cactus soil won't drain!

gvbraunJuly 24, 2014


I've been gardening indoors and out for 25 years, and I'm really stumped on this one so thought I'd see if anyone else has had the same experience.

I've branched out to cacti and am growing some from seed. I'd been told that commercial cactus soil often contains too much peat, so I mixed up some soil based on recipes from the internet: 20% commercial cactus soil, 40% perlite, 20% limestone screenings passed through 1/8" mesh, and 20% screened compost/worm castings. After mixing the damp soil was crumbly and wouldn't hold together.

I sterilized the soil in a 450 deg oven for an hour, then moistened it, put it in an 8" clay pots and ran about a gallon of water through it, then planted my seedlings.

My problem is, every time I water it just pools on the surface. After about 15 minutes a centimetre of water will be absorbed. Even when the soil is damp it still takes forever to drain, much longer than my "regular" container soils.

What did I do wrong?! I wonder if sterilizing by heating made the soil super-hydrophobic..I just can't imagine how a mix with so much perlite and gravel can be so poorly draining....


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I do not have much experience wih cactus or potted plants in general - these are in pots, right? However, I do enjoy starting vegetables and cuttings - so this is my take:

The fine portion of the mix seems way too high to me.

Twenty percent of the mix is castings/compost at 1/8 minus. Twenty percent more is limestone at 1/8 minus. - minus meaning pretty much down to nothing. Your limestone may have been washed, but that may still be too fine for your planting.

The cactus mix that you used at twenty more percent probably already had too much fine material. - which is what you were trying to avoid.

The perlite (that is 40 percent of your mix)also might have been too fine - bags that I have bought vary in quality. It may be worth it to screen some ingredients so that they have a minimum size- especially if they are supposed to help with aeration. So, for example, you could screen through a 1/4 inch mesh and then wash the dust through a window sccreen to get the proper material for your mix (the remainder would be good filler for less important mixes.)

The compost and castings may be better used in your inground garden. They are probably not helping your mix. They are too fine, inhibit drainage, and in a container environment are not able to provide the best nutrition to your plant.

Check out the container gardening forum (link below) for more information (ask the same question.)

Best of luck!

Here is a link that might be useful:

    Bookmark   July 25, 2014 at 6:23AM
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tropical_thought(San Francisco)

Peat moss normally contains a wetting agent. This must be some sort of chemical, because peat sheds water. So, what if your peat did not have a wetting agent? I am not a huge fan of peat. I think sand would be a better choice for growing cactus, but it's hard or impossible to find the bags of horticultural sand that they used to sell. Peat is only for plants like ferns that like a lot of water.

    Bookmark   July 25, 2014 at 1:12PM
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Peat is only for plants like ferns that like a lot of water.

That's not at all true. Peat is the primary component of a lot of different potting mixes, for everything from conifers to annuals and perennials to veggies to houseplants and cacti. It's what you ADD to the mix that customizes it for various different plant types.

I do agree that uniformity of particle size is probably hampering your drainage as is the sterilization process. There is no need to sterilize this mix. Omit the worm castings and compost and focus on the permanent drainage enhancements. For most cacti types, I would use 25% prepared cactus mix, 25% coarse perlite or pumice, 25% coarse sand, granite grit or Turface and 25% bark fines or fine orchid bark.

    Bookmark   July 25, 2014 at 3:09PM
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tapla (mid-MI z5b-6a)

First, that a bag might be labeled 'Cactus Soil' is no clear indication what's inside the bag is appropriate for cacti. Soils with a significant fraction of peat that don't also contain a surfactant (wetting agent) can be expected to start becoming hydrophobic as moisture content approaches about 30%, and grow continually more hydrophobic as moisture content declines. That can be fixed to a significant degree by adding internally porous materials that don't exhibit hydrophobic tendencies, even when the soil is completely dry. Two materials that fit this description are calcined (fired at high temps) clays and DE (diatomaceous earth).

Even in a completely dry soil, calcined clay, like Turface, and calcined DE absorb water on contact. The water can then diffuse throughout the remainder of the soil volume, 'breaking' the hydrophobic tendencies of dry peat and bark. If you water as described, then wait several minutes (10-15) before watering again, the soil will readily absorb the second application. These materials The larger, internally porous particles) need to be in the soil at above threshold levels, though. That means there can't be a high enough volume of peat in the soil to fill all the air spaces between the larger particles of calcined clay or DE.

To a significant degree, your soil will determine what kind of opportunity you'll be able to offer your plants to reach their genetic potential. It plays a big part in your ability to put a good fertilizer supplementation program in play, in your ability to water correctly, and in how much margin for error you have in watering and fertilizing. In short, you can choose a soil you'll forever battle for control of your plant's vitality, or you can choose one that works FOR you, providing the strong foundation your container plantings need. No matter how you look at it, a healthy plant is impossible w/o a healthy root system, and the soil is the foundation of every conventional container planting. Getting it right pays big dividends. Soils that hold as little water as possible between soil particles and as much as possible inside of particles are much easier to grow and keep plants healthy in.

The information at the link provided below probably represents the largest step forward a container gardener can take at any one time.

What I use for cacti, succulents, houseplants, and all my woody plant material:


Here is a link that might be useful: Much more about soils if you click me.

    Bookmark   July 25, 2014 at 10:27PM
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tropical_thought(San Francisco)

That settles it. If peat without a wetting agent has to be really wet, then it's not good for cactus, even if they try to push it. I live to think, what are desert soil in which cactus do well like? I might do something like go collect some native desert soil, but cactus impossible in my climate anyway, but that is what I could do if I was living in Arizona.

    Bookmark   July 26, 2014 at 12:41AM
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tapla (mid-MI z5b-6a)

..... and that would be a serious mistake. If soils collected where the plant is found in situ worked in containers, we'd all be growing in topsoil or soils from our gardens/beds ..... but we know that doesn't work, except perhaps in rare cases where the grower is skilled enough to micro-manage the situation after making choices that are generally not wise.

In a very high % of cases, soils that occur naturally are much too fine to be good choices in pots.


    Bookmark   July 26, 2014 at 9:59AM
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tropical_thought(San Francisco)

I don't desert garden, but it is impossible to find horticultural sand any longer. There is probably no way to get sand anymore other then collecting it.

I would suggest large grained sand, naturally, maybe mixed with some fines. But, I can't even run that test because they don't sell sand anymore and just getting sand for somewhere may have a of sodium. I assume beach sand has a lot of salt from the ocean. I have access to tons of beach sand at ocean beach and I am sure it is all bad. They added tons of new sand to ocean beach to make it bigger.

    Bookmark   July 26, 2014 at 4:08PM
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tapla (mid-MI z5b-6a)

You can use poultry starter grit instead of horticultural sand, or cherrystone a size smaller than #2 ..... or Manna-Pro makes a grit smaller than poultry grit - but these grits, even though very coarse by what we consider 'sand standards', are too small to be used in judiciously fabricated soils. It's out there, you just need to be innovative in how you search and what you search for.

Good luck!


    Bookmark   July 26, 2014 at 10:10PM
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tropical_thought(San Francisco)

That is good to know something can substitute for large grained sand. I am a fan of large grained sand because my soil is fine grained sand, and I am not supposed to add clay as that make concrete.

    Bookmark   July 26, 2014 at 11:26PM
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nil13(z21 L.A., CA (Mt. Washington))

Too fine, way too much organic matter, and it was made hydrophobic in the over.

I use pumice with a little calcined clay (turface) and thats it for cacti.

And if you need to sterilize medium use a pressure cooker.

    Bookmark   July 27, 2014 at 2:53AM
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