A question I recently received was about raised beds. An elderly lady was revamping her garden and was advised to make raised beds for ease but was concerned about the soil, asking ‘how often do you have to replace all the soil, as I wouldn’t be able to dig all the soil out every year.’ The purpose of the beds was to grow veg including ‘runner beans, broad beans, peas, carrots, beetroot, spinach and red onions.’ She also wanted to know which could be grown together in the beds.
Raised beds do seem to be the only way to grow vegetables but it is not essetial. What they do allow is easy organisation and you can improve the depth of the soil or allow the soil to be ammended to suit a wider range of crops. The term ‘raised bed’ covers a number of different structures from brick-built beds up to 1m high to allow gardening from a wheel chair to simple wooden-edged beds 15-20cm above soil level.
Most vegetables and all that this person wanted to grow, will be happy and productive in pots of multi-purpose compost and would grow in raised beds filled with the same stuff. However, because the beds all vary in height it gets complicated. It may be best to start with the filling material.
Native soil. It may be possible to fill the bed with soil from the garden. This is the cheapest option but there may not be spare soil to use. It may be that it has problems that the raised beds are trying to overcome, such as being heavy clay or full of weeds. The soil can be amended by a mixing in organic matter or sand.
Topsoil. If you buy this in bags it is hideously expensive. The only practical way to buy it is in ton bags or by the lorry load. But lots of rubbish is sold as topsoil and it must be seen before you buy it. I got some topsoil for my ‘alpine’ raised beds and saw it in a large barn but even so it contained tiny pieces of vetch and mares tail root that I am having to deal with. I suppose it could have been worse. But be beware.
John Innes compost. This is the perfect choice but not practical unless you have won the lottery.
Multipurpose compost. This is light and may be affordable but it will decompose with time because it is wholly organic matter. It is not ideal.
It is not a great idea to make these beds with the aim of replacing the soil every year or few years. It is better to aim to improve it every year and treat the soil as you would the soil in your garden.
My own raised beds are filled with native soil which I am improving every year with organic matter. In only a few years it is getting more manageable. If, like me, you have heavy, ‘square’ soil you could cover it in multipurpose compost to sow into and then dig it in at the end of the year.
The depth of soil and the height of beds is important too. If the beds are on soil they only need to be 15cm high. This allows you to improve the surface, where drainage will be perfect, and there is soil underneath that plants can grow into and get moisture. This depth is fine for all plants including rootcrops. If you make beds much deeper you then run into problems with watering in summer and tall, brick beds are going to need lots of watering. Most veg do not need soil 1m deep either so you could fill the lower 30cm or so with rubble but that is only going to make watering an even bigger issue in summer.
If the raised beds are placed on a hard surface such as paving or tarmac then they should be 20cm or even more in depth because the plants will be unable to reach lower levels of soil to reach moisture. Either make the beds deeper or use pots.
So, to answer the question directly, you should not have to replace the soil at all, though if you fill beds with multipurpose it may be necessary every three years. And all the plants should grow in the beds together.
Suttons Seeds Giveaway
For UK readers only – Suttons Seeds are giving away collections of six packs of their 2023 introductions. These are: yellow sunflower ‘Carmel’, cosmos ‘Apricot Lemonade’, Coreopsis ‘Incredible Seashells Mixed’, climbing bean ‘French Sunshine’, Tomato ‘Lily of the Valley’ and round courgette ‘Boldenice’.
You can enter by visiting here
That name of ‘Lily of the valley’ seems very strange for a tomato and because it is supposed to be a French Variety I had a look and it seems that ‘Brin de Muguet’ is indeed a French variety. I am not sure how the name is translated though because Googletranslate does indeed give, as I thought I knew, lily-of-the-valley as the translation for muguet but for Brin de muguet it came up with ‘a bit of thrush’. I might stick with ‘Sungold’.
‘Brin’ translates as ‘strand’ so maybe the name derives from the fact that the small tomatoes resemble a stem of the red berries of lily-of-the-valley in which case the name is even more dodgy since they are decidedly poisonous. I am sure the tomatoes themselves taste delicious.