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Complete Beginner’s Guide to the Best Raised Bed Soil

Complete Beginner's Guide to the Best Raised Bed Soil

Adding some raised beds to your garden this year? Great idea! But before you begin, you’ll need some good quality raised bed soil for your garden beds to make sure you’re off to a good start.

Growing plants, veggies, and flowers in a raised garden bed are different from growing them directly in the ground and for that, you need the best soil specifically for raised garden beds.

Using a raised garden bed has many advantages; it’s commonly said that raised beds produce about four times the amount of produce compared to the traditional method.

Plants seem more vigorous there in the early season, probably because the soil in a raised bed warms faster than that in the garden patch. As gardeners, we love early-season growth.

None of this is true, of course, if the soil in your raised bed isn’t at its best. And that’s the great thing about raised beds. You can dig them out and fill them as you like.

Think of them as a controlled experiment in which you’re looking for just the perfect mix of organic materials — including beneficial microbes and other living things — and naturally occurring nutrients like nitrogen and minerals.

If you’re about to embark on the journey of having your own raised garden beds, this is the article for you. We’ll talk about exactly what you need to consider when choosing your soil mix, and also share our favorite recipe that you can easily make at home.

But that’s not all, you’ll also learn how to amend your soil so that it works for any plant, vegetable, fruit, or flower you want to grow in your raised garden bed. So let’s get started!

Wooden Raised Vegetable Beds

What to Consider When Choosing the Best Raised Bed Soil

A raised garden bed is different from planting flowers and vegetables directly in the ground. Before you start shoveling dirt into your raised beds, keep the following points in mind:

Step: Determine Raised Bed Height

You can estimate the amount of soil you’ll need to fill your raised bed garden by measuring its size.

Fortunately, there are many practical calculators that can assist with this task while taking shape and size into account. In general, you’ll need more raised bed soil the higher and taller your garden bed is.

Gardening can be easier on the back if your raised garden beds are taller or have a tabletop design.

But increasing the height doesn’t necessarily mean an increase in cost for your raised beds. You can also reduce the amount of soil mix for raised beds by filling the bottom of a tall ground-level bed with filler, such as dead leaves or cardboard.

Step 2: Evaluate Plant Type and Root Depth

The ideal soil depth for the raised bed will be determined by the plants growing in the garden. Build a raised bed that is at least 6 inches deep as a general rule of thumb. This depth supports drainage and retains sufficient moisture for a variety of popular crops.

Some plants, however, have more robust root systems than others that grow deeper. For instance, if you want to grow root crops such as carrots, a depth of 12 inches is recommended.

The benefits of growing plants in raised beds extend to all types of plants, but vegetables benefit most from this method of gardening. By raising the growing area, you can keep weeds away, keep the soil warm, and keep it from getting packed down.

Gardeners also benefit from starting with pure, pH-neutral soil. But if you are using soil that has been used before, you should test it to see if you need to add any amendments or fertilizer to bring back the nutrients.

Here are depths for some popular crops:

6-inch Soil Depth:

  • Lettuce
  • Salad greens
  • Spinach
  • Onions
  • Leeks
  • Radishes
  • Strawberries
  • Basil
  • Chives
  • Cilantro
  • Dill
  • Mint
  • Oregano
  • Parsley
  • Thyme
  • Marigolds and other annual flowers

12-inch Soil Depth:

  • Beans
  • Beets
  • Broccoli
  • Brussels sprouts
  • Cabbage
  • Cantaloupe
  • Carrots
  • Cauliflower
  • Collards
  • Cucumbers
  • Garlic
  • Kale
  • Swiss chard
  • Summer squash
  • Turnips
  • Lavender
  • Rosemary
  • Sage
  • Borage
  • Calendula
  • Everything in the previous list

18-inch Soil Depth:

  • Eggplant
  • Okra
  • Peppers
  • Pumpkins
  • Sweet potatoes
  • Tomatoes
  • Watermelon
  • Winter squash
  • Everything in the previous two lists

Step 4: Consider Soil Components You Require

Although many gardeners have tried-and-true soil mix recipes, it’s usually advisable to stay away from super-cheap packages of raised bed soil.

A healthy soil mixture includes topsoil, a little quantity of substrate, and a substantial amount of compost that provides the nutrients that enhance soil conditions.

Most of the time, the cheapest bags of soil aren’t very good and are full of weed seeds, trash, and other unpleasant things. They’re also less likely to include beneficial nutrients your plant needs.

Topsoil is essentially a filler soil that is required for anything from gardening in raised beds to growing plants in indoor pots. It constitutes the majority of the soil mixtures used for raised bed gardens. It doesn’t have a lot of nutrients, but it’s a necessary part that has organic matter in it.

The substrate is only a fraction of the total soil used in a raised bed, yet it plays a crucial role. In a raised bed, moisture control is crucial, and this assists in doing just that.

Peat moss, rock phosphate, vermiculite, perlite, and coco coir are only a few of the possible substrates in soil mixes. Once again, the substrate requirements may vary depending on the plants you intend to grow.

In general, here are the three most important components of raised garden soil:

1. Compost

This is the source of all of the nutrients that are needed by plants. There are numerous types of prepackaged compost available, including mushroom compost, cow manure compost, chicken poop compost, and worm-casting compost.

If you have a compost pile at home, that counts toward your total too, as it is our go-to choice due to the fact that it is both cost-free and loaded with wonderful nutrients. If you’re interested in starting your own pile, check out our complete guide to compost pile here.

2. Vermiculite

Vermiculite is an essential component for proper drainage and aeration of your raised garden beds. The roots of plants don’t like to be in the water, so it’s important for the soil to drain water at the right rate, i.e., not too quickly or too slowly either).

When you mix vermiculite with compost and peat moss, you get just the right amount of aeration.

3. Peat Moss

Peat moss is basically dead moss that has been breaking down and decomposing for a long time.

It does a great job of assisting the soil in holding onto nutrients and water long enough for the plants to get the water they require. It also helps break up topsoil and compost that would otherwise be too hard and dense.

Be very careful not to add an excessive amount of peat moss, since this might cause the pH balance to become unbalanced and increase the acidity of the soil. If you’ve accidentally done that, lime can help balance the soil pH by raising it.

Step 5: Get Some Topsoil

Topsoil is different from garden soil. It is, as the name implies, the top layer of soil. This versatile soil is great for filling garden beds or prepping a barren lawn for growth.

It doesn’t have a lot of nutrients, but it does have important organic matter and serves a purpose.

Purchasing bags of topsoil to utilize in your raised beds? Pour the topsoil in first, as it will serve as a fantastic filler and foundation. The topsoil should then be covered with a more nutrient-rich combination of compost, peat moss, and substrates to nourish the plants.

Step 6: Mix in Some Fertilizer

The ideal soil for raised beds contains nutrients that support growth. Compost, worm castings, and sea kelp are all examples of organic and natural forms of fertilizer you can add.

You won’t have to worry about the soil becoming contaminated while using these fertilizers because they improve both its quality and its condition organically.

The slow-release qualities of organic fertilizers avoid potentially damaging buildup and protect your plants from an overabundance of nutrients.

Some soil mixtures for raised beds may contain synthetic fertilizers. They are useful for boosting nutrient levels quickly, but unlike organic choices, they don’t improve soil quality, texture, and conditions. Because of this, it is best to buy or make a soil mix with organic sources of nutrients.

Raised Bed Garden

How to Get Started with Soil for Raised Garden Bed

The easy way is to just buy topsoil and compost, in bags or not, and fill up the bed’s box. If you have your own compost, or can get reliable, organic compost — we were lucky to get it from a local, organic dairy goat farm — it’s worth making your own soil recipe.

That way, you’ll be able to fine-tune it for particular crops. Growing tomatoes? Make your soil slightly acidic, just the way they like it. Growing greens? You’ll want to keep the nitrogen low until you have germination.

If you’re using compost, make sure that it’s completely finished. If you’re adding manures of any kind, make sure they’re completely composted and are no longer “hot.”

Mix in other materials, like peat, pumice, or vermiculite if you’re looking for good drainage, or sand, which root vegetables like. The easiest way to make sure compost is garden-ready is to spread it in the fall, leaving it on the surface to finish through the freeze and thaw cycles of winter.

If you didn’t spread compost in the fall or just don’t have any to spare, you can make it on the spot and grow in it as you do. We’ve stuck a bale or two of straw in raised beds in the fall and were left with good results when we pulled the remnants off in the spring.

Not only does the bale smother any weeds that might try to poke up early, but it also conditions the soil beneath where it sits.

We even know someone that placed a bale right on the sod where he put his box in August and finished up with top soil when it came time to plant next spring. Personally, I’d go to the trouble to dig out all the sod ahead of putting down bales. But that’s just me.

If you’re putting in new raised beds this spring, why not put your first planting right in the straw bales? The craft of straw bale gardening has grown in popularity and for good reason. Some gardeners skip the box altogether and just grow in the bale.

Raised Bed Garden Kit

The Perfect Raised Bed Soil Mix Recipe (Weed-Free)

Good organic garden soil is the single most important ingredient for healthy, nutritious vegetables.

It is loose and fluffy — filled with air that plant roots need — and has plenty of nutrients and minerals essential for vigorous plant growth and bountiful yields.

Filling your raised beds is an opportunity to get high-quality soil and fine-tune the mix of fertilizers and amendments.

The following soil mix was developed by Planet Natural to fill a 4’ X 8’ raised bed one foot deep (32 cu ft).

5 bags Black Gold Peat Moss, 2.2 cf x 5 = 11 cf

3 bags Teufel’s Organic Compost, 3 cf x 3 = 9 cf

4 bags Worm Castings, 1 cf x 4 = 4 cf

3 bags ​Organic Chicken Manure, 1 cf x 3 = 3 cf

2 bag Therm-O-Rock Organic Vermiculite, 2 cf x 2 = 4 cf

3-6 lbs Azomite

1-2 lbs Kelp Meal

3-6 lbs Oyster Shell Flour

2-4 lbs All-Purpose Fertilizer

Have on hand all the ingredients for your soil mix before you start filling the beds, and pre-mix as much as possible, on a large tarp if necessary, to avoid pockets of peat, manure, or any other ingredients.

Note: Do NOT use pressure treated wood or railroad ties for your raised bed frame because of chemical leaching

Raised Vegetable Gardens

How To Amend Raised Beds For Better Soil Health

The recipe above is perfect to get started, and then you can adjust and amend your soil to fit the particular plants you are planning on growing!

As we mentioned above, a great option is to include a slow-release organic fertilizer in your raised garden bed soil

Once or twice per year, incorporating this into the soil improves the soil’s quality and nutrient content. Work it in at the beginning of spring or the end of fall.

After applying, cover the area with a thick layer of mulch or compost. Mulch is great for keeping the soil moist and preserving nutrients.

Soil amendments are great at helping change the soil pH, moisture level, or aeration of your soil to better suit the plants you’re growing in your raised garden beds.

Some other useful alternatives to the standard soil amendments are:

  • Consider using perlite instead of vermiculite in your recipe if your plant requires moist soil since perlite holds moisture better.
  • Bone meal contains some nitrogen as well as a significant amount of phosphorus.
  • Blood meal is an excellent source of nitrogen.
  • Epsom salts are high in sulfur and magnesium.
  • You can also use shredded bark, wood chips, and sawdust as they tend to break down slowly and will improve the structure of your raised garden bed soil over time.
  • Used tea bags, grounds, and composted coffee contain NPK. They are great additions to the compost pile.
  • Seaweed has a good nitrogen, potassium, and calcium balance. It also has a lot of potassium.
  • Alfalfa meal is an exceptional nitrogen source. It also contains several micronutrients as well as potassium and phosphorus.
  • Coconut coir, a more sustainable alternative to peat moss, keeps the soil moist. When gardening in dry and arid conditions, coconut coir is an especially useful amendment.
  • Sulfur contributes to the acidity of your soil. It also facilitates the absorption of calcium from the soil by your plants.
  • Conversely, wood ash reduces the acidity of garden soils. Ash contains a variety of minerals, including potassium.
  • Lime is also great at reducing the acidity of your growing medium neutralizing acid reactions in the soil. It’s a great way to counteract an increase in acidity due to a high level of peat moss in the mix.

And that concludes everything you need to have the best raised bed soil. Bookmark this page to come back to it and amend your soil over time.

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2 Responses to “Complete Beginner’s Guide to the Best Raised Bed Soil”

  1. Paul Armstad on April 22nd, 2015 at 8:57 am #

    The problem with using straw, at least for us here near Lewistown, MT, is that it is impossible to find straw that has been grown organically, so the straw contaminates the soil and crop.

  2. SQWIB on March 26th, 2019 at 10:12 am #

    That mix sounds awesome but it seems like a whole lot of stuff to buy for a raised bed and most if not all, isn’t even needed, folks can just use an inexpensive layering method.

    It’s better to build a soil if you have time, but if you don’t have time go with “The Perfect Raised Bed Soil Mix” in this article.

    Build the bed in the fall, start filling with whatever you can get your hands on. Make sure to layer.

    Here’s a few ideas… Pumpkins (plenty around after Halloween), hay, straw (plenty around after Halloween), Rabbit/Chinchilla bedding and manure, Urea (pee), grass clippings, leaves, pine needles, yard trimmings, newspaper, kitchen waste, compost, twigs, pond sludge, bio-char, ashes, old potted plants, old potting soil, egg cartons (paper), pizza boxes and cardboard, vacuum cleaner trash (especially if you have pets).

    Throw the weeds in there too unless they went to seed, and even then, you can soak the weeds with seeds in water for a week then dump in the beds, however the water can get stinky.

    Just use common sense when layering, like carbons to nitrogen ratio and when adding food scraps, cover with something like leaves, hay or soil. Your microbial activity will be thriving by the spring.

    In case of nitrogen tie up, just top the beds with a decent composted manure your first year of planting or you could plant a nitrogen fixing cover crop early spring and terminate when ready to plant. You can also use compost as a mulch even if it’s partially composted.

    For tall beds, line the bottom with rotten wood and start layering from there, use your native soil, layering and watering as you go. Using wood with fungi present will help a lot.

    I posted a few of my Hugelkultur builds that may help some make a decision on their next raised bed. You can check it out here:
    https://sites.google.com/site/sqwibsurbangardeningadventure/raised-beds-reconstruction.

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