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Getting a good soil mix for large planters

I have built some extra large planters, which are built into a deck frame i've built. Ive taken the inspiration from terrace gardens where they will build a large planter to add interest to the side of the decking. 

I have one large planter at the moment complete, its 3m long, 60cm deep and about 50cm wide, so its a good sized planter. Its been plenty reinforced, and to the sides, a 1200gauge poly sheet has been added to retain the ingretity of the bed, and also to protect the wood. The sheeting only protects the sides and laps at the bottom however, it is open to clay at the base in case of any root development or drainage needed. 

So far, i've had some old pea gravel hanging about, so i've filled the bottom of the planter about 10-15cm deep which should aid drainage but I'm now looking at what to put in it, so that come next year when it is filled with plants it'll be well fertilised soil. 

My thoughts are to buy a load of well rotted manure and pile it in, perhaps with topsoil. I have loads of clay soils which I've dug out of the garden, so wondered if I could pile that in with the manure to break it all down into good soil? Just another way to avoid wasting the heavy clay soils and to avoid having to replace it with topsoil? That may or may not be a good idea if it wont break down the heavy clay. I also have some sharp sand but understand that you need a pile of it to make a difference in heavy clay.


  • FairygirlFairygirl west central ScotlandPosts: 35,347

    The bed sounds good cc and similar to the raised beds I've made. Mixing the manure, topsoil and some of your garden soil will be a good start and I'd also add some compost to get a nice mix. It depends what you want to grow of course, but that will allow you to grow most things. If you're worried about drainage, add grit to the mix rather than the sand, though raised beds tend to drain quicker anyway. I've always gardened on clay and find this the best way of lightening the soil to allow me to grow a wider variety of plants.  We get a lot of rain so it's vital to stop plants waterlogging through winter and spring in particular. You could just mix some of the pea gravel through as well rather than just having it in the base  image

    It's a place where beautiful isn't enough of a word....

  • Steve 309Steve 309 Posts: 2,753

    If you're on clay, it might be worth thinking where the water's going to go when it drains out of the bed/pot/planter.  You won't want a big puddle, and you won't want to lose the dissolved nutrients it's picked up on its way through the bed.

    In one of my gardens we're growing squashes in the enriched soil next to the compost heap, which gets all its run-off.  The plants are enormous!  The intention is to put rhubarb there in a month or three.

  • Well I've got a drainage channel running underneath the deck one way to drain away pooling water however next to the planter is a border so likely some will drain into there.

    I've sourced some 3 years old horse manure (about 20 tonnes of it!) So needless to say I'll be getting plenty. I'm going to mix it in with the clay soils I already have rather than buying topsoil, and leave it over winter. Should I mix it up or layer it?

    I'm planting trachelospermum in 2 planters, to grow up a wall, bamboo and ferns in another (with maybe flax or similar) and an Acer in the other.
  • FairygirlFairygirl west central ScotlandPosts: 35,347

    Hi cc, it won't really matter how you apply it if it's all nicely rotted down. image

    Just be careful with your bamboo. If it's a clump forming one it shouldn't be too bad, but the other kind are quite invasive. I've had clump forming ones on heavy clay soil and they were never a problem, but on lighter soils they can get bigger and could take over your raised beds. If you can make a solid barrier round it with paving slabs on their edge or some extra timber fixed securely across the bed, to make a mini bed of its' own, that will make life easier. 

    It's a place where beautiful isn't enough of a word....

  • Clumping is definitely what I'm looking at. Fargesia Rufa is probably the variety i'm aiming for, with the height and aspect all considered. 


    I havent picked up the manure yet, but it'll Thursday now when I start working on the beds. 

    I wont be buying any plants until spring

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