7 Tips for Choosing the Right Soil for Your Raised Bed Garden – Abundant Mini Gardens

7 Tips for Choosing the Right Soil for Your Raised Bed Garden

Filling raised bed with soil

You are eager to create your garden, and you've decided to use raised beds. Now you need to decide what to fill them with – should you use potting soil (such as the square foot gardening Mel's Mix) or garden soil (topsoil)?

Here are seven tips to help you choose the right soil for your garden – based on your unique needs, resources, and priorities:

1) Do You Want to Create Rich Soil Quickly?

Potting soil, like Mel's Mix, is a fast and easy way to create rich soil for your garden. Blend the ingredients together, dump the mix into your beds, and you're done! It may take you several years to improve garden soil (topsoil) into high quality rich soil.

2) How Much Can You Afford?

Good quality potting soil can be pretty expensive – much more than good quality topsoil, especially when purchased in bulk by the cubic yard. I have local sources of quality ingredients for potting soil. Even though I don't have to pay for shipping, it can still cost me at least $55 (in 2014) to fill a 4 foot by 4 foot garden bed just 6 inches deep.

If you have to mail order many of the ingredients, it could easily cost twice that. However, a similar amount of bulk topsoil (not counting delivery) only costs about $16.

Mind you, it's quite possible to produce $60-150 worth of vegetables in that size garden bed in just one year. So you can get a return on your investment pretty quickly.

3) Are You Concerned About Contaminants?

Peat moss (or coir – shredded coconut husks) and vermiculite are weed- and disease-free. Good quality compost should be, too. With purchased topsoil, you rarely know where it has come from, or what soil-born plant diseases or invasive weed seeds it might carry.

There is also a concern about long-lasting herbicides in soil, manure, mulch, or compost (if it's not certified organic). For more information on this topic, read Don't Kill Your Garden With Compost!

4) Do You Prefer Soft, Lightweight Soil?

If you have disability issues, like I do, it may be easier for you to work with light-weight potting soil than with the heavier garden soil.

However, potting soil can be so light-weight that top-heavy plants like broccoli or peppers can fall over, as the light soil can't anchor their roots securely. This isn't as much of an issue when using potting soil in containers, as the rigid sides of the pots can often help prevent roots from being pulled up.

5) Can You Obtain Quality Ingredients?

Like many gardeners, you may not have a local source of high quality ingredients for potting soil. Peat moss is usually pretty easy to find, but good coir, high-quality compost, and large bags of vermiculite may not be. You may have to purchase them online.

Because many commercially bagged composts sold at big box stores are of very poor quality, even if you follow Mel Bartholomew's recommendation to blend several different composts together, your potting soil mix may still end up being nutrient-poor or have unbalanced nutrients.

Before you invest a lot of money purchasing ingredients for several garden beds, make sure they are good quality. There's nothing worse than spending days and a few hundred dollars creating several garden beds only to discover that your plants won't grow well in that potting soil!

I've heard of too many people following Mel's recipe, only to have poor results, even though other people have had a great experience with the same recipe. It's all based on the quality of your ingredients.

However, you can run into the same problem with regular garden soil. If you purchase topsoil, you risk having terrible soil delivered to your home, as companies will sometimes sell subsoil that has been screened to make it look like good topsoil.

I've had this experience myself, and I strongly encourage you (if at all possible) to test the soil you want to buy before you arrange to have it delivered. The same caution applies to bagged topsoil.

6) Do You Want Your Soil to be Long-Lasting?

One of the biggest benefits of using topsoil (garden soil) is the permanence of it. Once it's in place, it will remain there. And once you improve that soil's quality, it doesn't take much work to keep it in good condition.

Potting soil, however, is composed primarily of organic matter which decomposes down to water, carbon dioxide, and a few minerals. So it steadily disappears every year! A lot of new gardeners don't realize this.

I've had people tell me that their potting soil cost a fortune, but they were looking forward to gardening in those beds for many years. They didn't realize that they would be lucky to have 1/3 of their potting soil remaining in their beds after just 3 years.

In my climate in southwest Virginia, I've noticed that potting soil decomposes at a rate of about 25% per year. So I would expect to need to replace about 1.5” of potting soil every year in a 6-inch deep bed.

Most of that can be compost, but vermiculite doesn't last forever, either. It's an expanded mineral that tends to flatten over time when the potting soil is repeatedly dug up or turned over. Once vermiculite is flattened, it's no longer as effective at retaining moisture and nutrients, or in helping to aerate the soil.

7) Could Nutrient Imbalances Become a Problem?

If you use potting soil in same garden bed for many years, you risk building up nutrient imbalances. As I mentioned above, when potting soil decomposes, it will leave small amounts of minerals behind. Over time, some of these minerals will accumulate, and others will leach away with rainfall or irrigation.

This can cause major problems for your plants. When some nutrient levels become too high, it can prevent your plants from absorbing other necessary nutrients – even if those nutrients are abundant in the soil.

This shouldn't be a problem if you are using potting soil for only a few years in any particular garden bed. But if you plan to garden in one spot for a long time (say, over 10 years), you might want to consider using topsoil in your garden beds instead of potting soil or Mel's Mix.

You can also cause the same types of nutrient imbalances in regular topsoil if you apply too much compost, manure, or mulch for many years. More is not always better! The same caution applies to adding any fertilizers or lime to your garden if you haven't had a soil test to confirm that your soil actually needs those particular nutrients.

Once you develop extremely high levels of various minerals in your soil, it can take many years to correct this imbalance. Prevention is the key! Read A Simple Tip for a Healthier Garden for more information about the importance of soil testing.

So, Which Soil Should You Use?

Your very first decision should be to decide if you actually need raised beds at all. If you have deep, well-drained soil, you don't need raised garden beds in order to grow a lot of food in a small garden.

If you do choose to use raised beds, then you'll need to decide what to fill them with. Unfortunately, there is no single perfect solution for everyone.

If you'll be gardening in the beds for just a few years, you want a quick and easy way to create rich soil, and you have the money for high-quality ingredients (including adding more mix to your beds each year), then potting soil or Mel's Mix may work out great for you.

If money is a concern, or you plan to garden for many years in your beds, then using topsoil (garden soil) may be your best choice. The easiest and cheapest way to create raised beds with deeper topsoil is to till compost/peat moss into your whole garden area, and then shovel the topsoil from your paths onto your garden beds. I used this method for over twenty years, with great results – even with heavy clay soils.

In my current situation, we created garden beds 18 inches high, so that I could continue gardening with my disabilities. Because we needed many cubic yards of soil, and because we plan to garden in these beds for at least 20 years, we chose to use topsoil instead of potting soil – even though potting soil is easier for me to work in with my damaged hands.

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Looking for sources of high quality ingredients for potting soil or Mel's Mix? 

Here are two companies whose products I'm pretty comfortable with. I am not an affiliate for either one.  You should be able to locate good products from other sources, too. Best wishes for your garden! ~ Debra

7 Springs Farm in Virginia

Peaceful Valley Farm and Garden Supply in California

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So, what has been your experience in creating or using potting soil or topsoil in your raised beds? What has worked well for you, and what have you had problems with?

  • Steve says:

    I live in Florida and there are many nurseries that you can purchase topsoil from. i read you post where you said that if you were going to garden for many years it was probably better to use topsoil. I purchased a cubic yard of topsoil from a local nursery . I built the bed 4 feet by 4 feet and 12 inches high. Do you think I need to mix anything with the topsoil? It looked and felt dark and rich.

    Steve

    • Debra says:

      Hi, Steve! If the soil looks and feels good, that’s a great start. But there’s no way to know what it might really need until you conduct a soil test. Looks can be deceptive. Read this article for more information on this topic: A Simple Tip for a Healthier Garden I’m glad you created a good deep bed. That can really help increase your yields!

    • joe says:

      Steve, I also live in Florida. I would also strongly recommend sending a soil sample to the University of Florida Agricultural extension to have them run a soil test. They’ll give you the soil pH and micronutrients. Very accurate. Heres the link http://soilslab.ifas.ufl.edu

  • Steve Simpkins says:

    I have mushroom compost that is readily available to me . I can get a pick up truck load for 10 dollars. It was suggested I use this for my vegetable beds by mixing 4 parts mushroom compost with 1 part quality topsoil bought from a local nursery. What are your thoughts?

    Steve

    • Debra says:

      Hi, Steve! I think that’s too much compost. If the topsoil is good quality, you shouldn’t need to add more than 1 part compost to 3 parts soil. Soil that is poor quality could use perhaps 1 part compost to 2 parts soil. Adding too much organic matter can hurt your garden, not help.

      Even less compost would need to be added in future years to maintain the levels of organic matter. Good soil usually only needs about 1/4″ of compost added each year (maybe double that in very hot climates with sandy soil). Soil tests can really help to determine what needs to be added, and how much to add. Good luck!

  • Peter says:

    Hi Debra,

    Hope all is well. I had made my own Mel’s Mix a couple of years ago. Prior to that, it was top soil from my property.

    Before Mel’s Mix, I had great yield, after, not so much. Recently, I had my soil tested, and I was informed that my organic matter is very high (above 20%), above suggested range of 2-10%. In addition, Calcium, COpper, Iron, Manganese, Zinc, and Aluminum are all well above optimum levels.

    Would you suggest me mizing in some of my topsoil from my own property to bring everything back down into a better balance? Also, what top soil product are you aware of that would bring organic matter down. Thanks,

    • Debra says:

      Hi Peter! I’m glad you got your soil tested. I’m assuming you’re referring to soil you have in raised beds, and not containers? For almost all containers (with the possible exception of fabric containers like Smart Pots, people should use potting soil, and not topsoil – and, yes, potting soil will have very high levels of organic matter.

      But for raised beds, I highly recommend using real topsoil. Yes, you can mix real topsoil in with the Mel’s Mix to reduce some of the excessive levels of nutrients and organic matter. The organic matter will decrease (decompose) on its own over time, if you don’t add more each year. I’ve seen Mel’s Mix and other potting soils shrink by 25% each year. Some excess nutrients will leach out over time, though others won’t and may take many years to reduce levels.

  • Patty says:

    Hi Debra,
    First I want to say thank you.
    I have boxes filled with Mel’s mix recipe. I am in my 4th year of gardening (second year of retirement). I have rearranged and added mix through the years.
    How would I test soil in several boxes?
    I want to garden for several more years. Should I replenish with topsoil as needed?

    • Debra says:

      Hi Patty! If the soil in several beds is relatively similar, I would take samples for each of the beds, mix them together, and send it out as one sample to test. However, many of us create more beds over time, using very different soils. In that case, to keep the costs down, you can test different beds each year. However, be aware that some soil testing labs will NOT test potting soil, including Mel’s mix. I had good luck doing that with Logan Labs, though.

      For myself, if I planned to garden for many years in my beds, I prefer to use real soil (topsoil). If you want, you could gradually add topsoil to your beds as the Mel’s mix slowly decomposes. To make future testing simpler, try to add the same topsoil to all the beds each year. Good luck!

  • Wendy says:

    Hi Debra, my question is about soil or mix in sub irrigated planters. We will be building 4’x8′ raised beds and the bottom will have the water reservoir. We are hoping to use the beds for many years. In order to wick water up I think we need to use a soilless mix, is that correct? Thanks in advance for any guidance you can provide.

    • Debra says:

      Wendy, your project sounds like a great idea! I don’t have any experience yet with self-watering containers or sub-irrigated beds, though we plan to start using those containers this year. From what I’ve read, soilless potting mix is the best thing to use. However, for larger beds I’ve heard of a number of people recommending that folks use gravel or sand in the bottom of the bed, in the water reservoir area. Then put soil on top. Water will naturally wick up about 12″ high through the soil. Soil much deeper than that may have a dry upper layer. I’d just read up about your options and watch a number of YouTube videos on the topic. I wish I had some personal experience to share with you. Good luck!

  • Sam Wilkins says:

    You make a great point about choosing light soil to help plants anchor roots. My husband and I just moved into a new home and I would like to start a garden. After reading this, it seems like choosing a light soil would be the best way to go, especially because I want to grow broccoli.

    • Debra says:

      Hi Sam, what are you referring to a “light soil”? I consider potting soil (soil mix) to be a light soil that is NOT able to anchor roots very well. On the other hand, topsoil (mineral dirt) is usually heavy enough to hold plants securely, unless it’s very loose sandy soil.

  • Richelle says:

    Hi Debra. Great information, thanks. I was thinking about using a mix of topsoil and cotton burr compost for my raised beds. I am starting a backyard homestead and I hope to be using the beds for a long time. What do you think?

    • Debra says:

      Hi Richelle, that sounds reasonably good, though cotton is often heavily sprayed with pesticides. Best wishes!

  • Richelle says:

    I did some checking on the BtN website. They state that their product is “virtually chemical free” and that they don’t use arsenic as a defoliate but use sodium chlorate instead if needed. I guess I will choose to trust them and give it a try. Thanks!

  • Tomas says:

    I mix compost and peat moss with my own style of top soil / loam. Right next to a river we have moles/gophers that push up mounds of soil, if you leave that mound alone I ends up being a raised area and no field grass. I scrape up the mound to add to my garden mix then I try different ways to discourage the moles. Tomato leaves into their holes, water down the holes, some work some don’t.
    The mix seems good, the soil is free, the work is not bad every 3 days or so.
    And natural growth comes up instead of a raised dirt mound with nothing growing on it where the mole pushed up soil/loam …..
    Making do with what you have.
    Another idea, I use old, free garage door panels, most about 20″ high, for my raised bed sides. Sone are aluminum and insulated, most are wood that I line with plastic to last longer. Wish I could send you a picture of the beds …..

  • I’m making a garden in my back yard and I’m really excited about it! It’s going to have lots of fruits and vegetables in it. My goal is to get my kids excited about having these home grown foods and maybe they will want to eat them more often. How much soil do you think it will take to fill up a garden bed that is 3 feet wide, 6 feet long, and 4 feet high?

    • Debra says:

      That would require about 2.7 cubic yards – a LOT of soil! I don’t really recommend a 4 foot high garden bed, though. Your plants will end up well above your heads. No more than 3 feet high at most, but my 18″ high beds work really well for us and my plants. Here’s an article that has instructions on building our type of beds: https://abundantminigardens.com/how-to-create-an-awesome-raised-garden-bed/ Have fun, no matter how you choose to create your garden bed!

  • It’s good to know that I should check the nutrient balances in my soil before I lay it down in my garden. My husband and I just moved to a new house, and we want to put a garden in as soon as possible. However, I don’t want to just put any soil in, I want to make sure it quality. I will be sure to check the nutrients, so that I know that they are balanced.

  • debbie says:

    Hi Debra
    I am just starting out so this is all very helpful for me. I have raised beds made out of recycled pallet wood and I think due to treatments of wood I would need to put some sort of layer between the wood and soil. Can I use builders plastic or will that leach into the soil and harm my veggies?
    Kind Regards
    Deborah

    • Debra says:

      I frankly don’t know. But the plastic might be the lesser of evils. I really try to avoid the chemicals used to treat wood.

  • Alla says:

    Hello! I’ve been referencing the great information on your site for several years now while gardening in my small garden at the old house. Recently, we moved and I am in the process of designing and building a new, more ambitious raised bed garden. I really want to use concrete block to build the beds, as they are long lasting. However, some research revealed that there may be some issues with leaching of heavy metals from the blocks into the gardening soil. There is no good source of actual scientific data out there, but I certainly don’t want to take any chances.

    Was this an issue you considered before installing your concrete beds. There is the picture of some black fabric lining in your concrete beds. Is that out of concern for heavy metal leaching or for some other reason? How have you dealt with or recommend dealing with this issue.

    Thanks in advance for your input on this problem!

    • Debra says:

      No, I was not aware of this potential issue when I built these beds. The fabric was to prevent the soil from filtering out the cracks between the blocks.

      I suggest that folks try to purchase concrete blocks from a local company, after inquiring if they use ash or other potentially toxic ingredients.

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