Raised bed soil, compost question

crystalshoe(6)January 10, 2014

I am building raised beds after struggling with clay soil with lots of rocks for 5 years. I've decided to attempt SFG this year, if that makes a major difference in the soil choices.

I've read all sorts of threads on raised bed soil, and I'm thinking of going one of several ways. If I've understood what I've read completely, I could do one of the follow:
1) mix compost and soil in a ratio of anywhere from 50/50 to 25/75
2) Al's mix as detailed in an assortment of threads
3) A local vendor sells a raised bed mix that contains the following:

"80% of our screened compost mixed with sand, peat moss, rock phosphate and greensand"

Their compost is the following:
" farm animal manure, such as horse, chicken, cow, sheep, goat, and rabbit manures. These we gather along with whatever bedding material was used, such as sawdust, wood shavings or straw. We also mix in elephant, camel and other exotic manures from zoo........ fish scraps, shell fish, seaweed, paper, wood chips, spent bark mulch, wood ashes, mulch hay, flower, vegetable, and shrub trimmings and also food scraps. Our major bulking agent is leaves which we receive from local towns..."

Laziness makes me want to use the raised bed mix and be done with it. But, it is $80 a yard, plus delivery, which is a lot! (At least I think so. But, I'm cheap.) Alternatively, I could follow one of the plans above.

Any thoughts on their mix? I don't mind spending the money if seems good, because I'd rather spend the time planting than stirring dirt. Also, if anyone has other suggestions, I'd welcome those too. Thanks!

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If you trust your local vendor, then why not use the ready made soil mix? We make our own compost, here in Madison. Our native soil is high clay, and the garden, in particular, seems to benefit from annual applications of compost. However, it took 10 years of making compost and adding it to the garden, before the soil was easily worked with no clumps or clods of clay.

    Bookmark   January 10, 2014 at 10:33AM
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yolos - z 7b/8a Ga.

If you are using the SFG method, the soil recommended now is a soil-less soil. Composed of 1/3 vermiculite, 1/3 peat moss or coir, 1/3 compost using five different types of compost.

This post was edited by yolos on Sat, Jan 11, 14 at 20:17

    Bookmark   January 11, 2014 at 8:16PM
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nancyjane_gardener(Zone 8ish North of San Francisco in the "real" wine country)

I don't use the SFG type of gardening, but I do have to use raised beds due to a huge gopher infestation! AND I have hard clay.
When I start a new bed, I usually dig down as much as I can (we have about a 2 week timeline when we can dig in between gunky clay and hard as cement!) loosen the soil as deep as possible, then build the raised bed. We fill that with whatever manure we have available (We're in a horse/dairy/goat/rabbit area =) ),tHEN WE INSTALL THE GOPHER WIRE! Water it into the areas that have been broken up, then fill it to the top with An organic garden mix from our landfill (yes it is certified organic) If you don't have that option, there are soil centers that have certain garden mixes. I wouldn't go with the big box store, cause it's so expensive to fill with bags! I will top with a bag every one in awhile....
After that, I use my own compost , one bed spring, the other fall to top off the beds. I have a little tiny tiller to mix everything up each spring (very few worms are harmed during this action!)
I don't know if this helps you. Just my way of doing things! HTH Nancy

    Bookmark   January 11, 2014 at 9:30PM
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You will find all kinds of opinions on what soil to use in the garden whether a raised bed or not. Some regard raised beds as containers and feel they should contain a soilless mix while others look at raised beds the same as in ground plots that need the same soil.
I have used a soil mix in raised beds that is as close to loam as I can find with about 45 percent sand, 25 percent silt, 25 percent clay and 5 percent organic matter which
I amend with more organic matter so that comes to around 6 to 8 percent. That soil mix is available here for $20.00
per yard in 5 yard loads plus a delivery charge depending on the distance from the nursery where it is mixed.
Getting enough organic matter together to make a soil mix that is 25 percent organic matter can be very difficult, and a mix with more OM is even more difficult.

Here is a link that might be useful: What is loam

    Bookmark   January 12, 2014 at 6:34AM
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Thanks for your helpful comments. I've been thinking about them all morning. You hit on something that has been bothering me since I started reading about the raised beds. I didn't realize that the difference I was noticing in growth medium possibilities was actually the difference between viewing the bed as a container and viewing it as an extension of the ground. It's very helpful because I can easily sort the various options into those 2 categories, and that is a good start to sorting out my confusion.

So, I'm putting these beds on ground that is not particularly fertile for various reasons, but it's also not cement-hard clay or a patio. I'm guessing that there is probably no clear consensus on this, but are there any sort of guidelines for using one treatment or the other? Or, is there a clear benefit to doing one? (When I say treatment here, I'm referring to treating the bed as a container, or as an extension of the ground.) If the growth outcomes are more or less equal, and I'm going to need to add OM regularly no matter what, I'd prefer the 'cheaper' method.

On a different topic, when you buy your loam mix, do you know the percentage of the components because you get it analyzed, or because the nursery knows those percentages, or for some other reason? I've never bought anything by the yard except sandbox sand, so I don't know anything about the dirt buying process.

Thanks again for you help, and thanks to everyone else for your input, too.

This post was edited by crystalshoe on Sun, Jan 12, 14 at 14:34

    Bookmark   January 12, 2014 at 2:30PM
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I know the composition of the soil before I commit to purchasing because I obtain a 1 cup sample of that soil and test it with that simple test for soil composition.

Whether to think of a raised bed as a container or a planting bed is the size. I have always thought of a raised planting bed of 4 feet by 4 feet, or larger, as a planting bed and not a container, even if that bed was raised high enough for a wheelchair to fit under it.

Simple soil tests,

1) Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains’ too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.

    Bookmark   January 13, 2014 at 7:15AM
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marshallz10(z9-10 CA)

In my experience, bagged or bulk planter mixes are more often than not mixes of sand and organic materials, not straight soil amended by copious amounts of organic material. Even topsoil in bulk may contain a lot of filler sand. So I advise my gardening customers to find sources of quality topsoil when making raised beds. Using sand-based mixes will leave a planting medium with much of the sand at the bottom of the planter over time.

My other advice is to be sure to create a transition zone between the country soil and the new planting medium by physically mixing the two very different media. Not doing so creates for a time a discontinuity across which water and roots have difficulties passing.

    Bookmark   January 13, 2014 at 7:50AM
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Crystalshoe has not said what he or she will plant in this area.

How important is that in answering the question?

    Bookmark   January 13, 2014 at 3:55PM
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I am planning on planting vegetables: peas, beans, tomatoes, carrots, summer squash, swiss chard; turnips, and perhaps a few other things.

    Bookmark   January 13, 2014 at 4:19PM
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The soil needed for the vast majority of plants, vegetables or flowers, is the same. A soil well endowed with organic matter that is evenly moist but well drained, a soil with a pH in the 6.0 to 7.0 range and balanced nutrient levels.

    Bookmark   January 14, 2014 at 7:30AM
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A mix containing 80% compost seems to high on the compost. The stuff is great but could probably be diluted by half with some kind of soil. Plants don't need that much, and it will continue to decompose so that next year you'll have to add more again.

    Bookmark   January 14, 2014 at 11:51AM
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sand_mueller(z 7a, oklahoma)

I used to compost the Tulsa Zoo manure...at one time all of it. Its a great material and grew great plants. I composted it, screened it and grew all my plants in it. the key now is to avoid salty manures and the way too common Graze-on herbicide contaminated manures. animals raised off pasture; fed mainly hay offer the safest manure.

    Bookmark   January 14, 2014 at 5:45PM
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Since you yourself used the phrase "square foot gardening," I would suggest you consider using Mel's mix, which is equal parts (by volume) vermiculite, peat moss and compost. (Yolos mentioned this also.)

This will probably not save you any money in the first year, but it is the square-foot-gardening way. You can get good results this way, but it is not the only way. (There is no "only way" as far as I know.)

One good thing about Mel's mix is that it has good moisture retention, even though it is porous. That can be beneficial if you are in an arid area where you rely on some type of irrigation (or hand watering).

Whatever you do, good luck!


    Bookmark   January 15, 2014 at 12:58PM
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nancyjane_gardener(Zone 8ish North of San Francisco in the "real" wine country)

If you go to the container gardening forum, they will say that a raised bed is different from a container. Raised beds being connected to the soil beneath. Containers, however don't have the help of worms etc mixing the brought in soil with what lies beneath. Apparently containers need different soils and fertilizing techniques.

    Bookmark   January 15, 2014 at 9:07PM
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Keep in mind that Mel's Mix is meant to supplement the soil in one block which is a 4 foot by 4 foot plot. It is not meant to be the only growing medium.

    Bookmark   January 16, 2014 at 6:36AM
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tishtoshnm Zone 6/NM

I will say that I did not find Mel's mix to be very water retentive. In fact, it drained so well that I found myself watering 2x a day. There are advantages to clay and one of those advantages is that it can hold water reasonably well. YMMV.

    Bookmark   January 16, 2014 at 10:23PM
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I have found that if one adds adequate amounts of organic matter to sand (6 to 8 percent) the moisture retention ability of that soil is greatly enhanced.
Clay soils do hold moisture and without adequate levels of organic matter will keep that moisture away from the plants that are trying to grow in that soil. Organic matter changes that so the moisture becomes available to the plants as do the nutrients that the clay tend to also hang onto.

    Bookmark   January 17, 2014 at 6:23AM
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Yeah, the problem with clay is it doesn't just retain moisture, it really RETAINS it! :-]

    Bookmark   January 17, 2014 at 11:24AM
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I've just started some raised beds, just finished putting them in last weekend, whew loads of work! I used mainly compost due to trying to keep the cost down. I'm waiting for the arrival of azomite and worm castings to add to it in hopes that it will help the health of my soil. I've started a compost pile but it's not really doing anything as of yet. I'm hoping and praying that it's sufficient to start and amend as I go through the growing season (please, please, please!). I'm really tired of buying crappy produce full of more crap from stores, I want real produce! I'm glad I found this thread with so much information, thanks!

    Bookmark   January 17, 2014 at 11:53PM
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I've been reading the comments, but haven't had a chance to respond until today. I really appreciate the info everyone is providing. I have read so many threads on soil, but I'm still surprised by some of the things written here. I am planning on making several large raised beds, so whatever I use I need a lot of it. One of the things I have against Mel's mix is the huge need for vermiculite, which would be prohibitively expensive here. Also, I'm not crazy about using it to amend my soil. I'd rather truck in something usable and fill the beds with it. As far as SFG, I am not completely sure I'm using that exact technique. I just started reading the book, so I can't really say at this point that I'm buying into Mel's whole plan. (Not that I have anything against it, I just don't know enough yet.)

    Bookmark   January 18, 2014 at 8:45AM
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The purpose of the Vermiculite and sand in Mel's Mix is to provide something of large particle size to aid in drainage. In my sand Mel's Mix is of little value for that reason and I found that when amending the clay soils found in northwest Indiana and southeast Ohio, as well as Oklahoma, Texas, Tennessee, and other places, l all that was ever really needed was organic matter.

    Bookmark   January 19, 2014 at 6:40AM
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It sounds like Mel's mix is too expensive. So let's rule that out.

Al's mix will also be too expensive. So I think we can just rule that out, too.So that leaves two of the original choices:

1) Order pure compost delivered by truck and mix with existing soil to fill beds

2) Order raised bed mix for $80/yd and fill beds completely with raised bed mix.

I think it will be a lot easier to order the raised bed mix and just load it in to the bed. The compost approach will involve a lot of digging and mixing. One piece of information not provided so far is whether free compost is available, or would that need to be purchased also, and if so, what is the cost?

In addition, I just want to respond to a couple of things.

Mel recommends filling the bed completely with Mel's mix, and says that the bed can even be put down on concrete or other passive surfaces. So he clearly intends the mix to be used as a potting mix, not just as a supplement.

While it is porous, it also holds a lot of water, because the vermiculite has a lot of pores in it. And the peat moss is kind of like a sponge.

If you water too fast, it may just flow right through, but if you spray it on evenly over several minutes, it soaks up a lot of water and keeps it available for roots for a long time.


    Bookmark   January 22, 2014 at 6:14PM
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yolos - z 7b/8a Ga.

Kimmsr wrote "Keep in mind that Mel's Mix is meant to supplement the soil in one block which is a 4 foot by 4 foot plot. It is not meant to be the only growing medium."

Kimmsr - Mel has changed his formula. If you can afford it, the new formula for his mix is now 1/3 vermiculite (perilite if you can't afford the vermiculite), 1/3 peat moss or coir, 1/3 compost made up of 5 different types of compost).

I know it is expensive, but after trying to garden in hard dense clay, it is so pleasant to grow in Mel's Mix. But it is costly up front. The vermiculite and peat should last a long time and if you do your own compost, there is no further expense after the first year.

    Bookmark   January 22, 2014 at 9:40PM
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I've had success with layering partially composted manures plus bedding in fall or late winter on top of rocky soils. Switched some beds to boxes a few years ago & happy with results.

We used what wood we had on hand to build 4' x 11 or so boxes. I wouldn't go any longer that that because you'll be tempted to step across. Keep the path wider than you think you need between beds. Plants grow out of them, so 4' or 5' wide isn't too far apart when full in summer. Make sure your cart or wheelbarrow fits.

Mel's mix is quite loose & too free draining for our summer droughty weather plus my tomatoes didn't stand up. Mixed with some of my soil underneath boxes now perfect... well if the moles didn't tunnel through & bring me more rocks. Small rocks aren't a problem except for carrots, but you can grow a stubby variety fine.

One idea might be to trial the different soil mixes in the boxes to find out what you like. You could always buy more soil next year when the compost box has dropped volume.

Fill 1st box with compost only added for your peas/squash/beans bed. If you put a trellis on the north side the squash won't be shaded. Plant peas 1st followed by beans & don't even have to remove pea vines. Sheet mulch this bed in fall & add more purchased soil mix in spring or sifted soil without rocks, so you can have the garden you want.

Use the purchased soil mix (no rocks) for a 2nd box & 3rd box
2nd for chard & root vegetables
3rd box for tomatoes interplanted with your other vegetables desired like lettuces or onion sets to eat as green onions.

hope that helps.

    Bookmark   January 27, 2014 at 10:14AM
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Soil, whether for a flat garden plot or a raised bed, consists of the mineral portion (the sand, silt, and clay) and the organic matter. The growing medium for containers is a soilless mix. Vermiculite (or perlite) is an inert mineral that provides drainage. The peat provides a moisture holding medium but does nothing to provide nutrients. The compost is there to help provide nutrients the growing plants need and is the replaceable portion of a container mix. Without the compost one would need a "fertilizer" which is not a good source of plant nutrients, normally.

Ma Nature, every year, provides a fairly large amount of organic matter to be put back into the soil, except many people remove that OM and contribute to the pollution of our water by the method of disposal used (piling those leaves up on the banks of rivers as is done here). Then, in the spring, and to replace those leaves, those people go to the garden center and buy peat moss, a non renewable resource.

    Bookmark   January 28, 2014 at 6:51AM
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tapla (mid-MI z5b-6a)

Hydrology, not the size of the bed or container, determines whether you're growing in a raised bed (RB) or a container, and by extension, what type of soil you can/should use. A planting of daisies in a plastic or clay pot resting on your deck or patio is a container planting. Bury that pot an inch or two into the ground so there is continuity between the medium in the pot and the soil the pot is partially buried in, and it becomes a RB - no matter what size the pot is. The difference is, the earth will act as a giant wick, and remove water that would normally perch in a conventional container planting, but water movement within the partially buried container would closely mimic water movement in the earth or in the soil below. There is a caveat, however. The area where the pot is buried or partially buried would need to drain properly for things to work. If you were to bury the pot in clay soil that allowed essentially no percolation, the pot would simply fill up with water at every rainfall or irrigating.

If you're building a RB over a clay soil that drains poorly - don't amend the clay unless you can drain the area. If you do, water will percolate or run into the area you amended, leaving it saturated for extended periods (the bathtub effect). You can remedy this problem if you are able to cut trenches or use a French drain cut into the clay to drain excess water to a lower area away from where you're growing. Not amending the clay below your beds also allows any excess water in the RB soil to move laterally over the surface of the clay where it can evaporate. If you amend, the water won't move laterally until the amended depression is entirely saturated with water.

No matter how you look at it, you're eventually going to have problems with a RB over clay unless you can figure a way to use gravity or a pump to move water from the area. As soil life moves through the clay and increases the OM content of the clay below the RBs, that soil will become more porous than the surrounding clay. The pores will quickly fill with water when it rains or when you irrigate unless you have a plan to remove the water. The end game is, the water that collects below your RBs has to have a place to go. It has to drain naturally in a reasonable time, or be directed downhill or to a sump where it can be mechanically removed. Amending the soil beneath RBs just forces you to face the brunt of the problem sooner (in heavy clay soils).

BTW - you don't need to use a highly aerated soil like that you referred to in your OP in RBs. In fact, the highly aerated soils that perform so well in containers are probably not a particularly good choice for RBs because they have a steep water retention curve that will have you watering more than you'd prefer.

Keep in mind that the time you spend now, properly preparing, will save a lot of time and frustration later. How many beds? Size/depth? Budget constraints high, low, medium? Ready access to materials you think might work?


    Bookmark   February 6, 2014 at 5:13PM
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Hi Al,
Thanks for weighing in.

The beds I'm planning would be on a very slight slope. I know from some flooding we've experienced in the past that the incline is certainly enough to make water travel downhill once the dirt is saturated. Do you think the incline would be enough to address drainage problems?

Unfortunately, the yard has been under snow for weeks, and I haven't been able to try the mason jar test to determine the soil's composition. I'm certain the area of my existing garden is clay, so I assume the whole yard is. As soon as I can get to the dirt, I'll be testing to determine that percentage. I should note that I don't live in an area notorious for clay soil, so I don't imagine I'm facing the heavy clay conditions some areas experience. I think the failure of my current garden is certainly aggravated by clay, but it also might just be poor soil. Judging from the yard, I don't think the previous owners or builders ever actually did anything to improve the soil.

Regarding the budget, I don't have huge constraints, but I'd like the best outcome I can get for my money. My RB plans at the moment are two or three 4' x 10' x 1'D, and two 3' x 10 x 1'D. The smaller 2 are for the kids, so those may not be as deep, perhaps 6-8 inches. I haven't built or bought materials for these yet, so I can change my plans if needed.

I have free and abundant access to leaves and seaweed (although seaweed requires some effort on my part, basically gathering in buckets from the beach.). I have a small pile of shredded leaves, and can certainly shred more in the spring. I can get horse or cow manure from local farms, but may have to wait for some to become available. There is a chance I could get chicken manure/spent bedding. Other than that, I can get whatever garden centers and dirt/mulch places sell, usually: topsoil, mystery compost, "loam."
I'd love any advice you can provide for creating the ideal soil.

I'll send the kids out in the snow and see if they can dig up some dirt later on today and report back what I find from the jar test. I can almost see ground peeking through the snow!

Thanks for your info, I never even considered the drainage issues of the dirt under the bed.

    Bookmark   February 8, 2014 at 11:04AM
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tapla (mid-MI z5b-6a)

"Do you think the incline would be enough to address drainage problems?"Probably at the outset, but not so much as the beds grow older. For things to work as they should, the depth of any trench you cut should be at least as deep as the soil under beds that is impacted by the results of soil improvement. Soil improvement includes OM and added aeration in the soil beneath the beds. Keep thinking about how to eliminate having to deal with the 'bathtub effect'.

This is soil from my raised beds, which I use mostly for growing on material that will eventually become bonsai:

It's wonderfully productive, and you can tell just by looking that it has excellent tilth. That particular soil is comprised of pine bark fines, Michigan (reed/sedge) peat, sand, and unscreened Turface MVP. If I was making it today, I'd probably use Pine bark fines, reed/sedge peat or compost, some native topsoil or fine play sand, and some Turface Pro League, being sure to increase the mineral fraction. You'll want that (the mineral fraction of your soil) to be somewhere around 65-80%, and the rest OM. Plants will grow fine in more OM, but you would have lots of shrinkage annually. There's advantage in having the OM at least partially decomposed. I like the Turface Pro League as opposed to MVP because it makes the soil much easier to rewet, and its smaller size helps increase o/a water retention. FWIW - the first year you grow in your raised beds, you'll need to add considerably more N than in subsequent years.


This post was edited by tapla on Sat, Feb 8, 14 at 13:18

    Bookmark   February 8, 2014 at 1:16PM
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Do you recommend these be uncomposted or partially composted bark fines? Do they need to be screened? I'm looking to fill three 4x10 1.5ft raised beds, so screening enough would take the whole growing season!

Also, what is the ratio you would recommend for all of these?



    Bookmark   April 2, 2014 at 8:22PM
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tapla (mid-MI z5b-6a)

Well composted bark, like Fafard's aged pine bark, unscreened, will reduce N immobilization. Shoot for somewhere around a 75% mineral fraction and the rest organic material that will break down slowly or is already close to having all the cellulose broken down. Reread the post above yours for more.


    Bookmark   April 2, 2014 at 8:53PM
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