Finally graduating to custom soil mix

[cefalophone]

I will be heading to the nursery soon to buy my ingredients but have some questiongs before I do. Would pumice and coir be a good mix? Should I add anything else? Does pumice come in different sizes? If it does what would be a good size for both planting mature cacti and starting seeds in?

[cactushobbyman]

Speaking of soil mix for the greenhouse, I use what works best for me. I do over water, bad habit. So my mix is a 5-2-1. Five parts DE, two parts chicken grit, and one part sifted potting soil. The size is what ever comes out of the bag. My greenhouse is 100 degrees today and will be there until late Oct. I know some will say not to water when it's that hot, but I just tell the cacti it's a long monsoon season.

[Sutremaine]

The pumice should be as large as you can get it while still being mixable with the coir. The finer coir will be holding the vast majority of the water, so the coarser (and less water-holding) the pumice is the drier the overall mix can potentially be. If you find the coarse pumice too dry, you can add more coir. If you get fine pumice and find it too wet, you're stuffed.

Is pumice and coir a good combination? Well, you can get numbers for that.

1. Take a container and fill it to the top with water. Record the volume of water required to do this (A).
2. Dump the water out of the container and fill it to the top with your mix, compressing it as much as you would if using it with a plant.
3. Fill the container with water until no more will go in, and give the whole thing several hours to soak, refilling the container with water as the mix takes it up. Record the volume of water required to do this (B).
4. Poke a hole in the bottom of the container and leave the water to drain for several hours. Record the volume of water that drains out (C).

A = Container Volume (amount of water required to fill air spaces in whole container).
B = Total Pore Volume (amount of water required to fill air spaces both between and within soil particles).
C = Aeration Pore Volume (amount of water required to fill air spaces within soil particles).

Total Porosity = B / A
Aeration Porosity = C / A
Water-Holding Porosity = Total Porosity - Aeration Porosity

Total Porosity is how much of the space in a dry pot of mix is air.
Aeration Porosity is how much of the space in a wet pot of mix is never water. This is dead space in the pot, be it from the inside of a grit chunk, the inaccessible pores inside a chunk of perlite, or the tiniest pores inside a chunk of calcined clay.
Water-Holding Porosity is how much of the space in a wet, drained pot of mix is water.

Remember that neither pumice nor coir contain any nutrients, so you'll have to provide those yourself right from the first watering. CRF is an option, but adding the fertiliser with the water gives you complete control over how much fertiliser is in the pot. You'll also need either a pure salt fertiliser, or a way of providing the microlife that breaks complex chemicals into the simpler ones that plants can get at. That's a rather complicated subject, and one I'm still figuring out. Look into hydroponic products, or Dyna-Gro Foliage Pro. (In the meantime I leave everything outside and hope there's enough activity in the pots to break down any complex components of whatever fertiliser I'm using. Seems to work with cacti and succulents, but I wouldn't try it with a tomato.)

Or you could just use dirt/compost/bagged soil to provide the minor nutrients and microlife.

[cefalophone]

Thanks for the info. I do not have a problem with water retention at the moment. But after seeing Steve's results with just pumice and dg I wanted to give it a try. I also want to attempt growing more difficult species.

I use miracle gro tomato fert at half strength. Will this be acceptable for a mineral mix?

[Steve Johnson]

I use miracle gro tomato fert at half strength. Will this be acceptable for a mineral mix?

Sorry, but I'm a complete dummy when it comes to all the ferts that are out there. So when people say half-strength, quarter-strength, etc. it doesn't tell me anything. Let me know what the NPK values are and how much you're diluting, then I can advise you on what you're doing.

[cefalophone]

The npk is 18-18-18 I believe. Full strength would be one tablespoon in a gallon of water so I use half a tablespoon.

[Steve Johnson]

The npk is 18-18-18 I believe. Full strength would be one tablespoon in a gallon of water so I use half a tablespoon.

Good as far as it goes, but I forgot to ask for a little more detail. The 3 sources for Nitrogen in fertilizers are Ammoniacal (Ammonium) N, Nitrate, and Urea. Any fert manufacturer worth its salt should give you the Nitrogen breakdown on the packaging. With that info we can work from there, although at first blush I think the Nitrogen may be way too high for cacti and succulents they way you're diluting it. Let me know what you find out.

[BarryRice]

@sutremaine---once we've measured the various porosities for our potting mix, what do we do with that information? Are there guidelines on what the porosities should be for different cacti?

[iann]

Your 18-18-18 fertiliser at 1/2 tablespoon per gallon is about 350ppm of each nutrient. That's quite a lot higher than I like. I wouldn't even use this much for occasional fertilising, let every time.

[Steve Johnson]

Your 18-18-18 fertiliser at 1/2 tablespoon per gallon is about 350ppm of each nutrient. That's quite a lot higher than I like. I wouldn't even use this much for occasional fertilising, let every time.

Using cephalophone's fert, I'd say on the top of my head that 1/4 tsp. per gallon of water should do it. 58 ppm -- sound about right?

[peterb]

Too much nitrogen, no matter the source. Over a short time, there might be dramatic benefits, but over the long haul, crazy things will start to happen with a lot of the cacti.

Also, I would entirely avoid coir if possible, as I have said before.

peterb

[Steve Johnson]

Too much nitrogen, no matter the source. Over a short time, there might be dramatic benefits, but over the long haul, crazy things will start to happen with a lot of the cacti.

Also, I would entirely avoid coir if possible, as I have said before.

peterb

Both excellent points. The only reason why I bring up Nitrogen sources is that Urea in ferts won't do much good, at least for desert cacti. I don't know the specifics on cephalophone's fert, but for the sake of discussion let's say that half of the Nitrogen is coming from Urea. Without the presence of bacterial activity in soil, that leaves her with an effective NPK ratio of 9-18-18. This throws the major nutrients out of balance, and regardless of dilution she'll be looking at a different set of problems. I really can't say anything further unless and until she's able to determine whether or not the amount of Urea in her fert should be a cause for concern.

[cefalophone]

Today I went to the california cactus center. I was a little overwhelmed trying to decide what to buy for my mix so I went with their premade soil mix. It is a lot fluffier than what I have been using and seems like a good mix for my echinopsis sp. but not for the species I want to add. The woman helping me said it was a mix of pumice and mostly grit but when I opened it upon getting home it looked like mostly compost. They said(according to the woman) there was no peat. So I believe it will make a good base. After some thought a straight mineral mix would not be ideal for me at this point. I am going to focus on the plants I have at the moment, and after at least two years give the harder species I want;navajoa, a shot. I will be going again tomorrow or later this week to buy some pumice and make a 75% pumice/25% their soil mix. My only complaint about their soil is the bark but I have no problem sifting it out.

I really like the look of Steve's plants and he is a great gardener, but I do not have the attention to detail he has. I neglect my plants a lot so for the moment I believe a slight compost/grit mix will be best for me. I like only having to fertilize my plants once a month. Aquaculture is a little bit beyond me for the time being. Although I understand and know how to do the concept I am a bit too lazy for it.

Here are the specifications of my fertilizer
Total N
1.4% amoniacal nitrogen
14% urea nitrogen
2.6% Nitrea
Available phosphorous 18%
Soluble Potash 21%
Magnesium total .50%
.50% water soluble magnesium
Copper .05%
.05% water soluble copper
Iron .10% chelated
Magnese .05%
zinc .05%

p.s I am a guy by the way

[Sutremaine]

@sutremaine---once we've measured the various porosities for our potting mix, what do we do with that information? Are there guidelines on what the porosities should be for different cacti?

I've been looking for numbers on that. For soils (for which the most non-advertising information is available), a total porosity of 50% is okay. That's for plain ground soil. A pot-sized chunk of soil in the ground may have a whole mountain to drain into, whereas that same pot-sized chunk dug up and put into a pot has only thin air to drain into. So you need to make the mix drain more freely to compensate. 60% to 75% starting air is given as good here. I don't really understand the point of giving an upper bound -- anything greatly exceeding the 75% figure is unlikely to hold adequate amounts of both air and water when saturated and drained, which would make the initial range restriction unecessary.

Once you've watered the pot fully and it's drained, you need water and air for the roots, and water also for you (a bucket of broken glass will contain enough water and air after watering, but you'll have to come back with the watering can sooner than you'd like...). I haven't been able to find any soil-based figures for water-holding porosity (also known as field capacity), but the link above gives 33% each of air, water, and solids as a target.

Might be interesting to start a topic and see how peoples' different mixes compare.

As for growing stuff in these finely-tuned mixes, I don't think any two species of terrestrial plant have different enough root biochemistry to need different porosities in order for their roots to function. There are a couple of cases I'd like to try for myself before recommending the same mix for everything. The first is fine roots transplanted from a fine mix, especially if those fine roots belong to a rot-prone species. They may not appreciate the coarser particles, or rather the amount of water required to keep their floating roots moist (a plant grown from the start in the coarse mix would naturally crawl its roots across the particle surfaces, where the water is). The second is tap roots belonging to species that just can't say no to water. I've grown parsnip-rooted cacti in a coarse, open mix kept well-watered, but never anything explodey like a Lophophora. I believe that if you deliberately kept pouring the water on, the top half would split but the bottom half would keep on trucking. I have test plants for both these cases, though I'm not going to go out of my way to overwater the Lophs.

[Steve Johnson]

Today I went to the california cactus center. I was a little overwhelmed trying to decide what to buy for my mix so I went with their premade soil mix. It is a lot fluffier than what I have been using and seems like a good mix for my echinopsis sp. but not for the species I want to add. The woman helping me said it was a mix of pumice and mostly grit but when I opened it upon getting home it looked like mostly compost. They said(according to the woman) there was no peat. So I believe it will make a good base. After some thought a straight mineral mix would not be ideal for me at this point. I am going to focus on the plants I have at the moment, and after at least two years give the harder species I want;navajoa, a shot. I will be going again tomorrow or later this week to buy some pumice and make a 75% pumice/25% their soil mix. My only complaint about their soil is the bark but I have no problem sifting it out.

Oops, sorry about that...

Anyway, I hate to bag on the CCC, because they're really nice people. However, really nice says nothing about whether or not their growing advice is good. I think the CCC mix is okay, but for the life of me I can't understand why they don't tell people that it needs to be mixed in with a lot more pumice for many desert cacti. Wish they gave me that info right up front. I wouldn't be thrilled about the organic material either, but you can just pick out the bigger chunks -- the 75/25 pumice/CCC mix should be fine.

Here are the specifications of my fertilizer
Total N
1.4% amoniacal nitrogen
14% urea nitrogen
2.6% Nitrea
Available phosphorous 18%
Soluble Potash 21%
Magnesium total .50%
.50% water soluble magnesium
Copper .05%
.05% water soluble copper
Iron .10% chelated
Magnese .05%

Great for growing tomatoes, but horribly out of balance for cacti and succulents, especially under pot culture. The roots of your cacti simply won't be able to use Urea as a Nitrogen source. The Ammoniacal N and Nitrate in your fert are all they'll get, so your effective NPK ratio will be 4-18-18. I've been using Dyna-Gro All Pro 7-7-7 for a year, and here's a nutrient profile your cacti can live by:

Dyna-Gro_7-7-7_specs.jpg (33.14 KiB) Viewed 2091 times

You can find it on Amazon here:

http://www.amazon.com/Dyna-Dyna-Gro-All ... B004A2BJ86" onclick="window.open(this.href);return false;

I know it sounds kinda expensive, but IMO worth every penny for the wonderful results I get. Besides, you'd be diluting just 1/2 tsp. per gallon of water, so a gallon of the liquid concentrate will last you for a nice, long time.