved drainage.

Worst of all is plain "Rölunda" (a mix of light peat moss and peat humus, see above); for this, adding sand helps a lot even at 30% (even though it seems to get worse at first). This is what I used to have for a lot of plants. Switching to the mix with 50/40/10% Solmull/vermiculite/sand was intended to improved drainage. It looks like I was better off before. I hope the lower weight makes it a bit easier for roots to penetrate..

There are some weirdly big variations for the mix with vermiculite, I can only imagine it is due to vermiculite falling apart and rearranging. They way I understand it, vermiculite size 2-4mm and sand size 2-4mm should have exactly the same drainage behaviour, since drainage depends on size of grains. But vermiculite is clearly worse. I'm thinking it could be due to either it falling apart or being soft, or both. Perhaps it is a bad idea to rely on vermiculite for improving drainage. Perhaps one should never use vermiculite. At least not with peat moss. Not even sifted. Still vermiculite has been common since the 50s, especially with peat. Perhaps they had a bit coarser peat, and/or more loosely packed mix?

Solmull is the only 100% light peat moss available in Sweden. Maybe it is a bit fine textured, or maybe it mixes with vermiculite (and sand) in an unfortunate and unpredictable way.

I'm thinking the pic by Spomer above is a good attempt at explaining how there must be so few of the fine particles that pores between big particles don't get clogged. However, is it really even that simple? One thing that might have an impact is shape of grains - coarse peat moss and sand both give drainage but maybe their shapes fit together too well? And another thing: Imagine a mix where 50% of the big pores are filled by fine particles, throughout. But what's to keep the fine particles from moving down, so that the bottom half has 100% filled pores and the top half 0%? (However when I look at the soil after the experiments it looks more like fine particles being on top.)

Sure, using pure Solmull would give poor soil when it starts to break down. But all organic soil breaks down - hard to say when but you can probably expect over 2 years for good peat (and maybe 3 years for coir and 4 years for bark, and then it breaks down fast). If you plan to repot within 2 years, any of the choices should suffice. Unless you plan to leave the old soil in the rootball, in which case it doesn't matter much which organic soil you chose, but could be good if a large part is coarse mineral.

I have read a number of books and articles about soil and to me it is weird how I've never come across anyone investigating or even discussing the intrinsics and dynamics of soil mixes regarding drainage. Maybe because researchers and pros focus a lot on outdoors plants and short-lived small potted plants.

I don't know if 1-4 minutes per cm is to be considered poor drainage but is is worse than I expected. I'm not really happy with any of the alternatives here, except maybe pure Solmull. Whichever mix one plans to use, I wouldn't rely on anything but a test. I also think it is weird how I never see anyone performing drainage rate tests such as mine - it is so simple, and reveals the truth perfectly.

I'm also thinking that possibly good drainage doesn't necessarily equate good aeration. I'm thinking if, say, the bottom 20% is clogged by fine particles in all pores, but the top 80% free from fine particles. Air comes mostly from the top surface, so the top 80% would be well aerated. At the bottom there probably soon are many big roots which suck up the water quickly and thus improves drainage by removing water. Or the plant could avoid the bottom 20%. I read it is common trees in parks have very superfical roots because of very dense soil due to people walking there a lot.

IMG_0018.thumb.jpg.f3d0f204fd935616371f0

Edited by David_Sweden
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