OK, you have built your raised bed but now you have to fill it with a soil mix. This is where you need to be thinking before you build your raised beds of how deep to make them. A 3 foot deep bed takes six times the soil mix than is needed to fill a 6 inch deep bed so get your planning right at the start. I am going to discuss what I did when I sent my first High Density Gardening raised bed up.
My beds are 6 inches deep and I am amazed at what I grow but these are on the top of an old lawn. I would think about a deeper bed than this if you are building on a concrete base.
Gardening
If you followed the square foot gardening method, it recommends a third peat, a third vermiculite and a third home made compost. If you want to be organic you should not use peat and although it is 100% organic it is not a sustainable product. The other problem you will have on first starting up your raised bed garden is that you are unlikely to have any home made compost. Finally, vermiculite is made from a rock and is heated to a high temperature to expand it into the state we are all familiar with. I guess this is organic but the use of the high temperatures, over 1000 centigrade is not very environmentally friendly.
What I did when I built my first raised bed in my High Density Garden plot was to create a mix of the following ingredients. Top soil, peat, coir and well rotted horse manure. I mixed these in roughly equal quantities of a quarter each.
The top soil is just soil I had lying around from earlier gardening projects but I also bought a couple of bags from a local garden centre to finish the top of the raised bed off. However, do not make the same mistake I made. My father decided he would help out by brining me some top soil. He lives about 20 miles from me and what happened is that I ended up with weeds in my raised bed which do not grow round here. The topsoil I bought in bags had been sterilized so there were no weeds there.
The peat I used was old peat I had used in the greenhouse to grow tomatoes and cucumbers in. I know I should not use it and this year I have tried organic growing medium but I am not as impressed with it as with peat. I only used peat when setting up the raised bed vegetable gardens and this was peat I had used in the greenhouse to grow tomatoes and cucumbers in. I now grow my toms and cucumbers in organic growing medium and reuse this in the garden as well.
The coir I used is great stuff and is made from the waste product during coconut harvesting. It is 100% organic and is fully sustainable. I buy it in compressed blocks and re-hydrate it. The coir also has a great moisture holding capacity and this helps in your raised beds.
I used well rotted horse manure in the bottom of the beds. This is put down in a thin layer and then the other ingredients added on top and I just mix it all together as best as possible but at the same time trying not to disturb the horse manure in the bottom of the bed.
Every time I now add a new crop I just mix some of my home made compost in to the soil mix which does two things. The first is that it adds a source of food for the plants and the second is that it make up for settling of the soil mix and soil mix lost on plants.
By using a soil mix like this one it is very light and very friable or easy to work. Plant roots can easily grow down into your soil, moisture gets down to the roots easily and the added coir helps to retain moisture in the soil mix and the final thing is that it is very easy to pull weeds out as they do not have a good hold in this light soil mix when young. If you get the soil mix for your raised beds right it makes gardening much more enjoyable and easier and your crops will be healthier for it.
One final point is to get your home made compost pile started as you will need to use this in the future to add to your soil mix. You can download a free worksheet on how to build a hotbox composting bin from the High Density Gardening website here.
Raised Bed Gardening - Getting the Soil Mix Right
Ric Wiley is an internet writer and gardener. His website about High Density Gardening can be found at http://www.highdensitygardening.com/home.html and his latest ebook is High Density Gardening.