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Making Thumbs Greener One Raised Bed At A Time
The Perfect Soil Mixture For Filling Your Raised Bed
January 4, 2013 by _Corbyn_
(http://www.gardeninginraisedbeds.com/author/admin/) _Leave a Comment_
If you research what various experts say about this topic, you will find
that they don’t all agree.
And in fact, you should keep in mind that the best soil mixture for any
raised bed can and sometimes should vary from one gardening situation to
Different Soil Mixtures For Different Situations
For example, certain plants require a different level of soil pH from that
preferred by other plants (blueberry shrubs need an acidic soil mix, for
Also, your local climate may dictate a different mix than that required by
someone in a very different climate (if you live in rainy Seattle, you
will want a mix tailored to drain well…if you live in arid Arizona, you will
probably want a mix that retains moisture as much as possible).
One of the great benefits of raised beds is that you can easily create
your own perfect soil mix from scratch for your given situation, instead of
trying to figure out how to amend your native soil (which involves sending off
samples to be tested and then determining what it needs).
All Purpose Raised Bed Soil Mixtures
In this article, I want to highlight two very different recommendations
from two prominent gardening experts concerning a good all-purpose soil
mixture to fill your raised bed with.
Mike McGrath of NPR’s You Bet Your Garden recommends a raised bed mixture
consisting of 50% screened topsoil and 50% high-quality compost. These two
ingredients should be mixed together well (not layered), and once in place
will never need to be tilled. I have tried this mixture myself, with good
Another prominent raised bed gardening expert, Mel Bartholemew (author of
the popular gardening classic Square Foot Gardening), recommends that you
fill your raised beds with something he calls Mel’s Mix.
Mel’s Mix is (by volume) 1/3 coarse horticultural vermiculite, 1/3 peat
moss, and 1/3 blended compost.
He recommends mixing the ingredients thoroughly and using at least 5
different sources of compost to provide a variety of nutrients (bagged compost
often comes from a single source, such as cotton burrs or chicken manure, so
it is a good idea to get a variety of sources and of course make your own
compost at home to serve as one of your five sources).
Mr. Bartholemew says that the compost in his mix provides all of the
fertilizer or nutrients that the plants will ever need, and that the only thing
you will need to add to his mix over time is a little more compost each
season (as the original compost breaks down and is used up by your plants).
He also says that his mix only needs to be 6 inches deep, never needs to
be tilled, and that your raised bed bottom can be lined with landscaping
fabric or cardboard so that the roots of the plants don’t even reach into your
As an aside, I might note here that some sources would call Mel’s Mix “
soilless” because it does not contain “field soil”, like that found in your
yard or garden. Other sources say that if a mix contains compost (as Mel’s
does), then it cannot be considered soilless.
Regardless, Mel’s Mix is essentially a high-quality form of potting soil,
much like what any professional grower would use to grow container plants
A Good Rule Of Thumb About Good Garden Soil
Also, I might add a tip here about garden soil that I heard recently and
thought was very interesting: one should always be able to sink their
forefinger all the way to the knuckle into their garden soil.
This simple test will tell you if your soil is as soft, airy and “friable”
as it needs to be, and illustrates the difference between proper garden
soil and, say, your lawn, which is probably so packed and dense that you can’
t sink your finger into it at all.
“Big” Problems With Big Tall Raised Beds
I want to talk about something that I think is hugely critical to think
about before you start building and filling your raised beds.
First, I love TALL raised beds that place the level of the soil somewhere
around waist high. The convenience of such a raised bed over a structure
that is literally 6 to 8 inches off of the ground is just monumentally
important to me from the standpoint of making the raised bed gardening
experience so much more enjoyable.
But there are a couple of really big problems that arise when you make a
tall raised bed:
1. it costs a lot more to fill a waist-high raised bed with an
expensive premium soil mixture than it does to fill a 6 to 8 inch high raised bed
2. a tall raised bed filled with soil requires substantial
cross-bracing to keep the sides from bulging outwards (or totally failing) from the
pressure of the heavy soil mixture inside it (this issue is exacerbated when
the soil is really wet and from “frost heaving” – the expansion of wet
soil when frozen)
Now I want to mention a couple of options that address these issues
concerning tall raised beds.
Option One: Lasagna Gardening
You could fill most of your tall raised bed with a mix of organic
compostable materials (2 parts shredded leaves to 1 part grass clippings would be
ideal) and then top that off with 6 to 12 inches of your preferred soil
mixture. You will want to put a few layers of newspaper or a single layer of
cardboard in between the compostable materials and the soil mix to keep the
mix from filtering down into the compostable materials in your first
This is referred to as “lasagna gardening” and achieves three key
objectives at once – piling up material to be composted, raising the level of your
soil to a very convenient height, and saving you money by not having to
fill the entire container with expensive mix.
It also negates to a large degree the outward pressure on the sides of your
raised bed because their is much less heavy soil inside it. However, I
would still recommend some cross bracing of your raised bed.
Over time, the compostable materials at the bottom of your bed will shrink
in volume as they break down into compost. Then you can simply add more
soil mix or compost to the top to keep the height you desire, and you will
eventually have great compost below your mix to feed your plants in future
Option Two: A Raised Bed/Compost Bin Combination
An alternative (but similar) raised bed design to the lasagna gardening
idea would be to build a tall raised bed structure, fill it mostly with
compostable materials, and then have 6 inch deep plastic or wooden containers
filled with something like Mel’s Mix (or some other premium potting container
mix) placed across the top of the raised bed structure (so the raised bed
structure is really a compost bin designed with an open top into which you
can fit large planting containers).
With this idea you don’t have to worry about your compostable materials
mixing with your soil mixture – they are kept totally separate.
But, you still achieve the raising of your planting surface to a very
convenient height, and you have created a place for storing your compostable
You would also be able to remove your planting containers easily from the
raised bed structure to:
1. aerate, remove, or add to the compostable materials underneath
2. simply move the planting containers to a sheltered spot to protect
them from oncoming inclement weather (a late or early freeze, violent
storm, etc.) or a shady area where you are more comfortable working
So I hope you can see that raised beds can take on a number of forms, each
of which brings with it a significant change in functionality!
Preparing The Raised Bed Site Before You Fill It
Before you begin filling a raised bed, many sources recommend that you
remove any existing vegetation and till the native soil in the area where your
raised bed will be.
This will improve the ability of the bed to drain. It will also allow
deep-rooted plants to sink their roots deeper than the medium with which you
fill the bed.
You could even improve the native soil with compost when you till it. It
might be your only chance to work this area since it will be covered with
your raised bed.
Protect Your Raised Bed From Critters!
It would be a good idea to invest in some galvanized hardware cloth (with
no larger than 1/2 inch square openings in the mesh) to place under your
raised bed and on top of your native soil. You will want to extend the
hardware cloth beyond the outer perimeter of your bed a foot or so.
This will keep rodents and other creatures from burrowing under the sides
of your raised bed to make a cozy home inside of it.
I intentionally neglected this step when I built my first raised bed and
(wouldn’t you know) I had something (probably a mouse) make a small burrow
under the side almost immediately after I filled the bed with soil.
The two mixes recommended by the two experts in this article each could be
said to contain the basic “building blocks” of a good soil mixture. There
are certainly a number of other popular soil amendments that could be
added to your mix, depending on your needs.
For example, most experts would recommend adding an organic, slow-release,
balanced fertilizer of some kind to the soil once or twice a year (early
spring is ideal but you could also fertilize in the fall), either lightly
worked into the surface or covered with compost or mulch.
Other possible amendments (and their uses) include the following:
* greensand – a slow-release source of potassium and micronutrients
* perlite – an alternative to vermiculite which does not hold
moisture as well
* soybean meal – a slow-release source of nitrogen
* gypsum – improves drainage
* rock phosphate – a slow-release form of phosphorous and
* alfalfa meal – primarily a source of nitrogen but also phosphorous
and potassium and micronutirents
* sulfur – increases soil acidity and makes calcium available to
* dolomite lime – increases soil alkalinity and adds calcium and
* wood ashes – decrease soil acidity and are a source of potassium
and other nutrients
* epsom salts – a source of magnesium and sulfur
* blood meal – a source of nitrogen
* bone meal – a source of phosphorous and some nitrogen
* aged (at least 6 months) manure – a source of nitrogen and minerals
* seaweed – a source of all three major nutrients but especially
* coffe and tea grounds – a source of all three nutrients and also
great for the compost pile
* shredded bark – improves soil structure and break down slowly
* wood chips and sawdust – improves soil structure and break down
I should note that if you have free access to any of these amendments then
of course you should think about adding them!
A Special Soil Mixture For Starting Seeds
I should add that if you will be direct sowing seeds outside in your
raised beds, you might want to consider topping your soil mix with something
more suitable for sprouting seeds. Such a medium should be finer textured than
your soil mix, moist and spongy.
A perfect recipe would be Mel’s Mix without the compost (just the
vermiculite and the peat moss), an inch or so deep, on top of your raised bed. Once
your seedlings are up, you could then layer some compost on top of your
bed to complement the seed-sprouting medium.
Or you could simply carefully screen the compost and leave it in the Mel’s
Mix (again, the idea is to make your seed starting medium a finer texture,
and some bagged composts can be pretty rough textured with bits of bark and
Don’t Forget The Mulch!
Finally, remember when you are filling your beds to leave some room for an
inch or two of mulch.
This mulch doesn’t have to be added early in the season when the weather is
mild and you are trying to warm the soil, but you will surely want to add
it later to combat the summer heat and weeds, retain moisture, and as way
to minimize nutrient loss from your soil (which can occur from the exposure
of bare soil to the elements).
Also, remember that many experts now say that the very best mulch of all
is an inch or two of pure compost. I was skeptical of this at first,
thinking that the compost would be much more susceptible to weed formation versus
something like a bark mulch. I was shocked to see almost no weeds at all
in my compost mulch through the growing season.
If you like the look of a bark mulch, at the least you could cut back on
the cost of this mulch by using much less of it, simply sprinkled on top of
a compost “base”. I used to think that I needed an inch or two of this
bark mulch everywhere (many gardening sources really preach the use of woody
mulches), but again the compost is a superior mulch by itself so a
sprinkling of the pretty bark mulch would suffice to beautify the bed.
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