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Any Other Topics: Is blended soil to BS3882 okay for using for alpines in a very large raised bed?

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Started by: Paul Lewis

Go to latest contribution by Paul Lewis, 31 July 2011, 22:46. Go to bottom of this page.

Contribution from Paul Lewis 31 July 2011, 10:50top / bottom of page
I need some advice!

Hello to fellow AGS members! I am after some advice from members in the sun-baked south east!

I have recently moved to North Kent from North Wales and I am creating a new alpine garden for my girlfriends family. The area receives low rainfall (650mm per year on average) and the underlying rock is chalk. I am in the middle of a large project to renovate a very large raised bed and to create a large alpine planting scheme. The bed receives full sun for the majority of the day, as it is south facing. Part of the bed receives some early morning shade. The bed is 19 metres long and ranges from 2.3 to 2.8 metres wide. The raised bed did contain a variety of mature conifers, which had exhausted the soil and little else would grow in the bed. These have all been removed along with about the top 15 to 2 inches of tired root-filled topsoil. Underlying this soil was a fill of pure chalk rubble ranging in size from 20mm to pieces the size of a football. This will provide a good underlying drainage layer. I have a large quantity of of Kentish Ragstone with which to landscape the bed and will be having several tonnes of 6-10mm ragstone chippings delivered to dig in and also top dress the planting scheme. I have estimated that in order to refill the raised bed with a suitable growing medium, that I will need to have about 15 tonnes of 'soil mix'. A local company which provides skips and recycled aggregates can provide a blended soil mix for 11.50 a tonne plus delivery. This 'blended soil' contains recycled topsoil, sharp sand and organic compost and apparently conforms to the British Standard BS3882. I have had a sample of this soil and done a PH test on it. It is markedly alkaline (above PH 8). I also potted some up and did a drainage test on it. It seems to drain okay, but not as well as other compost mixes I have made up for alpines. Also the blended soil has quite a clayey smell and smells a bit sour? The soil will squeeze into a fist sized ball when wet and hold it's shape, but will crumble again when dry.

Obviously we are in a very low rainfall area and the conditions will be vastly different from my projects in North Wales, where I have always 'majored' on drainage for any alpine scheme! Also this soil mix will be overlaying very sharp draining chalk and I will be digging in lots of ragstone chippings into the mix.

Sorry for the long post, but I would like some advice on the blended soil and particularly the BS3882 specification.

I am also concerned that the recyled topsoil may be quite clay based, hence the smell?

Any advice would be gratefully received before we go ahead and purchase many tonnes of soil!!

Thank you

Paul. Membership Number :-080158F

Contribution from Tim Ingram 31 July 2011, 18:24top / bottom of page

Paul - what a fantastic project! I would advise that you talk to Mike and Hazel Brett of the Mid Kent AGS Group who have a very extensive, and successful alpine garden at Loose in Maidstone (see 'Old Orchard' in the Kent NGS Yellow Book). I would think that the soil mix would be OK but would need a large amount of chippings mixed in (ca. 50%) to create more of a scree, and then a good deep top dressing with pure chippings. So much depends on what you want to grow and another approach may be to have different areas for different types of plants. Thus there are quite a few that would grow in pretty much pure chalk (Frederick Stern's book 'A Chalk Garden' is a great reference); on a small scale for choice alpines I have made a sand bed (described elsewhere in the Online Discussion, and stronger plants would do well in a gritty loam mix. David Hoare, well known for growing saxifrages, gardens on chalk and has a nice bed planted up with silver saxifrages, pretty much in the natural garden soil. I'm not sure the British Standard Number means a great deal in the context of making a bed for alpines (apart from conforming to the general standards for planting schemes). Personally I have always had compost, either in bulk or bags from Eden Park at Sittingbourne and found them very reliable and of high quality (but possibly too costly for your project?).

Contribution from Paul Lewis 31 July 2011, 22:46top / bottom of page

Hello Tim. Thank you for your detailed response to my post. I would be very interested in seeing Mike and Hazel Brett's garden! I have only been down in Kent for a few weeks so I am just getting my bearings and finding out more about the area. As well as growing alpines as a hobby, I also work as a landscape and garden designer. The project to revamp the large raised bed is a very exciting project and as it is in a prominent position at the front of the house, it will be a good advert for my work.

I have been told that the best place to source a bulk amount of Kentish Ragstone chippings is from Gallaghers near Maidstone.

I think I will go ahead with your suggestion of mixing more chippings into the soil mix. I will order a lesser amount of the blended soil mix and a larger amount of ragstone chippings. I think the rotovator is going to come in handy!! Thank you again for your advice. I will search out that book you recommended. I will also post some pictures as the project progresses.


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