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Container Gardening

what soil mix for raised beds?

5 years ago

last modified: 5 years ago

I know raised beds aren't the same as container gardening, but since there is no sub forum specifically for raised beds on this forum, I figured this would be the best place to post this topic.

What type of soil mix should I use for raised beds? I know it depends on the crop you're growing; I'd be doing peppers in mine, and supposedly peppers prefer (for lack of a better term) soil that is 'light and fluffy'. Is this true?

Here are two recommendations of what to use for raised beds:

1. 30% peat moss, 30% coarse vermiculite, 30% compost, and 10% sand.

2.50% top soil, 40% compost, and 10% sand (when it says top soil, does it mean potting soil, i.e. miracle gro)

I am very bad at math, so for a 4' x 8' bed, what does '50% top soil, 40% compost, and 10% sand' translate to in gallons (how many gallons)?

If use the one with compost, do I still need to add fertilizer? If so how much for a 4'x 8' bed, and should it be a slow release fertilizer that continuously feeds the plants (i.e. osmocote). If yes to using slow release fertilizer, how much for a 4' x 8' bed (2 cups, 3 cups, etc.)

Featured Answer

val rie (7a - NJ)

5 years ago

Your garden is an oasis tapla :)

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Comments (9)

User
5 years ago
There is another ongoing thread just a few below the one you started that discusses this:

https://www.gardenweb.com/discussions/5637069/best-soil-for-hedge-plants-grown-in-raised-beds#n=15

Hope it helps!
gardengal48 (PNW Z8/9)
5 years ago
To simplify the long and involved discussion linked to above, a raised bed is just a elevated garden planting bed.....so you just want to fill with garden soil and one that is well amended with some fertile organic matter. Of your choices, #2 is much better than #1....but not ideal (too much compost and no need for sand). Better would be 75% topsoil or garden soil and 25% compost.

A 4'x8' bed that is 12" deep will take over a cubic yard of soil so it is far less expensive to buy a bulk soil product if you need to import the soil to fill rather than purchasing any bagged product ($50-75 for a cubic yard of bulk soil compared to $5-9 for a cubic foot of a bagged product.....and you'd need 27 of them!!). A landscape or garden planting mix or raised bed soil mix should be available anywhere that sells landscaping supplies and bulb soils and mulches.

You may still need to add fertilizer at some point during the growing season but that is never a given so it makes no sense to give some arbitrary formula for what and how much to apply now. Even the occasional application of Miracle Gro via a hose-end sprayer will suffice.
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You will find a lot of disagreement about whether sod needs to be removed or not, but if you plan on putting a raised bed, 6 plus inches, over the sod there is no need to remove it since the raised bed will smother the grass. On the other hand I have created numerous ground level planting beds simply by covering the existing sod with newspaper/cardboard and covering that with shredded leaves and that has killed the grass growing there and left the soil in a very good workable condition. There is nothing wrong with clay that adequate amounts of organic matter will not help, in time. Texas A & M does a good job of soil testing, and you have the added benefit of having a resource readily available to answer questions from both the horticulturist and maybe Master Gardeners. Visit the campus and you will find there are demonstration and trail gardens, not just farm studies. In addition to the soil tests done by Texas A & M you might want to dig in with these simple soil tests, 1) Soil test for organic matter. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. For example, a good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top. 2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drainsÃ¢ÂÂ too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up. 3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart. 4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer your soil will smell, to9 a point. Too much organic matter can be bad as well. 5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy. to see what your soil looks like.
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You need almost a cubic yard of soil per bed. That's a significant investment if you are going with some custom soil mix. But it is really not necessary to get too carried away with what you fill the beds with. All you need is soil - like ordinary garden soil - and some organic amendments like compost or composted manures. If you don't have excess soil sitting around at home - and who has 6 yards of soil going begging for a home?? - then search out some bulk soil suppliers in your area. Most will carry something like a "garden" or "landscape" mix that will suffice nicely, often being an OM enriched product straight from the supplier. Even plain old topsoil (whatever they choose to sell under that vague designation), as long as not filled with rocks and roots, will work perfectly well for a raised bed as long as you incorporate a fair amount of OM in the mix. Focus on the bulk suppliers - bulk soils are far less expensive than bagged product and you would need a ton of bagged stuff to arrive at 6cy. For a raised bed soil, you don't need peat moss and you don't need vermiculite. Peat moss will increase soil acidity, which may not be desirable, and can retain excess moisture. Vermiculite is not advised for any inground (or even container) plantings as once it becomes saturated with water, it collapses and impedes good drainage. And it is an unnecessarily expensive additive as well. Just plain ole "dirt" and a good quantity of OM - 10-15% of the mix - will provide a very successful start to a raised bed garden.
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Amy Wickett
Original Author
5 years ago
last modified: 5 years ago
If I were to use the raised beds, one of the crops I would plant in them would be tomatoes, peppers, and eggplants (not necessarily all in the same bed).

When Preparing Tomato bed apply 3 lbs. of tomato tone per 50 sq feet and work into top 4”-6” of soil.

I found a PDF for tomato tone, and these are the instructions it gives for how to use it:

For Single plants mix 3 T (tablespoons)of tomato tone into the soil when planting

For potted plants, mix one part tomato tone to 30 parts soil mixture (1.5 cups tomato tone for every 8 quarts of soil)

FEEDING:

Apply tomato tone after plants are well established (10-14 days after planting) and then twice a month during growing season (May-Aug)

ROWS: 1 cup each side per 5’ of row

Single Plants:3 T per plant

Potted plants: Apply 1.5 t per 4” diameter pot (1.5 T per 12” diameter)

The thing I am confused about is it doesn't give any instructions for how much to use in a raised bed; the closest I get is 'apply 3 lbs. per 50 sq feet'. Is the area of a raised bed (by 'area' I mean how much soil it can hold) square feet or cubic feet? I am pretty sure it's square feet, but I can't remember, so I wanted to double check to be sure.

Also a really dumb question but what do you mean by 'top soil'? There are bags labelled 'top soil' and other bags labelled 'potting mix' (potting mix would more akin to miracle gro/something similar). Which one should I use?

Also how much can you actually grow in a 4' x 8' raised bed with plants? I realize this depends on what you're growing in the raised bed but for me it'd be veggies. The veggies (no I am not growing to grow all of them at once nor am I going to grow all of them in the same bed) would be:

*Greens (lettuce, spinach, arugula, watercress, baby kale)

*Brassicas (cabbage, napa cabbage, cauliflower, broccoli)

*Root veggies (turnip, daikon [large white Asian radish], carrots, potatoes)

*Zucchini

*Winter squash (only one variety, butternut)

*Tomatoes (slicers, cherry, and romas/paste) (please specify how many plants, both trellised and untrellised I'd be able to grow in a 4' x 8' bed; most of mine are indeterminate varieties, but some are determinate varieties, specifically the cherry tomatoes)

*Eggplant

*Peppers (both sweet bell and hot chile varieties)

*Snap beans (bush type)

*Snap Peas (both pole type and the variety that don't require trellising)

*Herbs (flat leaf parsley, rosemary, basil, culantro [it's similar in flavor to cilantro but it's NOT cilantro, it's much stronger in flavor])

Also I am wondering, can you grow corn in a raised bed? I know it's generally not advised to grow it in a raised bed, but the variety I have is a dwarf type (by dwarf, it's 2'-3') and says right on the pkt that it can be grown in containers.
tapla (mid-Michigan, USDA z5b-6a)
5 years ago
Admission - I didn't read anything but the OP.

You're going to want something with enough mineral content that it doesn't break down in a couple of years and gas off as CO2, leaving you with an amazing amount of shrinkage.
This ^^ is what's in my raised beds and it's amazingly productive. Tilth is excellent. Anything I've added over the years (pine bark), has been added as a top dressing to avoid N immobilization. It's made of Pine bark, Michigan (reed/sedge) peat, unscreened Turface MVP, pine bark, builder's sand. I wish I'd have added more inorganic material when I made it, but over the years I've corrected that issue by dumping used gritty mix from my bonsai repots onto the beds.

Al
Nil13 usda:10a sunset:21 LA,CA (Mount Wash.)
5 years ago
last modified: 5 years ago
Raised beds with open bottoms just need topsoil that is mostly mineral soil with 5-10% organic matter. So sandy loam.
tapla (mid-Michigan, USDA z5b-6a)
5 years ago
I agree that's all that's needed, but I can say that what I've been observing is remarkably superior growth compared to what's in the soil in the rest of my yard, which is the same as what's under the 8" deep RBs ..... and I'd rate that soil as excellent. You can see that things aren't suffering much in the underlying soil.

Al
Nil13 usda:10a sunset:21 LA,CA (Mount Wash.)
5 years ago
Of course plants grow more vigourously when they have a surfeit of nutrients but too much organic matter just leaches those nutrients into the ground water.
tapla (mid-Michigan, USDA z5b-6a)
5 years ago
There's nothing in the beds that can't be found in a forest. No guilt here.

Al

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