[johnny5]

Hey, guys & girls another newbie to the site. I am preparing 3 6ft by 3ft raised beds for the new year can someone please tell me the best mix of substrate for these please, my back yard is totally concrete so i am starting from scratch.  thanks johnny5.

[Yorkie]

Welcome to the site 

I'll move your question over to Grow Your Own as it's about growing your own 

Feel free to pop back into the Welcome forum to introduce yourself 

[SG6]

From the description you are making a square container on top of concrete. No interface to the ground (worms). OK?

You will need a reasonably heavy compost, soil based. The material will have to retain the nutrients and the light stuff, potting, seed compost etc, simply is too light. Also they tend to be a fairly inert material with nutrients added, which pretty soon are used or washed out.

May be best to mix top soil with manure (well rotted), also throw in a fine gravel for drainage. Thinking here is that whatever capacity it will be could be expensive to fill with lots of JI No3.

Problem will be maintaining the nutrients, when it rains or you water it then some will get washed out and need to be replaced over time. Plants will take up their share also. A bag or two of rotted manure dug in over the winter should solve it, also things like chicken poo pellets.

Not sure of the intended depth, so cannot say if you should consider a  base layer of any sort, then the "growing" level, although in reality these will get mixed over time I would say. If less then 18 or 20 inches then just the same mix throughout.

As long as whatever you select as the main ingredient is sufficently "heavy" then a lot of the remainder is simply to add nutrient and to get a structure that is easy to grow in and  light enough for hand tools as I suspect that these are what you will use most of the time. Don't forget the gravel or course sand however. It helps drainage but also maintains the structure of whatever you arrive at.

[savbo]

are you going to line them with weed fabric to stop the soil washing out? tricky to get a good seal between raised bed and concrete...has a brick bed in my first house and the mortar kept breaking down and the soil would dribble out...

[Yorkie]

Surely you don't want too good a seal - raised beds should always have some drainage, firstly for the sake of the plants, but also because wet soil is very heavy and will push outwards on the walls of the bed if it cannot drain properly.  If you look at retaining brick walls, they have gaps in the mortar to allow this to happen.

[johnny5]

Thanks for your replies, all this will be taken on board, spoke to my friendly local stable manager well rotted manure is on order. If anybody has any more info please feel free to reply.