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