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Water-Saving Containers, Part 2 – Secrets of Water-Saving Soil


Forget the movie. This is the secret.

For some reason, discussions of soil type are often missing from water conservation discussions. That’s a pity, because it really makes a difference.

I started out by trying to grow in containers that were filled with my local clay soil. When I got nothing but wizened and dying plants, I realized that didn’t work. So over the years, I’ve evolved a dirt system that does; I use it, with variations, for pretty much all the containers that I water year-round.

Basic soil: I don’t recommend just dumping some dirt into a container. I do use small amounts of local soil in the containers I plant: this adds some local mycorrhizae, minerals, and other things that plants like (mycorrhizae are kind of like yogurt culture; a little will spread. If you want to know more about mycorrhizae, click the link).

The rest of the soil is your basic potting soil. If I’m feeling lazy and rich, I buy the kinds that have amendments already added. If I’m feeling thrifty and self-sufficient, I get the basic kind and add amendments. The important thing to remember is that soil with lots of humus holds water. Soil with clay holds water, too, but it has no air in it and will turn into an impenetrable block if it dries. Sandy soil will let water run through before plant roots get a chance to drink it. A dose of compost helps everything; it’s a kind of soil-builder and amendment in one.

The usual amendments: Because commercial potting soils tend to lack minerals, I add a handful of azomite (a kind of super-rich-in-minerals rock dust from ancient sea beds) to each large container. Lately, I’ve become enamored of calcium, so now I throw in a small handful of dolomite or oyster shell lime. Acid-loving plants need either a commerical amendment or lots of used tea-leaves. (That’s black tea; others aren’t usually acidic enough.)

In containers where I’m growing plants for flowers and fruit, I put in fertilizers that encourage bloom, usually a pre-mixed organic fertilizer with mycorrhizae, which definitely up the fluffy lushness of my container plants. I sometimes vary these fertilizers depending on what I’m planting, but honestly I often just wind up using the same phosphorus-intensive fertilizer for everything. (Phosphorus is the middle number on fertilizers, and it makes roots and fruit – which also means flowers.)  For heavy feeders, like roses, foxgloves, and lilies, I may add some extra nitrogen fertilizer, such as alfalfa meal, but a lot of my plants get their nitrogen from the humus and foliar feeds alone.

The unusual amendments:  Besides the more-or-less ordinary amendments, I also add earthworms to every pot I own. Earthworms are little fertilizer-factories, taking minerals in and excreting them in a rich, nutritious form the plants can take up easily. They also aerate the soil – important in containers, where dirt tends to pack down hard. I rarely transplant my large containers, and I think the reason is that by adding some fresh soil on top every year, and keeping earthworms in it, the soil gets refreshed without the work of a big upheaval.

The big water-saving secret: The final soil secret is water-conserving polymers, such as TerraSorb. These come in the form of little crystals; you can hardly believe you paid so much for a tub of them, until you try the experiment: put a tablespoon of the crystals in a big mixing bowl and pour on the recommended amount of water. Then watch them swell and fill the bowl.  (The picture at the top of the post shows them lying on a plate.)

This is what the crystals do when they’re mixed in with your soil, gradually releasing water they’ve saved up. You mix them in the soil dry;  when you water, they swell up. It’s sort of an interior version of bottom-watering, keeping soil moisture levels even, and it’s especially good for water-hungry plants. I tend not to use the crystals for plants that like good drainage. On the other hand, if I have plants that like their feet in water, I create a layer entirely made of the crystals on the bottom, and then cover it with crystal-enriched soil.

Eventually the crystals disintegrate into a potassium-based substance that acts as a fertilizer. You can renew them by injecting them into the soil with something resembling a caulk gun (a pro gardener told me about this; I haven’t seen them. But you might be able to improv with a caulk gun). Or you can just dabble little holes with your hands and sprinkle more on top when you renew the soil; that’s the method I use.

Another version of this is mats, made of essentially the same material, that you put on the bottom of smaller pots and planters. I tried this in a seed flat once, but wasn’t as successful as I’d imagined.

Important point: This last water-saving secret won’t work unless you have the good, humusy soil. So all of you who thought you’d make a shortcut and just get the crystals: it won’t work. Sorry. The plants need the whole package.

Next post: keep water from evaporating with mulch – and a bonus idea.


In a way, my whole blog is about low-water gardening; that’s the reason I got involved with tulips, and I already loved natives and Mediterranean herbs. During June, my posts will all be about conserving water in the garden. This gives me scope to cover everything from containers to cityscapes, soil to site to sprays, and of course portraits of more of those stellar plants that spread their glories with little or no watering. (Hint: the “Wild Plants” category will give you quite a few more; so will the “Bulbs” category.)

{ 19 comments… add one }

  • Town Mouse June 15, 2009, 6:04 pm

    And here’s an interesting tidbid from a class I took on succulents in containers. Unless they’re huge plants, you can fill the bottom 2/3 of the container with whatever comes handy. “Trash” said the nice lady. Then fill the top with rather sandy soil. Water infrequently. Terracotta’s just fine ;->

    I’ll have to try those crystals, though. Sounds fascinating.

  • Pomona Belvedere June 15, 2009, 6:29 pm

    That’s a great tip, TM. I’d heard of filling bottoms of containers with lighter things than soil, but wasn’t sure how that would work. For succulents, it seems it would be great, and I like the idea of using trash!

  • Pomona Belvedere June 15, 2009, 6:36 pm

    Btw, I do use terra cotta for all plants that like it dry: Mediterranean bulbs and succulents, plus some Med herbs. It’s just plants that actually like water who suffer from terra cotta.

  • tina June 15, 2009, 7:47 pm

    Where can I get azomite? I mix my own potting mix using perlite, peat, and top soil. Sometimes compost and the water crystals, but need to add more fertilizer so I’m interested in this azomite. Fascinating throwing in earthworms.

  • Monica the Garden Faerie June 16, 2009, 3:55 am

    It’s also important to either add water to the polymers first or mix them witht he soil first and then add water before adding plant material… otherwise you MAY discover you added to much when the entire pot erupts the next morning. Hypothetically.

  • Cyd June 16, 2009, 6:29 am

    These posts have been so informative and interesting. I save coffee grounds and eggshells to add calcium. When I planted my tomatoes I added a handful of ground eggshells in the hole. My neighbor does huge annual pots every year and uses plastic bags filled w/ packing peanuts in the bottoms. This is really helpful when we go to move these beasts. I also collect earthworms from under my mosh pit (the organic leavings from dinners and such). I love those crystals, I will look for them.

  • Pomona Belvedere June 16, 2009, 12:48 pm

    Tina, I get azomite at Peaceful Valley Farm Supply, which is local, but ships all over the country. Buying a heavy bag of minerals means serious shipping though, so you might try googling it and see if you can find some nearer to where you live.

    Monica, theoretically you should be right about soaking the polymers first: my friend who told me about the caulk-gun injection method said that the first times landscapers tried it, they got worried calls about this strange, maybe hazardous substance erupting from their clients’ yards. In actual fact, I’ve tried mixing in polymers wet and polymers dry, and I now always do it the dry way. Why? Wet polymers are fragile and tend to break up as you mix the soil, which knocks a lot of time off their useful life. Mixing them in dry and watering never seems to make my pots explode – maybe because I leave a couple of inches to spare at the top to provide filler room. And you’re right, I should have mentioned that.

    Cyd, very useful tips, I didn’t know coffee was high in calcium (and yet another rationalization for coffee drinkers). Since I’ve become a calcium devotee recently, I’m going to think about working your suggestions into my garden. And we have yet another keep-your-pots-light tip (another reader emailed me with the suggestion of using trash for the same purpose!). I don’t think I’d remove any of the earth from some of my pots – roses and lilies and hollyhocks really need their root room – but there are many situations where the plants would do just fine with packing peanuts (or whatever) as filler.

  • Pomona Belvedere June 16, 2009, 12:50 pm

    Sorry Town Mouse: the trash tip was yours! Apologies: I’m dealing with multiple articles and multiple emails lately. And it’s a great tip.

  • Sylvia (England) June 17, 2009, 2:32 am

    Really interesting Pomona. I buy multi-purpose compost for all my summer pots and use crystals (and fertiliser pellets) then I just top it up with fresh compost for the winter. I have never thought of adding earthworms though I often find they have made there way into the longer-term pots for which I use John Innis Compost. I don’t use garden compost or earth because they are full of weeds. Our government is trying to wean us off peat but the substitutes are different to water – I find them harder to tell if the pots need water. With our last few wet summers it is so easy to overwater containers!

    Best wishes Sylvia (England)

  • Joy June 17, 2009, 2:37 am

    Hello there Pomona !
    Now this is truly interesting and I know I am going to learn great tips from you too ! Thank you : ) Joy

  • Pomona Belvedere June 17, 2009, 1:34 pm

    Sylvia, I was interested to hear your planting recipe. That’s a good reminder about the type of compost you use, as I can personally attest that regular garden compost will get you a fine crop of weeds in with whatever you planted.

    Joy, nice to have you here, and I very much enjoyed learning from you blog, too.

  • Alice Joyce June 17, 2009, 2:35 pm

    Just sent a few tweets your way. I can admit to having used the trash idea in planters. But per my tweet, the crystals didn’t work for me. I would need to start over to give them a proper test. Alice

  • Pomona Belvedere June 17, 2009, 6:50 pm

    As I tweeted to you, Alice, I’d be interested in knowing what went wrong with your crystal experiments (sounds very new-age, doesn’t it?). My mind is busily working away wondering if it’s climate differences (but I just got an email from Sylvia in England, who uses them, and hasn’t had problems even with wet summers) or…well, I’m just wondering what it might be.

  • Daffodil Planter June 18, 2009, 10:48 am

    Great post in a wonderful series PB! I do little container gardening and this is so helpful. I’m starting to wonder–just how many containers do you have? Sounds like acres worth.

  • Helen at Toronto Gardens June 19, 2009, 2:19 pm


    When our youngest was outgrowing diapers (she turns 16 on Monday), these gels were just hitting the horticultural scene. I had the bright idea to try using a diaper in the base of my window boxes. Unused, of course! Gels are what make these diapers so super-absorbent, and I figured anything made to go next to a baby’s skin would probably be okay for my browallias. I don’t think I’d use them for growing edibles. Anyway, it worked quite well.

    Once I’d used up my supply, I didn’t retry the experiment. However, if you have a lot of containers (I don’t), the cost of a pkg of Huggies or Pampers is a lot cheaper than some of the horti-gels, and could be shared with neighbours or friends.

    Not that neighbours can’t also be friends; or friends be neighbours.

  • Pomona Belvedere June 19, 2009, 4:28 pm

    DP, not quite acres, but they do creep up on me. It’s a way I can garden without getting out of hand on the water thing. And every year, I have to try just a few more plants – which means a few more pots – so it goes.

    Helen, great idea! I bet these would work just as well as the mats. Someone told me, in fact, that there’s a fire retardant based on the same gel; you put it on your house if wildfire’s near.

    It’s ideal if your neighbors are friends and v.v., but as neighbors go I’m happy to settle for good neighbors, period. (Where did that expression come from anyway?)

  • lostlandscape(James) June 20, 2009, 7:52 am

    I have some potted plants that I want to dry out a bit, so I haven’t bottom-watered them. With these the difference in potting mixes is beginning to become apparent. Some of the home-store inexpensive stuff (high in peat) leaches tea-colored ooze for months, while some of the higher-end stuff (one mix I like is Edna’s Best) lets water to flow virtually clear. The former kinds also dry to a pet brick that gets hard to water once it gets to that state, with the water finding a quick path around the rootball. I should try incorporating some of the crystals to see if they’d keep the bricks from forming, and if they do they should be easier to re-wet.

    As far as earthworms–Any pots I leave on my greenhouse floor get colonized by earthworms before long.

  • Nia October 24, 2015, 11:47 am

    Hi I have successfully grown toatmo plant from seed they are kept in my greenhouse as they are in large pots I do make sure they are watered well as they tend to dry out quickly and the toatmos are doing well. also feed every 4 days but I will be away on holiday would it be ok to bring the the plants outside the greenhouse thank you for advice and also for my toatmo club news emails

  • Sandu October 24, 2015, 12:23 pm

    Hi Brenda,I think they will be ok outside for a week or so. Keep them aganist a wall or the side of the greenhouse to protect them a little. You could spray with Bordeaux Mixture or Systhane if you you want to be sure to protect them from fungal disease, but a week should be ok if they are strong healthy plants.Regards,Nick

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