Efficient backyard garden – Part 5: Soil

soil garden boxes

Now that our garden boxes are built and placed, it’s time to fill them with soil!

You may think all you need to do is go down to the local hardware store and pick up some basic potting soil.  You could, but you likely will spend more money and won’t have the highest quality ingredients.

This post will look at calculating how much soil you need, different popular soil blends, and finally filling our garden boxes with soil.  Let’s get started!

Garden box soil calculator: How much soil do I need

Before we start diving into what we are going to put into our boxes, we need to figure out how much material we need!  If you are in the United States, most bags of commercially available soil will be measured in cubic feet (usually labeled CF or ft3)

We just need to do a simple volume calculation to find out how much soil we need.

[box](Height (ft) *width (ft) * length (ft)) * number of same size garden boxes = total project cubic feet[/box]

The final result is how many cubic feet of growing medium that we need.

Let’s use my 4’x4’x7” garden boxes as an example!

[box](4’ * 4’ * (7”/12”) = 9.3 cubic feet per box

9.3 CF * 4 garden boxes = ~37.4 cubic feet total[/box]

There you have it.  In order to fill my boxes, I am going to need a total of 37.4 cubic feet of material.  Granted, I will buy a bit more than needed in order to top off as I go and account for any spillage.

If you are ordering larger quantities of soil, or ordering from a direct supplier, you may have to calculate cubic yards.  The equation is very similar:

[box]Height (ft) *width (ft) * length (ft)) / (27) * number of garden boxes[/box]

All that changes is dividing our garden box volume by 27, which is the total of calculating one cubic yard (3ft height * 3ft width *3ft length)

Now that we know how much soil we need, what do we actually put in our boxes?

Common Soil Blends

Now for the interesting part.  As someone who had only used top soil or potting soil before this project, I had no idea there were so many different soil blends…

…and how hard it was to decide on a final mix.

This section will likely evolve over time as I experiment and learn about different growing mediums.  However, for the time being, I wanted to at least highlight the three most commonly recommended blends I came across in my research.

Mel’s Mix

Far and away, this is the most recommended mix for garden boxes.  Across numerous forums, Reddit posts, and websites, Mel’s mix was lauded as the king garden box soil blend.  This three-component mix designed by Mel Bartholemew (pioneer of the square foot gardening method) has an almost cult-like following.  It’s designed to be lightweight and airy – allowing seeds to germinate and spread roots easier than traditional soil.  The mix drains well and will pretty much remain weed free.

Here’s a breakdown of the formula:

Mel’s Mix formula:
  • Vermiculite: 1/3
  • Peat moss: 1/3
  • Compost (optimally from multiple sources): 1/3

So, what’s the downside?

It’s really expensive.

mel's mix premixed bag
$30 for 1.5 cubic feet? Wow…

Home Depot has it for a whopping $29.38 for 1.5cu ft. That means it would cost over $700 to fill my four boxes with this mix.  Now it’s no surprise the premixed stuff would be expensive, but SEVEN HUNDRED DOLLARS expensive? Hard pass.

The clear solution is to make it yourself.  The mix is simple and designed to be mixed at home.  Again, here’s where things get tricky.  While the peat moss and compost are readily available at big box stores, the vermiculite is not (or if it is, it’s in tiny expensive bags).  The cheapest I could find vermiculite in my area was for $7 a cubic foot off Craigslist.  Doing some quick math, that still means in vermiculite alone I would need around 12.4 cubic feet (a third of our 37.4 CF total), netting out to $87.27 worth of vermiculite.  Adding in the cost of compost and peat moss brings the total cost up to $152.72.  Way better than $730, but still expensive.

Let’s also not forget that this is the lowest price found from some random dude on Craigslist.  The local farm and garden supply stores I checked had better pricing than the big box stores but were still expensive.

In other words, while Mel’s Mix might be a divine growth medium for your plants, the availability and cost of it is going to be HIGHLY location dependent.

You can read more about Mel’s Mix here:

Lasagna gardening

The DIY soil method.  Lasagna gardening involves creating soil by layering yard waste and a few additives.  You build your garden box soil exactly as you would an Italian lasagna – layer by layer of different materials.  Best of all, you use items you likely already have in your yard or house.

Unlike Mel’s mix, there is no set formula.  Instead, most people use a layering system of grass, cardboard, straw, compost, and mulch to create their desired result.  The main concept is to layer “browns” (cardboard, shredded newspaper, compost, etc.), “greens” (grass clippings, scrap food, garden clippings), and repeat over and over until you reach the desired height.

lasagna gardening layers
General flow for building a lasagna garden from GardensAll.com

Then, you let nature do its thing.  Water the lasagna garden as if it had plants in it, and a few seasons later you have nutritious soil ready for planting. In theory, you would start a lasagna garden in the fall while cleaning your yard of fallen branches, leaves, and yard trimmings.  Then in the spring or summer, you would be ready to plant.

Therein lies the downside – you have to wait a considerable amount of time to get planting.  While this might work great in cooler climates, I’m ready to grow in late fall / winter.  Additionally, to make this method worthwhile, you need to have a solid supply of raw material to work with.

Thing is, I just tore my entire yard out and don’t have any clippings to work with.  Whoops.

If you live on a large property with trees that shed their leaves every winter, this is probably a great low-cost mix.  With patience, you can outfit your garden boxes at a fraction of the cost of buying soil.

Unfortunately for my purposes, this is a dead end.  As time goes on, I might start a composting area and save clippings to try lasagna gardening.  However, this isn’t feasible in the present.

You can read more about lasagna gardening here:

Triple mix: topsoil, compost, and peat moss

Similar to Mel’s Mix, Triple Mix is a three-component soil blend.  The main difference is vermiculite is subbed out for top soil.  This results in a significantly cheaper soil blend.

Many pre-blended potting mixes sold in big box stores are variations of this mix.  However, like our previous example of the pre-mixed Mel’s mix, it’s usually more expensive.  Moreover, many blends use more peat moss than top soil and compost.  Peat moss is great at holding moisture, but doesn’t have a whole lot of nutritional content for your plants.  Thus, blending your own is the more cost effective way to go (shocking, I know).

Here’s a breakdown of the formula:

Triple Mix formula:
  • Top soil: 1/3
  • Peat moss: 1/3
  • Compost 1/3

Considering our previous price breakdown of vermiculite, this holds promise for the budget gardener.  Top soil can be had for as little as $1-$3 a cubic foot compared to the $7 lowest price vermiculite I could find.  Triple mix provides a great nutrient balance and good moisture control at an affordable price point.

What I went with

The cost factor was extremely shocking to me.  I figured soil would be very cheap, but I couldn’t be more wrong.  I didn’t want to drop big dollars on Mel’s Mix for my first gardening attempt and really didn’t want to wait six months to give the lasagna method a go.  Therefore, I went with the triple mix blend.

I used a modified version of the recipe found in this article.   I found this soil blend to be a nice nutrient mix at an affordable price point.

My end garden box soil recipe looks like this:
  • 40% top soil
  • 30% peat moss
  • 30% composed cow manure (compost)

All of these components are available at my Home Depot less than three miles from my house.  I calculated how many bags I needed using the percentages above, and went to Home Depot to weigh my car down with soil.

Filling the garden boxes with your soil blend

Now that I’ve hauled my bags of soil home, it’s time to get filling!  However, we want to make sure our soil is well mixed.  We don’t want to create odd layers or pockets of different materials.

Bags of soil ready for mixing!
Bags of soil ready for mixing!

To do this, I laid out a ground tarp and slowly started mixing bags together.  As the tarp got full and the mixture started to look consistent, I moved the combined soil into the garden boxes.

Slowly combining dirt to make a combined blend
Slowly combining dirt to make a combined blend

This is a somewhat slow process, as you want to mix everything together well.  However, the final result looks amazing!

Soils mixed and spread
Soils mixed and spread
Watered down and spread out
Watered down and spread out.

After filling, I raked down the boxes to even the soil out.  Finally, I hit each box with a little water to clean off the sides and bind everything together.

After filling your boxes, don’t walk around the beds.  We want the soil to be loose and airy.  Compacting the soil by walking on it is the opposite of what we want to do.  If you have mounds in the middle, use a rake to lightly move the soil around to fill the edges of the box.

Finally, I decided to use string to section off the boxes into 1′ growing sections.

Looking pretty good!  Now, let’s look at what it cost us to fill our boxes.


So how much did all that soil cost?  Check out the breakdown below:

Item  Price Quantity Total
Steer manure 1CF  $    1.09 11  $      11.99
Top Soil 1CF  $    2.37 15  $      35.55
Sphagum peat moss 3CF  $  12.48 4  $      49.92

Total spend on garden box soil: $97.46 (or roughly $24.37 per box).

Again, the total cost surprised me.  When I initially thought about putting the garden boxes together, I figured soil would come cheap.  Turns out there is actually some planning to be done if you want to give your plants the right nutrients and retain moisture.  Who would have thought!?

It’s starting to look like we can get planting!  Some people may be inclined to do so, but we are going to discuss installing a drip system next.  No forgetting to water for us!

Continue on to part 6: installing a garden box drip system


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