Looking for advise on engineered container soil

EibenJanuary 27, 2011

Hello All,

I have been lurking on this forum off an on for quite some time but this is my first post so take it easy on my. I am trying to come up with an engineered soil blend to be utilized on a large scale vegetated retaining wall project. These vegetated retaining walls are made from concrete and have an internal volume of about 1 cubic foot. The modules have drip irrigation incorporated in them and will be dosed with collected storm water run off.

I have read many of "Al's" posts on his soil mixes and have incorporated lots of his "philosophy" in what I am trying to accomplish.

Now I am sure some of you are probably thinking just mix up the soil to one of Al's recipes and call it a day, however there are a few requirements which i have which alter some of the ingredients.

I am trying to come up with a mix that is primarily composed of "waste stream" ingredients. I have access to lots and lots of reclaimed crushed red brick which i would like to use as the "turface" portion of the "gritty" mix. Red brick has a very favorable CEC and a slightly basic pH. I also have access to some pine bark fines, and different post consumer composts.

My questions start with the utilization of crushed granite. I understand that it has nice fractured edges to promote good drainage and slowly releases trace minerals over time, but what else is it doing? Why use it instead of just more turface (or in my position more crushed brick)? I am trying to avoid materials which have been mined or that have lots of embodied energy such as heat expanded/fractured materials such as turface and stalite unless they are waste stream.

The other thing about this project is the overall scale. We are talking about many many many of cubic yards (gulp) so the ingredients have to be as economical as possible. (I will be mixing up smaller test batches to get my mix right).

Plant selection is still a bit in the air but will be mostly hardy perennial flowers and grasses.

Thanks everyone!

I look forward to my future participation in this great forum!



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rhizo_1 (North AL) zone 7

Hi, Ben...what an interesting sounding project.

Pardon me for being a bit slow on the uptake this morning, but can you explain it again? I'm thrown off by the "1 cubic foot" measurement. Is this retaining wall to be built of small sections? And how long a life span to you require of this mix?

The granite is added to create porosity. The air spaces within a soil (in ground OR in a container) are as important as the solids. It's the pores that supply water, nutritional elements, oxygen, etc. Granite's eventual erosion into elemental particles is a non-issue in containers as potting mixes are changed out too often to see any appreciable results from the decomposition of granite.

I'm a little concerned about the use of brick. It seems to me that brick behaves quite differently when surrounded by moisture at all times. I'm not positive about this, but a little red flag has gone up in my brain. Fractured brick, especially, would be very porous and likely to absorb water and disintegrate. Wouldn't it?

    Bookmark   January 27, 2011 at 3:19PM
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tapla (mid-MI z5b-6a)

Haydite is essentially crushed brick & very stable, used in substrates. It's the same material that Espoma's Soil perfector was, or is, if they still package it.

Ben - is this by any chance the same project that Kristin Rhodes, who is associated with the International Design Clinic is working on?

The function of both the granite and Turface in the soil you're referring to are twofold. First, they are screened (or purchased pre-screened) to a particular size to eliminate the layer of saturated soil at the bottom of containers that accompanies heavier soils comprised of finer particles. Enhanced aeration and drainage are inherent in the larger particle size. Both products are incorporated (along with a small organic fraction) so that water retention can be adjusted to suit climate, growing styles, and plant material. Increasing the volume of Turface and decreasing that of granite increases water retention, with the reciprocal also being true. Keeping the bark fraction limited to no more than 1/3 of the total volume is what ensures the gritty mix's durability. With a large organic fraction, the soil can structurally collapse over time, but with the primary fraction being a combination of stable mineral products, collapse is virtually impossible.

I think you need to set some parameters to get the best help. It's important to know things like how long the soil needs to be serviceable, and if the plants will be rotated regularly. What kind of a nutritional program do you have planned - or does it revolve around an all organic approach or are soluble fertilizers/slow release/controlled release fertilizers optional.

Hydrologically speaking, are these containers, or small raised beds. This has a considerable impact on how you would go about putting together a substrate. Is water conservation a priority, or is the supply unlimited?


    Bookmark   January 27, 2011 at 4:31PM
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here is a link of a project using the modules. (maybe you can see it?) How does one post pictures on this site? Each module holds about a cubic foot of media for planting. The lifespan of this mix is indefinite so there needs to be a decent amount of permanent structural integrity to the mix. I have read that brick is fairly non-friable due to the high heat that goes into the production (much like turface). It does pick up and hold onto moisture/dissolved salts pretty readily. I have found a few scholarly articles on the properties of red bricks but do not have very much hands on experience with it. I know that it has been utilized in green roof applications in europe for quite some time.
If the granite is just used to aid in drainage than couldnt you just use more turface/red brick and less pine fines to get the equivalent moisture retention? I should have a few truckloads of brick next week and will start testing some mixes. I really appreciate any feedback!



    Bookmark   January 27, 2011 at 4:35PM
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Thanks Al!

I would consider the modules more like raised beds, as the backs are open to soil/drainage material. I maintain a good number of these systems and usually fertilize once or twice a year. I have used both nutricote 14.14.14 type 140 as well as all organic ferts by "Fertrell" that use poultry manure/feather meal/kelp meal etc.. These plants will be never changed out unless due to mortality. It is too costly to think about refreshing soils in an application like this. I am not sure if it is the same project as I am not involved beyond the aspects of making the plants grow.
The retaining wall will be utilized to evaporate/evapotranspire lots of collected water. The wall will be "trying" to use as much water as possible without visible seep or causing plant health concerns. The primary goal of the wall is to utilize site storm water to prevent run-off so the name of the game is to purge water more than to conserve water.


    Bookmark   January 27, 2011 at 5:01PM
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tapla (mid-MI z5b-6a)

"If the granite is just used to aid in drainage than couldn't you just use more turface/red brick and less pine fines to get the equivalent moisture retention?"

The granite shares equally with the other ingredients in promoting drainage by virtue of its size. If you replaced the granite with an equal volume of, say, calcined DE in the same size, water retention would increase dramatically, but drainage (flow-through rates) would remain unchanged. You'll have to tinker with the ingredients to get the water retention you need, and it appears that the primary considerations will be particle size and internal porosity of the particles.

Will you be back-filling as you lay up the wall? IOW lay a course & backfill ... repeat. How significant is the critical angle of repose of the substrate going to be, if at all?

Have you considered that w/o being able to regularly replenish any organic fraction of the soil that you'll soon have an entirely mineral substrate and will essentially be growing in the ground, but hydroponically? Is the wall constructed such that you'll be able to utilize mulch and/or other organic amendments?

Just trying to get a feel for what/how you're thinking.


    Bookmark   January 27, 2011 at 5:14PM
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tapla (mid-MI z5b-6a)

Our posts sort of crossed, but now I think I'm starting to understand better. It's safe then, to say that the assessment of growing hydroponically is pretty close to accurate; & there will be little to no opportunity to replace any lost (broken down) organic component(s)?


    Bookmark   January 27, 2011 at 5:19PM
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Al, you are correct about how the wall is constructed. The media inside each module is relatively flat once it has been placed in the module with the back opened to a "geo-web" which holds the back fill out of the module. These modules could probably get a dose of mulch once a year of some sort but just laid on top as the maintenance costs quickly escalate. Al, what drawbacks could you see for eliminating the granite? Would having a greater water retention with the same air filled porosity create any problems? I know this is largely dependent on species selection, but isn't it good to have higher water retention and higher cec of the media without compromising porosity if you are looking to minimize nutrient addition, water dosing frequency, and run off in a largely mineral based media. Thanks again!

    Bookmark   January 27, 2011 at 6:02PM
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tapla (mid-MI z5b-6a)

I don't see any drawback in eliminating the granite. The gravitational flow potential in a wall that high is great, so if the substrata allows decent percolation (or it's tiled?) the lower limit of particle size pretty much depends on how much aeration you desire. I'm thinking if you can remove the dust, a mix of particulates from 1/16 - 1/8 would provide a pretty rapid flow through rate, and you could easily go smaller. If you don't remove the dust, you could make allowance for it's downward migration through the media and it's affect on the bottom levels/layers of soil by not planting in the first few courses to eliminate the possibility of issues with saturation.

Everything isn't perfectly clear yet, so some of this is just throwing stuff to see what sticks.

FWIW - the rest of your thinking (immediately above) is accurate and in accordance with my thoughts.


    Bookmark   January 27, 2011 at 6:24PM
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Thanks again Al,
The particle sizes are right in line with what I was thinking. It is pretty amazing how much information is in your collection of posts deposited throughout the internets. I will try to remind myself to update this post with some of the test mixes.


    Bookmark   January 27, 2011 at 7:42PM
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