Builders Gypsum/Plaster

Is builders gypsum suitable for use as a soil improver? As opposed to buying more expensive horticultural gypsum.

Specifically I'm looking at 'British Gypsum Thistle Board Finish 25kg' sold in Wickes for £5.48 per bag.

Is there likely to be any additives or differences between the two types?

Posts

  • Allotment BoyAllotment Boy North London Posts: 1,814

    Oh yes there will be a lot of difference, the builders one will have agents in it that help it to bind & cure & so set solid, it will also be very alkaline. It will ruin your soil- its a similar problem with builders sand vs horticultural grade the latter is heavily washed & cleaned so there is no salt in it. 

    AB Still learning

  • Thanks for the reply.

    Do you know if any of the main DIY outlets i.e. B & Q/wickes/Homebase etc sell horticultural gypsum?

    Only iv tried a few searches online with the big chains but it only seems to return results for the builders type. I don't really want to buy from an online retailer.

  • Allotment BoyAllotment Boy North London Posts: 1,814

    You may find it in the Garden section it is often sold as "Clay Breaker" but usually in quite small 2 or 3 kg boxes. I take it you have heavy clay to deal with if you have a large area to do then horticultural grit dug in will help and ironically so will manure & compost. I say ironically because many people think that as clay holds water the last thing they want is something else that does the same but in fact its the best method. There are several other threads on the site on the same topic.  Using gypsum alone will break the bank if not you. 

    AB Still learning

  • Thanks for the info Iain. 

  • Maddangler, Gypsum is the hydrated form of plaster, (calcium sulphate) so it is a fact that ’when the dry plaster powder is mixed with water, it re-forms into gypsum.’ Sorry I know my Chemistry :) ... you can check via a quick Wikipedia search too... so as long as there are no additives in that Wickes/B&Q mix (ie not a Bonding poster) then it is absolutely fine to use.

    If you have a clay soil like I do then mixing a dried (unhydrated) form of plaster you’ll find it hydrates itself easily in contact with damp soil, which then is ‘Gypsum’... basically what I did a few years back was turn 15-20 cm of soil and liberally spread the plaster, then turned the soil again trying to break up any large chunks of clay, I also added a sharp grit/gravel mix to aid aeration along with some very mature horse manure and worm castings from a Fishery — this was no mear feat (plus a couple of trips to the Physiology )

    My soil has never been so easy to crumble, no more gray glacial clay chunks!!  ** Please bear in mind that if your clay soil has lumps of chalk (think blackboard sticks) then neither gypsum (aka. Plaster of Paris, aka plaster) will work as it is the Calcium which causes flocculation - grouping of particles into small grains with air spaces between. 

    in which case you’re left with grit/gravel and humus to work in.

    It is a massive undertaking to change the property of a soil both in cost and labour. It would always be easier/cheaper to plant the trees/bushes and plants which are adapted to growing in clay, but with determination it can be done! Good luck

    Last edited: 15 February 2018 23:07:31

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