Mixing Your Square Foot Garden Soil

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The Perfect Soil

Mel's Mix - The Perfect SFG Soil

A successful, flourishing and productive Square Foot Garden is quite easy to achieve, even if you ignore everything else except this advice: Don’t Skimp on the Soil!

Quality soil is the number one reason that a garden succeeds. For years people have been trying to improve their existing garden soils by adding fertilizers, mulches and compost, digging and tilling to loosen the soil and to improve drainage, adding chemicals to balance the pH level and then having to start all over again the following growing season. No more.

With Square Foot Gardening we create our own perfect soil mix right from the start and we never have to worry about using or trying to improve our existing garden soil ever again. So, what is the perfect soil?

The perfect soil
Think of a sponge, it just keeps soaking up water until it is fully saturated and then anything extra simply drains out the bottom. The perfect soil is just like a sponge, enabling your plant roots get all the water they need without ever being drowned in a waterlogged soil. The soil needs to be ‘light and fluffy’ to enable the plant roots to form properly and to “grow without effort” and it also needs to be well aerated. Finally, the soil needs to provide nutrients, minerals and trace elements that the plants need to grow and flourish.

OK, so how do you figure out what all the ingredients are, that you will need to achieve everything mentioned above? Well, you don’t have to. The creator of the Square Foot Gardening method, Mel Bartholomew, has already done it all for you. We create our perfect SFG soil by mixing Peat Moss, Vermiculite and Compost in equal quantities. It’s called ‘Mel’s Mix’ and it’s really that simple!

Canadian Sphagnum Peat Moss

6 ft3 bales of Canadian Sphagnum Peat Moss

Peat Moss
Sphagnum is a genus of between 151-350 species of mosses commonly called peat moss. Canadian Sphagnum Peat Moss comes from the more than 270 million acres of natural peatlands in Canada.

Peat moss is a natural, organic soil conditioner created from decomposing plant material. It’s large cell structure enables it to absorb air and water like a sponge. Although peat moss does not contain nutrients, it adsorbs the nutrients added to or already present in the soil, releasing them over time as the plants require. This saves valuable nutrients which are otherwise lost through leaching.

It helps to:

  • Save Water – Peat retains up to 20 times its weight in moisture, and releases water slowly as your plants need it.
  • Aerate Heavy, Clay Soil – Peat moss allows for proper root growth by loosening and aerating soils.
  • Bind Sandy Soil – By adding body to sandy soil, peat helps it retain moisture and nutrients.
  • Reduce Leaching – Peat moss reduces leaching of nutrients already in or added to the soil, releasing them over time.
  • Protect Soil – Peat moss protects soil from hardening and adds organic material.

Peat moss decomposes slowly over several years compared to compost which typically decomposes within one year. It has a reliable pH (3.4 to 4.8), is environmentally friendly and free of insects, weeds, seeds, salts and chemicals.

Vermiculite
This is also a natural material and is obtainable all over the world. It is a mica rock mined out of the ground. Large commercial vermiculite mines currently exist in South Africa, China, Brazil and several other countries. Once the rock is collected and ground into small particles, it is heated until it explodes just like popcorn. Like popcorn, vermiculite is light and full of nooks and crannies enabling it to hold a tremendous amount of water while still keeping the soil loose and friable.

Vermiculite is graded into several sizes – fine, medium and coarse – and is also used in a variety of other applications from fireproofing and insulation to absorbing hazardous liquids for solid disposal. The coarse grade is used for agricultural purposes as it holds the most moisture.

Compost
Good compost is sometimes called ‘Black Gold’ due to both its value and its scarcity. Compost is absolutely the best material in which to grow your plants. It has all the nutrients needed for plant growth. It’s loose and friable and easily worked. It holds a lot of moisture yet drains well. It’s easy to make and yet hard to find. The best compost is homemade and for that reason, one of the first things we should all be doing in our Square Foot Gardens is starting our own compost heap.

But, since we’re all starting out here (and assuming of course that you don’t already have a compost heap), the compost that we have easiest access to, is store bought compost. Unfortunately, most of these composts are single ingredient byproducts (like horse or kraal manure) that some local company has simply collected and bagged.

We took the advice of many who have before us and went out and bought as many different types/brands of compost that we could get our hands on. A few bags of kraal manure, a few bags of rose mix compost, a few bags of mushroom compost, a bag or two of germination compost and three  different ‘no-name’ brands just to get as much variety in our compost as we could. We found that the quality of these different types varied greatly, from light, dry and dusty to soggy, dark, stinking clods of half decomposed manure. We removed all the obvious half decomposed chunks and mixed the rest of it all together and actually came out with a very decent resulting compost.

Our Perfect Soil

Kathy enjoying the 'Light & Fluffy' feel of our own perfect Square Foot Garden soil

Mixing your soil.
We took a groundsheet from one of our old tents, laid it out flat on the lawn and used it to mix our ‘Mel’s Mix’ on. We simply poured out the required quantities of each ingredient (compost, peat moss and vermiculite) into the middle and then picked up the one end of the groundsheet and pulled it over to the other end so that the soil inside rolled over to the edge of the groundsheet. We repeated the process in the opposite direction and then back again, rolling the soil mixture back and forth and folding it in on top of itself. Once mixed it was easy enough to drag the whole groundsheet the short distance to our SFG box and upend the contents into the box.

A word of warning from the wise (and from personal experience) – Take care when mixing the ingredients though, both the vermiculite and the peat moss are very dry, light and dusty and easily gets up your nose – Do Not try doing this on a Windy Day!

How much to mix.
Working out the various quantities to mix can be a little confusing and frustrating (especially for those of you who, like me, are mathematically challenged). The compost usually comes in bags measured in cubic decimeters, the vermiculite in kilograms and the peat moss we managed to get bales of 6 cubic feet/170 liters which is compressed @ 2:1. Now divide that lot into equal amounts volume wise ;-(

I found a nice little online measurement conversion website (www.convert-me.com) that helped me tremendously when trying to work out the equivalent volumes of each ingredient. Cubic decimeters is the same as liters, so 30 dm3 of compost is the same as 30 liters. The 6 cubic foot peat moss bale is 170 liters that expands to double it’s compressed volume when opened and lightly sifted by running your hands through it. Ok, so that’s 340 liters total per bale (170×2).

The vermiculite was a little more difficult to gauge due to its extreme lightness. A 2kg bag ‘looked’ to be around 15 liters (compared to the size of a bag of compost) but then I had the brilliant idea of getting my hands on a 25lt plastic bucket and simply using that to add my vermiculite to the mix. I was not far off by the way – a 2kg bag of vermiculite fills roughly three quarters of a 25lt bucket.

Figuring out the volume needed for each box, and in total, was the easy part. A 1×1 meter box (3×3 squares) that is 20cm deep (0.2m) equals 0.2 cubic meters in volume (1 x 1 x 0.2). A 2×1 meter box (6×3 squares) that is 20cm deep equals 0.4 cubic meters in volume (2 x 1 x 0.2). Add up the volume of all your boxes and then divide by three (ingredients). Then use the conversion website to convert cubic meters to liters.

A final word of soil advice.
The perfect soil that we are creating here will probably be the most expensive part of your Square Foot Garden but it is without a doubt the most important aspect of the whole process. If you skimp on this part you will be sorry a few months down the line. It is definitely worth spending the money to get the correct ingredients in the correct quantities and getting this part right. If you follow the process correctly and as explained here, all the other advantages of SFG will all fall into place naturally and you will have a most enjoyable and profitable Square Foot Gardening experience.

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37 Comments on “Mixing Your Square Foot Garden Soil”


  1. Tony says:

    Thanks best discription I’ve read re Square foot gardens and Vermiculite

    • Nazeem says:

      Ditto. I can’t seem to find Vermiculite (in bulk) or Peat Moss. Any suggestions on where to source? I live in Pretoria.

      • Mark says:

        Thanks guys.

        Nazeem, we got our Vermiculite in 2kg bags from Safari Garden Center in Lynnwood Rd (Pta East) – they did mention that they could get bigger bags but that promise never materialized. You can also try Benoni Farmers Supply – Hardware Division (they had coarse vermiculite on their product list but not in stock when we tried them).

        The Peat Moss was a little more difficult to source but we managed to find a company based in Cape Town who imports in bulk for big agricultural companies. We managed to purchase 2x 6 cubic foot bales and had them shipped up to Gauteng.

        • Ridwan says:

          The Peat moss bails are available from MIT Gardening distributors in Pretoria. http://www.mitdistributors.co.za
          Cost is about R300 per Bail ex VAT and Delivery

          • Ulrich says:

            I tried MIT Gardening today for the peat moss and they refused to sell it to me since I am an end user and not in the trade.

            I bought two big bags of Vermiculite from Lion Bridge in Lynnwood Ridge Mall (not far from Safari Nurseries) a year ago. Their price seemed to be very reasonable, but I have forgotten how much it was.

            Due to the scarcety of peat moss in SA I substituted compost and some good garden soil. Mel Bartholomew either in his book or on his web site mentions that people in developing countries will have problems obtaining peat moss and should substitute compost. My harvest was spectacular!

        • Nici Richter says:

          It just simply blows my mind that you can even suggest that people use peat moss. The wetlands are totally dependent on peat moss to filter water and create stability in the eco system. Peat moss may not be removed from its natural environment in SA. It is illegal. So now you advertise Canadian peat moss and create an ecologcal disaster in Canada. What kind of ethics does that reflect? Nici

          • Mark says:

            Hi Nici

            Thanks for the comment and I appreciate your concern but… 😉

            I agree that in some areas of the world peat moss has been massivly exploited in the past. In the UK and other European countries a large portion of their peat bogs were destroyed long before sustainability was a business practice. They are now in a mode of reclamation rather than sustainability. Other parts of the world are in a similar situation (which is why it is illegal to harvest peat in many countries at the moment). Also due to this previous exploitation, there is now a lot of misinformation out there regarding the sustainability of peat moss from properly managed peatlands in countries like Canada and New Zealand.

            Both the New Zealand and the Canadian Peat Moss industries are highly controlled and regulated. There are more than 270 million acres of peatlands in Canada, of which only 0.016 percent of that is used to harvest the peat moss. In addition, new peat develops 60 times faster than what is harvested. When peatlands are managed as wisely as the New Zealand and Canadian ones are, peat (from these countries) becomes the ultimate renewable resource! This is the reason that we use and promote the use of only the Canadian Sphagnum Peat Moss.

            More info from the Canadian Sphagnum Peat Moss Association.

            Also read this Q&A With the president of the Canadian Sphagnum Peat Moss Association (CSPMA) on the use of peat moss and the environmental effects of its harvesting.

  2. Hallo everybody!

    I got some really special volcanic rock dust that will really add to your harvest. Finely powdered volcanic rock emulates the unique fertilising qualities of volcanic ash. The wide spectrum of minerals and trace elements which the rockdust provides is the final piece of the puzzle

    I’ll be happy to supply some demo stock so you can do back to back comparisons with your square foot gardens. Please contact me on 011 882 0570 to talk about rockdust, bat guano, worm castings and other organic soil ameliorates & fertilisers.

    Regards

    Jacques

    • Louise says:

      Hello Jacques
      I would love to know where you obtained your volcanic rock dust. I believe it makes a huge difference to crops and I am struggling with poor soil in my suburban vege garden. I am from Durban.
      Regards
      Louise

      • Mark says:

        Louise, you can purchase the volcanic rock dust from our new webshop (OrganicSeeds.co.za) but I’m not sure what the shipping costs would be (due to the weight of the bags, we are presently only offering it for personal collection from our home office in Pretoria East). If you are interested just let me know and I’ll get a quote for shipping it down to you.

        Regards
        Mark (SFGSA)

  3. Mark says:

    @ Ulrich
    Yes, I tried MIT Gardening too and they said the same thing. We are now thinking of offering the Peat Moss in bulk ourselves but our problem is going to be in the delivery of the stuff. If anyone in Gauteng is looking for Peat Moss and is willing to collect themselves (from Pretoria East), then please feel free to contact me through the Contact Form on this blog (with your contact details) and I will get back to you with prices.

    @ Jacques
    Thanks for the chat via phone. I will most definately be testing your volcanic rock dust soon and will post the results and the photos on this blog.

  4. Shingi says:

    Been trying to figure out the dimensions and literage of the germination mix to no avail – can anyone just tell me how many CUP FULLS are required.

    • Mark says:

      Shingi, only if you tell us how much mix you need in total 😉

      I am not sure which “germination mix” you are referring to but if you mean “Mel’s Mix” then that is made up of 1/3 Peat Moss, 1/3 Vermiculite and 1/3 Compost – so 1x cup of each ingredient would give you 3x cups of Mel’s Mix.

      We use the same Mel’s Mix for all our seed germination trays as well as for our raised beds, boxes and growing containers.

  5. Amanda says:

    Can’t you just mix 1 x 25L bucket of peat moss, 1 x 25L bucket of compost and 1 x 25L bucket of vermiculite? That’s the same volume, and no calculations required?

    • Mark says:

      Hi Amanda
      Thank you, exactly what I was trying to say in my previous comment but shorter and to the point 😉

      That is also how we mix our own Mel’s Mix.

      But, mixing it is the easy part… trying to figure out just how much (total mix) you’ll need for a certain size box/raised bed and then figuring out how much of each ingredient you need to buy (peat moss – either in liters or in cubic foot bales; compost – in cubic decimeters and vermiculite – in kg’s) is a tad more difficult for some of us, thus the reason for all the calculations in my post 😉

  6. Amanda says:

    Well, my SFG is not going all that well. I got a nice big (8kg) bag of vermiculite, and I’ve experimented with mushroom compost vs SuperPlant no-name compost. The SuperPlant mix started off a little dubiously – all my brassica’s were developing yellowish leaves that dropped off. Not all the leaves at once, mind, just one or two at a time. But that seems to have fixed itself, and everything in that box is growing nicely. Including the aphids. Go Bayer Insecticide, go!

    At one point, I ran out of peat moss, but since mushroom compost supposedly has peat moss in it, I ended up just mixing 1:2 vermiculite to mushroom compost. That mix has not worked so well though – I can’t tell if it’s because the spot the box is in might be a little too shady for vegetables, or if its just too moist (root rot?) – some of the plants look “wilted” (and eventually lose their leaves), even though the soil is still moist if you stick your finger in it. Of course, I’m also in Cape Town, in winter. I only water the plants now when there have been at least 3 consecutive dry days, and then only if the soil feels dry-ish.

    If my plants can survive the winter shade and rain, great. If not, I’ll just give that up and plant a shade-flower garden instead.

    • Jane says:

      Hi everybody – re compost: I have been using a new organically certified compost from Jacklin Organic. It is excellent!!! Have a look at their www for more info: http://www.jacklinorganic.co.za
      They are in the process of getting their products into the nurseries, so look out for it or contact them directly via their web.

  7. Naomi says:

    Halo everyone – would any of you know if one could use vermicompost instead of standard garden / shop compost in your SFG garden? I am struggling to find peat moss in my area (Goerge/Oudtshoorn) found “Super Flora” a German import @ R83.00 per 20L – sounds very expensive and I guess I need quite a bit. I plan to build 6 – 4×4 boxes and prepare them all at once with the “perfect soil” Would be grateful is anyone is willing to share information. Kind regards, Naomi

    • Carol says:

      Hi Naomi – did you get any response? I see you are also from here (lol) Any hints? Any successes or failures to void? Love to hear from you! Carol

  8. Mandy says:

    I saw someone in Joburg looking for vermiculite and peat moss. Both are available in sufficient quantities at the garden shop on Main in Bryanston. Or should I say they were available until I went there this morning….

  9. Conrad says:

    This is probably an extremely stupid question, but you mentioned Peat Moss is compressed 2:1. Does this mean a 30 liter bag will actually give me 60 liters? Or is this just wishful thinking on my part? 😉

    • Mark says:

      Hi Conrad
      Just wishful thinking I’m afraid. Only the big 6cu/ft (cubic foot) bales that the Peat Moss comes in originally are compressed @ roughly 2:1 (the bales hold 170lt of compressed Peat Moss which, when sifted will give you around 290lt).

      The smaller bags that you buy from the garden centers, plant nurseries, our own online shop etc are already sifted and “un”compressed so if it’s a 30lt bag then 30 liters is what you’ll get.

  10. Conrad says:

    Thanks Mark. Good thing I budgetted for without that wishful idea!

  11. Conrad says:

    I just want to check if I ordered the right thing from the nursery. The Peat Moss bag just says Peat on it. Is this the right thing? Its not the block.

  12. Conrad says:

    Ok well. According to a Square Foot forum peat is just a more decomposed form of peat moss, but does the same thing. I’m gonna mix it today. I hope its the right thing!

    • Mark says:

      Hi Conrad. Correct, the more decomposed form is often called ‘Sedge Peat’ and is much darker in colour, heavier and smellier than Sphagnum Peat Moss. The block that you mentioned in your previous comment would normally be the Coir block which, while similar to peat moss in form and function, comes from the fibers and husks of the coconut (it also does not hold as much water as the Sphagnum Peat Moss).

  13. Conrad says:

    Well. It is pretty light in colour and very light in weight. I was suprised at how light it is. And it doesn’t smell at all. So I guess it must be peat moss then.

    I’m just amazed at how much the level dropped after I watered it. I should get the peat that I didn’t get (as per the email i sent you) tomorrow. So hopefully I’ll be up and running tomorrow evening. I have no patience when it comes to waiting for something I’m excited about. Hehe.

    Thanks for all your help.
    Much appreciated.

    • Mark says:

      My pleasure Conrad. I know all about being impatient… that’s why we started our own garden at the beginning of winter 😉

      Yes the soil mixture will subside a bit as it absorbs the water and compacts a little from the weight of the water, unfortunately I have not yet worked out a simple way of predicting by how much it will subside by (I would have to run extensive tests on various sized boxes with soil mixes of varying moisture content to begin with).

      It is also surprising how much water it takes to saturate the soil, I must have stood for over an hour with the hosepipe on a 1m x2m box – 15 minutes of direct spray on the box barely wet the top 10cm’s of soil mix.

  14. Carol says:

    Greetings from arid Oudtshoorn.. Nor can I find peat moss, vermiculite – plenty compost though. I’m building quite large vegge gardens to help train and feed PLWA and this sqfg looks like the most sensible method. Thanks! But where o where to start?

    • Mark says:

      Greetings Carol

      Peat Moss is imported into this country (mainly Canadain Sphagnum Peat Moss or, as Naomi mentioned, the Supaflora blended brand from Germany). Most of the bigger garden centers and nurseries will stock one of the two. The Supaflora in 20lt bags (expensive though) and the Canadian Sphagnum in 2lt, 5lt or 10lt bags (also pretty expensive). If you are planning a large garden then these are a waste of time – they work out to be far too expensive when buying in small bags like that.

      The Canadian Peat moss is also available from a limited number of suppliers in 6 cubic foot bales (170lt compressed at roughly 2:1. We get 290lt out of a bale once it has been sifted). Unfortunately the suppliers only deal in bulk and only with registerd resellers. We sell the bales to the public here in Pretoria (see OrganicSeeds.co.za for prices) but unfortunately we do not have the shipping facilities to get them to you in Oudtshoorn.

      You have to start your garden with a plan. If you haven’t grown veggies before then I advise to start small. You are more than welcome to drop me an email with your contact number (use the contact form here) and I can give you a call to chat about your plans further.

      • Joan Mervyn-Smith says:

        Can someone please tell me where I can get Supaflora Peat Moss.
        I live in Benoni and work in Johannesburg.
        I used to get it from Flora Farm (Boksburg) but they are apparently not stocking it anymore.

        Regards
        Joan Mervyn-Smith
        email: dvinejms@netactive.co.za

        • Mark says:

          You can try calling ‘Supaflora’ themselves and find out where your nearest stockist is, or maybe try MIT Distributors. If I remember correctly MIT Distributors also distribute the Supaflora peat blend in the Gauteng region. I see that both of their websites are down at the moment though… hmmmph, typical!

          Supaflora: http://www.supaflora.co.za
          Tel: 021 851 8811 or 083 375 5315
          MIT Distributors: http://www.mitdistributors.co.za
          Tel: 076 733 1558, 076 733 1564 or 012 940 4264

          Also try Norsag: http://www.norsag.co.za
          Tel: 021 403 6588 (Supaflora importers in Cape Town)

  15. Marius says:

    Great site guys! We’re busy with a 4×4 square for salads and such and we were wondering what your views are on the compressed cocopeat/coir bricks? Is it an acceptable alternative to sphagnum?

    Thanks!
    Marius.

    • Mark says:

      Hi Marius
      Thanks for the compliment. I have not used the coir bricks myself (at the time we were building our first boxes, I thought that they were too expensive for the quantity that was required) but, according to various people who have used coir, it is an acceptable alternative to peat moss. The only thing to remember is that the peat moss holds water a little better/longer than coir so you might have to water more often.
      Mark

  16. Debra James says:

    A few years ago when I started my SFG long before the book was available here and I had only seen Mel on a telly program, I could not source peat moss for love or money. I spoke to a nursery owner and he told me to try bark until we could source something else, it worked like a charm. Cetainly kept my costs down while I experimented with what worked and my salad veg and flowers have always been beautiful.
    Just saying 🙂



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