Square Foot Gardening Project: Step 2, Making Dirt

Reader Contribution by Allan Douglas
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In this, the second installment of this series, we will make the “dirt” or Mel’s Mix needed to fill the boxes we built last time.  Mel’s Mix, as stipulated by Mel Bartholomew, author of All New Square Foot Gardening is made up of equal parts peat moss, compost and coarse ground vermiculite.  The peat keeps the mix “loose” making it easy for plants to grow and develop roots, the compost enriches the mix; providing lots of nutrients for the plants, and the vermiculite holds water; making it available to the plant roots for a longer period than regular soil would.

The first order of business in this dirt making project is to visit the local garden supply store.  No, that’s not true; the FIRST order of business is to locate a garden supply store that carries the commodities we will need, and preferably in the quantities specified in the book.  And this is where the problems started for me.


First off, call the local garden centers and ask if they carry the compressed peat, course ground vermiculite, and multiple brands of compost.  Compressed peat (according to Mel) should expand to twice its volume when removed from its bag.  Course ground vermiculite does a better job of storing water than medium or fine, and because this stuff is expensive, buying it in the big (4 cubic foot) bags will save money.  Compost is made by breaking down vegetable matter with aerobic bacteria.  If you have a large compost pile of your own, you may use that instead of buying compost.  But you want to be sure your compost is “done” before you use it.


I started making compost from leaves, lawn clippings, kitchen waste (vegetable and rinsed egg shells only – no animal waste) and fireplace ash last spring.  I learned quite a bit along the way.  There are three major concerns to watch as you make compost; 1) keep the pile moist, but not wet.  A little water helps the bacteria grow, too much turns the pile into s sodden, stinking mess. 2) Aerate the pile by turning the pile with a pitch fork or by using a tumbler; the bacteria you want to cultivate needs oxygen to live.  If oxygen is not available, the aerobic bacteria dies and anaerobic bacteria begin breeding.  Anaerobic bacteria will produce compost too, but they take much longer and smell like a garbage dump while they do their work.  And 3) allow the compost to “cook” until the material is a rich, crumbly, black matter.  The inside of a compost pile will reach 150° Fahrenheit, which is hot enough to kill weed seeds if kept at that temperature long enough.  Also, mixing compost that is not done composting into your garden soil can kill your plants because the bacteria that are “rotting” your compost will attack the roots of plants growing in it as well.  Once the process is completed, this is not a problem. 

Commercial compost is usually made from a single source for each brand of compost.  Brand A may use the cast-offs of a mushroom farm to make compost, Brand B may use the hay and manure from a dairy farm barn, Brand C may compost saw-dust from a mill.  All are fine, none are complete.  Mel recommends mixing a blend of at least five brands of commercial compost to get the best nutrient mix.

If you make your own compost, get a wide variety of materials to go into it.  Yard clippings, fall leaves, kitchen scraps, and expended plants from your garden (except nightshades: eggplant, peppers, potato, tomato) are all excellent fodder for your compost mill.  You can add horse or cow manure (but not dog or cat poo) for added richness.  If you know a woodworker, sawdust can be added too, but avoid black walnut if at all possible, black walnut trees produce the chemical juglone in their root systems that kill certain families of plants, and this chemical will be present in the wood produced from a black walnut tree as well.  Common garden vegetables that will not grow within 50 feet of a walnut tree, or in soil containing juglone are cabbage and other cole crops, peppers, tomatoes, eggplants, asparagus, rhubarb and potatoes.  For details, Read This.


How many quarts are in a cubic foot?  I didn’t know, and neither did our “expert” at the gardening shop.  Likewise, if a commodity comes in a 35 pound bag, converting it to cubic feet is a matter of guesswork.  A little standardization in the production of soil additives would be a very handy thing but, alas, that is not the case.

I went in armed only with a shopping list based on the formula Mel gave.  For my 6 boxes I would need 2, 3.9 cubic foot bales of compressed peat, 4, 4 cubic foot bags of vermiculite, and 16 cubic feet of compost divided between at least five different manufacturers.

The peat came in 3 cubic foot bales, not 3.9 and it turned out that this brand did NOT double it’s volume when released from its binding.  The vermiculite was available only in 8 quart bags.  Compost was available in 1 cubic foot bags, but they had only one brand available.  As a result I came up short on peat and vermiculite, and the compost is all from a single brand for the first year.  I’ll add my home-brew compost to fill the boxes starting next year.  Last year’s batch got tilled into the garden soil last fall.  Had I known then that I would be doing this, I’d have kept it, but I didn’t.

Weed Barrier

Attaching a weed barrier cloth to the bottom of your boxes is highly recommended, especially if you are fortunate enough to be able to simply plunk your boxes down on top of your lawn and fill them with the soil mix.  I’m using this gardening method primarily to deal with the problem of a sloping garden plot, so I had to dig the boxes in to level them up as much as possible.  And, the spot where the boxes are going was a garden last year, so there is no “lawn” to deal with.  But I decided to install the fabric just the same.  Since the boxes are already dug in, I stapled it to the inside of the boxes rather than trying to pull them back up and attach it to the bottom.  A 100 foot roll of 4′ wide weed barrier cloth only cost about $12.00.  What I don’t use in the garden will go under Marie’s flower gardens as she landscapes around the house.

Doling and Mixing

Once we gather our materials we set them into our boxes in the quantities needed.  Mel recommends using a large tarp to roll the Mel’s Mix around to combine it, but since I’m on this project by my lonesome, and because of our geographic features, I decided to mix up my Mel’s Mix right in the boxes.

Open the bags and pour out the contents.  Had the peat been what it was supposed to be, I would have used 1/2 a bale per box.  It wasn’t so I ended up using a full bale and ran short.

Our “expert” guestimated the 8 quart bags of vermiculite to be about 3/4 of a cubic foot.  They turned out to be no more than half a cubic foot, and somehow my in-my-head math turned out the result that to get 2½ cubic feet each for 6 boxes I would need 12 bags.  Wrong!  The calculator (which was lounging at home) says it should have been 21.  And the bags only contained a half a foot not three quarters, so I would have needed 30 bags at $6.00 a bag or $180 just for the vermiculite.  I think I’ll pinch-hit on this one and water a little more often.

The compost came in 1 cubic foot bags – even I can do that: 2½ bags per box, 6 boxes equals 15 bags.  Actually the formula requires 2.6 cubic feet of each item, so I’ll get an extra bag to divvy up, but to keep the math simple enough for my rotted brain to handle I was using 2.5 as my target number.  Compost is cheap: $1.40 a bag.

I used a flat-nosed spade to mix the mix.  I chose this tool because a regular garden spade, with its “pointy” end might snag the fabric and tear a hole.  I worked carefully and gently and it took a while but in the end the results were quite good.  Because of the shortage of peat the boxes are not quite full.  I’ll buy more and work it in during the coming week.

Next Up

The next step will be to make the grid and work out where I will plant what.  We will address that in the next episode.  Join us again next time, same plant time, same plant channel!

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