Recipe for a well-drained but rich container soil

bill585July 27, 2014

I am new to the garden forum and have spent a few weeks reading old forum messages and other web pages, trying to come up with a well-drained but nutrient-rich potting soil.

I have read Al's 5-1-1 soil treatise and many of the threads commenting on it, and about his gritty mix. I think I have a good handle on the basic concepts of a healthy soil.

I am growing vegetables (tomatoes, hot/sweet peppers, zucchini, okra, collards) in moderate to large size containers using MiracleGro container soil. I planted these in the spring, before I began thinking about devising my own soil mix. The plants are doing well considering I only get sun from 11 to 4 and that they are in pots. I also have a few varieties of young fig trees that are also growing in pots (some in MG, some in a soil mix from the seller).

While I see the value of Al's 5-1-1, I am looking for a bark-based mix that is well-drained but doesn't require constant fertilizing.

Here is my potting soil mix:

2 parts pine bark (fine, not composted)
1 part sphagnum peat
1 part manure/humus
1 part lava rock (1/8" to 1/2")
1 part pea gravel (1/16" to 3/16") or perlite (fine)
1-2 tbsp lime (dolomite, prilled)

* I'm going to add slow release fertilizer to this mix in the future.

I have repotted some perennials: asters, gaillardias and shasta daisys (all in about 10" pots), with this mix as a test to see their reaction. If they respond well I will use it on my fig trees and future vegetables. I repotted the flowers only yesterday so there hasn't been much time to see the results. I does seem to drain relatively quickly and hold some water.

I hope I haven't droned on here. Does anyone have a comment or advice on this mixture? A way to improve it? Like I said, a mix that drains well but provides nutrients and air without constant fertilizing.

Thank you,


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tropical_thought(San Francisco)

How did you get the humus? Did you make that yourself?

    Bookmark   July 27, 2014 at 10:41AM
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tropical_thought(San Francisco)

How much do you consider constant fertilizing? Its not like one has to do it each time you water. I don't know maybe once a month? Is that really too much work, compared to all the work over all involved, it seems like fertilizing is the least amount of work for me, compared to other tasks like pruning and removing older pieces of plant material, tending snails, watering, weeding etc...

    Bookmark   July 27, 2014 at 10:45AM
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The humus is GardenPro dehydrated manure/humus from Home Depot. When my compost bin gets ready I'll start to use that too.

I wasn't sure about all of this, but I guess I consider fertilizing once or twice a week to be constant and something I'd like to avoid. I would rather add slow-release fertilizer when making the soil and then fertilize once or twice a month with compost, manure or compost/manure tea. I don't know what is the best fertilizer for vegetables or how often to do it.

I read Al's document on fertilizing his 5-1-1 mix and he says it is best to fertilize every time you water, using a low concentration of water-soluble fertilizer.

I'm doing container gardening on a patio so I think tending the plants isn't as time consuming as a regular garden. There are hardly any weeds (maybe because I used MG potting soil) and I haven't had any slugs at all.

    Bookmark   July 27, 2014 at 1:59PM
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tropical_thought(San Francisco)

ok, in that case, yes, I do know some people like Jerry Baker may have said to use a something like Schultz Plant Food each time you water. I just can't say for sure, I don't grow food crops, but I almost never feed them anything due to composting a lot. Probably will be ok, not to feed them. I don't know about using humus as food because I am not sure how much nitrogen it contains and it would not be released slowly. You may be able to get away with adding it less or not at all. Just try and see if you like the results then carry on, if not then change. I wish I could say more, but I am not sure. Does not that humus you buy have a lot of sodium? What about using those little balls like Osmocote? I never use those any more, they are way too strong and you can end with nitrogen burn.

    Bookmark   July 27, 2014 at 4:55PM
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Just add a CRF to your mix (or AL's) when you mix it up or plant your containers. In most cases, a CRF like Osmocote will last the entire growing season, limiting the need for supplemental fertilizing to a minimum.

I will reiterate what Al and other experienced container gardeners (and that includes me) have said previously. You want to moderate the amount of organic matter you include in your potting mix - it continues to decompose and breakdown and adversely affect drainage and porosity. Second, while they contain some nutrients, compost and manure are NOT fertilizers and are unlikely to provide sufficient nutrient concentrations to be effective in that regard. A liquid fertilizer will do a much better job at this, especially if using only to supplement a CRF.

And Miracle Gro potting soil is widely regarded as being rather inferior, all things considered. Too uniform in particle size, too moisture retentive and too likely to develop drainage issues. The "rich" part of a container soil should always be supplied from outside, by the gardener, rather than from the soil itself.

    Bookmark   July 27, 2014 at 5:05PM
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tropical_thought(San Francisco)

I totally agree, compost is not good for containers. It is better not to have compost in them, but sometime if I transplant into a container from the soil and it has compost in that soil, compost ends up in the container. I can't say, it is always a bad thing, but if I was to grab up some compost and put in a container and stick in a plant, that plant would surely die right away.

    Bookmark   July 27, 2014 at 6:55PM
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So there is no container soil mix that uses a higher ratio of organic matter that could ever stack up to 5-1-1? Even if it is discarded after one growing season? 5-1-1 will undoubtedly yield more fruits and vegetables?

Some other questions I had:

Why do growers use compost or manure at all if their nutrient content is so low?

Is it true uncomposted pine bark will draw most of the available nitrogen from the soil so it should supplemented with a nitrogen fertilizer? Would a 3-1-2 ratio slow-release fertilizer be enough?

When adding dolomite lime to pine bark and/or sphagnum peat soil, how precise does the application need to be? Will adding 2 tbsp instead of 1 tbsp per gallon of soil have a negative effect?

Thanks for all your help

Thanks to you, too, tropical_thought for the info on compost

    Bookmark   July 27, 2014 at 10:39PM
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nil13(z21 L.A., CA (Mt. Washington))

"So there is no container soil mix that uses a higher ratio of organic matter that could ever stack up to 5-1-1? Even if it is discarded after one growing season? 5-1-1 will undoubtedly yield more fruits and vegetables?"

It depends on what you mean by stack up. 511 works great. A lot of commercial growers will use 311 for increased moisture retention. You might also see mixes of coposted bark with coarse sand added for incresed water retention. They have more time to pay attention to their plants and water can be a major expense. One of the most common hobbyist problem is overwatering. 511 helps with that. Commercial growers don'ttend to overwater because of the waste and also leaching waste. As you can see, the ideal mix depends on a lot of factors. It is the principles that matter.
Also, I discard primarily organic mixes each year regardless of ratio.
If you want to add more bark go ahead and do so, just pay closer attention to how much you are watering. If you want a richer mix, add more CRF. Iirc, al's recommendations are for 1#N/cuyd. Bump it up to 2 or 3 #s/cuyd. If you want to add fertilizer less often, use a 6mo CRF instead of a 3mo CRF. If you don't want to provide the roots an easy time of it my providing a dilute fertilizer solution every watering, bump the concentration from about 75ppm N to 150ppm N and fertilize every other time. If you don't want to dispose of all that organic matter every year, use just gravel and calcined clay. If weight is a concern use pumice and DE.

Adding a bunch of fine compost is just going to defeat the purpose of having all that coarse material. Why are you using lava rock and gravel or perlite? Lava rock is expensive compared to gravel or perlite.

    Bookmark   July 28, 2014 at 10:12PM
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I probably don't know enough about gardening to ask the right questions or to even know what I really want. Originally, what I really wanted was a homemade version of MG container soil - but something that drained better. I don't hate MG soil (yet), but I would like something more cost effective and I like the idea of mixing my own soils and being able to adjust the ingredients.

That being said, this spring I planted say twenty various vegetable seedlings from Home Depot into containers with MG container soil and they all immediately perked right up and are now large healthy plants. (The yellow squash were ruined by some beetle but the rest are ok.) To me this indicates MG is a legitimate soil. Thus, I thought I could home mix something approximate to MG, but make it drain better and be more root friendly. I also thought that with a more organic soil the plants would suffer less than if I used 5-1-1 and didn't adhere to its strict fertilizing schedule. I also bought a few small fig trees (1 to 2 feet high) and wanted to repot them into something other than MG but didn't want to screw them up with my own bad mix.

I think I misunderstood pine bark. Because it decomposes fairly slowly I viewed it as almost inorganic (maybe in its first few years?) - as not holding much water, but providing faster drainage and a few air pockets. Does it actually hold a substantial amount of water? I'm using a fine, uncomposted bark that seems a little stringy. I didn't screen it, but picked out the large pieces.

What is in 3-1-1? Where on the web do I find the water retention and porosity levels of different soil elements? Or do you know these offhand?

I used lava rocks because I read a few reviews raving about their effects. One where somebody dumped their unwanted lava rocks into their herb bed and it exploded with growth. It also seemed reasonable that their porous nature really would aerate the soil better than gravel or perlite. And I liked that it would hold considerably more water than gravel and was lighter. Also, my Home Depot has bags of pea gravel and lava rock each for about $4.00 while the perlite was, I think, $7 for a very small bag, finely ground.

Thanks for the information in your last reply.

    Bookmark   July 30, 2014 at 12:07AM
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You are asking some very good questions.......keep it up :-))

Organics in container gardening are highly overrated. It is really important to understand the concepts behind container growing and how radically they differ from inground gardening. The key elements are fast drainage and aeration.......once you have achieved those two primary factors, everything else can be massaged to your specific needs. Durability and textural content of the potting mix should be your focus - nutrient levels are addressed by fertilizing and watering is key but these will always be outside factors under the control of the gardener, not the soil mix.

The reasons significant percentages of organics in a potting mix are not advised are multiple. First, organics continue to decompose and breakdown, squeezing out pore space, reducing to a uniform and small particle size and then impacting the fast drainage required. Second, depending on source the moisture retentiveness may be excessive. Third, their nutrient content, if any, is low so it makes sense to evaluate exactly what they are providing in the way of benefits compared to what they may detract from the potting mix.

There will likely always be some percentage of 'organics' in a potting mix - without any there would be NO moisture retention! But it is important to consider the source - peat is already nearly fully decomposed so no further breakdown is expected - particle size won't change (but is small already). Both coir and bark fines (usually composted bark fines) are very slow to decompose and add both a degree of moisture retention as well as durability.

Why do growers use compost or manure at all if their nutrient content is so low?

Because this kind of organic matter is vital to inground growing conditions - not container growing. Organic matter adds the textural component that most garden soils lack, it lightens and adds aeration and it encourages a healthy soli biology. And it does have nutrient value although limited compared to a "fertilizer". That's why compost and other organic matter are considered soil amendments, not fertilizers. Their importance and relevance is directed at inground gardening, not container gardening

If you haven't already, I would strongly urge you to read this excellent and extremely long running post I've linked to from our recognized container gardening guru, Al (tapla). It pretty much explains it all :-))

Here is a link that might be useful: everythg you could possibly need to know about container soils.

    Bookmark   July 30, 2014 at 2:34PM
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nil13(z21 L.A., CA (Mt. Washington))

"There will likely always be some percentage of 'organics' in a potting mix - without any there would be NO moisture retention!"

This is not accurate. There is moisture retention in inorganic components. Straight turface will be far too wet for many plants. Ball clay is sometimes used by aquatic growers and you would be hard pressed to find an organic material that could even come close to retaining as much moisture as ball clay (zero air space though).Even plain sand will retain more moisture than people would guess (plants languish in pure sand because of insufficiet air space). You absolutely do not need an organic component in a container medium in order for it to retain moisture.

    Bookmark   July 30, 2014 at 10:31PM
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nil13(z21 L.A., CA (Mt. Washington))

Ok bill, your recipe is a bit of an 'everything but the kitchen sink' type recipe. I woud suggest simplifying everything and learning about how all the ingredients work. Designing your own container medium is about meeting your needs with the least expensive materials that are readily available. Sometimes your needs force you into more expensive materials like lava rock but unless you have a really good reason for using it, perlite is probably a cheaper and lighter way to increase drainage. There would be no reason to mix the two unless you needed a little extra bulk density (read that as increased weight) to prevent blow over from wind, but then gravel or sand would probably be a cheaper way to increase bulk density.

If you are ok with MG, I would recommend you start with a mix of 2 bark: 1 peat: 1 perlite with a good balanced CRF with micros added at 2-3#s N/cuyd. Then fertilizer every few weeks with a balanced fertilizer at the recommended rate (typically about 300ppm N) IMO, you will have too much moisture, not enough air space, and ypu will be leaching more nutrients than I would like but it would probably most closely replicate the experience you are used to. Sometimes people change too much and then blame poor results on the wrng factors.

Then you might try increasing the air space by increasing the bark component and seeing what effect that has on your plants. Or maybe you sub out finished compost or coir for peat. The problem with bagged premix is that you have no idea what the ratios are and so don't get a chance to figure out what the properties are of the individual components. Once you learn the basics, any exploration of oter materials will render a much clearer image for you.

    Bookmark   July 30, 2014 at 11:05PM
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Thanks to both of you for pointing me in the right direction. Today I checked the roots on the flowers that I had potted last week as a test of my mix and sure enough they were a soggy mess. I'll go ahead and use Al's 5-1-1 recipe and see what happens. From everything I've heard it will work well.

    Bookmark   July 30, 2014 at 11:54PM
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