You have decided to make raised beds and now need to fill them with soil. What is the best soil for raised beds? How often does it need to be replaced? How much compost should be added? Should you mix in some garden soil? I’ll answers these and other question in this post.
Before we start, lets define the term raised beds. I discussed raised beds in Raised Beds – Pros & Cons, and defined them as having walls. I will continue to use this definition in this post.
Soil for Raised Beds
Before looking at specific soil mixtures it is important to understand the purpose of the soil. You can take two different approaches, either the raised bed is just another container or it is an extension of your garden.
As a container, you can add any number of soilless mixes. They will provide a very airy soil that dries out quickly and needs regular watering. They contain little or no nutrients and therefore needs regular fertilizing. These high maintenance soils are great for growing large plants in small spaces. Some raised bed vegetable growers like this approach – I guess it gives them something to do every day.
The other option is to treat the raised bed as an extension of your garden in which case the soil will be mostly real soil (sand, clay and silt). It will not dry out as much and it won’t need as much fertilizer. The lower maintenance may result in slightly lower yields but I am not convinced you will notice a big difference.
One big difference is cost. The second option is much less expensive initially, and even cheaper long term since the soil retains its level much better.
There are many options and mixtures that you can use. You could simple buy bags of potting mix for containers which are usually peat based with some perlite added. This will cost you a fortune.
You can also follow any one of many recipes and make your own mix. Mel’s mix, developed by Mel Bartholemew (author of the popular gardening classic Square Foot Gardening), is a popular one and contains equal amounts of peat moss, compost and vermiculite. I am sure this mix will grow great vegetables, but you will need to water a lot.
These kind of mixes have a number of problems.
They are very expensive compared to soil-based options.
Horticultural vermiculite can be difficult to find and at one point it did contain asbestos. It should be asbestos free today, depending on where it is mined. Some people report that it degrades into dust over time, loosing its ability to create an airy soil. It is not something I would add to my garden.
Peat Moss and Compost
Both of these ingredients are organic material that decompose over time. That means the level of soil in the beds goes down every year and you will need to add more annually.
One issue with Mel’s mix is the amount of compost used. Normal soil has about 10% organic matter, by volume. Mel’s formula has 60% organic matter of which 30% is compost – that is way too much. Even if it does not harm your plants all of that compost is a waste of nutrients.
Real Soil Option
The soil option fills the raised bed with soil that is very similar to what you have in the garden. If your garden soil is good, and you have enough of it, just use that. There is no need to buy anything.
If your soil contains too much clay, add some builders sand to it. If it is too sandy, add some clay soil.
If you don’t have enough soil, then buy some top soil. Compared to soilless options this is quite inexpensive but may not be available in all regions. To learn more about different types of soil have a look at Soil and Compost – Selecting the Right One. Also have a look at Top Soil, Compost & Tripple Mix – What’s the Difference?
Because soil is mostly sand, silt and clay, it will not settle nearly as much as the soilless option, but you can expect some settling to take place during the first year.
The clay in real soil will retain water much better than soilless mixes.
Add 10% compost to the soil to both loosen it up and add nutrients.
Perched Water Table
A perched water table is created when two different types of soil are layered on top of one another, especially if the particle size of the soils are different. Water has trouble going through a perched water table.
This situation can be created at the bottom of the raised bed where the two different soils meet. If you create this, water will not drain properly from the raised bed. It will trickle down to the bottom of the bed and sit there. The same thing happens in a pot or container if you place stones at the bottom. This phenomenon is described more fully in the book Garden Myths.
To prevent a perched water table, loosen the soil at the bottom of the bed, and mix the native soil with the soil you will use in the bed. This provide a more gradual change in soil particle size, preventing the problem.
Calculating the Amount of Soil for Raised Beds
Almost everybody underestimates the amount of soil needed to fill raised beds.
Bulk soil is usually sold in volume and in North America the common unit of measure is the yard. This is even true in Canada which is partially metric – but not for bulk soil and many other things. A yard is actually a cubic yard, which is the soil that fits into a container that is 1 yd x 1 yd x 1yd = 1 cu yd.
Use this formula to convert the size of your raised bed into yards.
Height (ft) x width (ft) x length (ft) / 27 = yards of soil
A yard of top soil weighs about 1,150 Kg, or 2,530 pounds. So if you are buying by the bag, a yard is 46 x 25 Kg bags, or 63 x 40 lb bags.
Bulk soil can be sold in cubic meters, which is soil that fills a container that is 1m x 1 m x 1m = 1 cu m.
Use this formula to convert the size of your raised bed to cubic meters.
Height (m) x width (m) x length (m) = cu m of soil
A cu m of top soil weighs about 1,500 Kg, which is equivalent of 60 x 25 kg bags.
Dealing With Old Soil
People using containers and raised beds are told that “soil gets old”. That is a myth. Soil does not get old and does not need to be replaced unless you have contaminated it with high levels of fertilizer or pesticides. This is especially true of real soil as opposed to soil-less mixes, but even those can be used for a very long time. They don’t get old either – they just decompose.
Used soil usually lacks nutrients and organic matter since both are used up as plants grow. The best think to do is to add a couple of inches of compost each spring by layering it on top, as a mulch. Don’t dig it in, since digging soil damages soil structure. This compost increases the amount of organic matter, which decomposes over time providing nutrients. If you follow this advice the soil will never be old.
1) Raised Beds; http://ext100.wsu.edu/gardentips/category/raised-beds/
2) You Don’t Need Expensive Soil for Raised-bed Gardens; http://www.tri-cityherald.com/living/home-garden/marianne-ophardt/article32186457.html#storylink=cpy
3) Photo source; Wicker Paradise