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Question: My spouse just built me a new raised bed. I am so excited. It is still empty of soil. Can you suggest a way to develop my own soil without having to buy expensive bags of fertile soil at the store? My bed is 12 feet by 4 feet in size and 18 inches deep. How much soil will that take?
Answer: The cost to fill a raised bed with bagged fertile soil (planting mix) adds up quickly. The volume of soil you need is 12 feet times 4 feet times 1.5 feet (length times width times depth equals volume), which comes to 72 cubic feet.
The larger bags of fertile soil mix usually are sold in 1.5- to 2-cubic-feet-volume bags. To fill your bed, you’d need 36 bags of fertile mix, which cost at least$5 to $8 per bag at most stores. You could spend $200 or $300 if you go this route.
Bulk soil delivery from a landscape supply place delivers fertile mix in units of cubic yards. One cubic yard is 3 times 3 times 3 feet or 27 cubic feet. Your bed has a 72-cubic-foot capacity, so you would need to order almost 3 cubic yards of soil. It is at least $30 dollars per yard plus a delivery price, which is cheaper than bagged soilbut still a financial bite. I’d say the amount of bulk soil you need delivered would cost you well over a hundred bucks.
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Making your own soil is way more economical. But what you don’t pay in money, you’ll have to pay with patience. But in return, you’ll get satisfaction and be way more earth-friendly if you develop your own soil from materials at hand.
Where to start? Well, here’s how I’ve done it. Over the years, I’ve filled several of my raised beds this way. I use a kind of modified hugelkultur method. Hugelkultur is a German term meaning “hill culture” or “hill mound.” This method originated in Europe. It is a way of composting both woody and non-woody organic material in the bed while you grow things on top. There is an American version of this style of garden-bed building called “lasagna gardening.”
Whatever you call it, here is how I have done it, several times. It works for me.
First, I line the bed with hardware cloth (to keep burrowing mammals out) and then with weed barrier cloth.
Next, I go to my pruning and brush piles that I have, and I take a couple of large wheelbarrows of coarser material and put that on top of the hardware cloth and weed barrier cloth-lined bed. It fills the bed up to about 4 inches.
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Then I pile thin layers of leaves, grass clippings, straw, coffee grounds gathered from my local coffee shop, good wormy compost, some of my native soil and some lime or dolomite sprinkled in there.
Finally, I top the bed with 2 inches of nice fine-textured fertile mix that I purchase in bags from BiMartor somewhere like that. Then I can plant. It usually takes about two to three bags of purchased fertile mix (1.5 cubic feet each) to cover the bed surface to a depth of 2 inches.
Proponents of hugelkultur claim that you can use cardboard, petroleum-free newspaper, manure, branches, bark or whatever other organic material you have available, as long as you top your bed with finished fertile soil to plant your veggies or flowers. Avoid using wood that is toxic to plant growth or human health, such as pressure-treated wood, black walnut or cedar.
The woody stuff breaks down in a year or two if you only use finer branches. I always have to remember not to dig too deep or try and turn the bed over the following year, as some of the coarse material might still be decomposing.
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Fans of this method claim that the advantages are many. The woody material is slow to break downand releases nutrients for a long time. The decomposing material can generate some heat and warm your soil. Soil drainage and aeration is way better than using the parent soil most of us have in our yards. Since there is so much organic material, it will hold on to moisture longer and need watering less. Plus, this method is dirt cheap.
To learn more about hugelkultur, see Permaculture magazine’s article: permaculture.co.uk/articles/many-benefits-hugelkultur. And, a few years ago, some Polk County Master Gardeners made a comic book about how to do lasagna gardening, online at extension.oregonstate.edu/polk/sites/default/files/PCMG_Organic_No_Till_Gardening_Comic_Book.pdf.
Carol Savonen is a naturalist and writer. She is an associate professor emeritus at OSU and tends a large garden in the Coast Range Hills west of Philomath with her husband and dogs. She can be reached at Carol.Savonen@oregonstate.edu or c/o: EESC, 422 Kerr Admin. Bldg., OSU, Corvallis, OR 97331.
