[calypsored]

Hi

This is my first season on my allotment in East London - covered the area in October 09 with agricultural fabric to supress the weeds after turning over. Just added 6" mushroom compost/horse manure mix with each trench while double digging and removing the couchgrass and nettle roots.

The soil is still very clumpy (moderate clay). The ph tester I have is showing the soil as neutral to slightly alkaline in different places. I have read that gypsum in its agricultural form can improve soil texture without altering pH values but I cant find no reasonable value supplies for the amount I'm after.

Would builders finishing plaster be an acceptable substitute. I would be using the Thistle brand and on page 3 of the manufactuers pdf for the product for the ingredients (http://www.british-gypsum.com/pdf/DS-110-01%20Thistle%20MultiFinish.pdf) it lists calcium sulphate hemihydrate as the primary ingredient without showing any additives that I can see I should avoid. 

There is mixed info not so much on the use of gypsum but on the acceptability of using buiders based gypsum products (finishing plaster, plaster board in compost etc) on the allotment. Just wondering if anyone can shed any further light.

[aelf]

hemihydrate version is better known as plaster of paris. As your soil is neutral to slightly alkaline, adding calcium sulphate would increase the alkilinity of the soil. You would probably be better off adding lots of organic matter to the soil to improve the soil texture and maintain its neutrality. I would be very wary of adding building waste - the drawbacks may outweigh the benefits.

[Rangerkris]

I have heard of people going to the plot and having a drink with lunch but never getting plastered at the plot.

[Pompey Spud]

I think you maybe getting too technical and worried about the soil.

Just keep adding the organic stuff and nature will take its course.

Rotation plans will help as well to know what parts to add manure or, to lime.

[calypsored]

Do I add lime if the soil is already neutral to slightly alkaline? I thought as this was already the pH value of the soil I should add gypsum as it does the job of lime without altering pH values.

I really was just looking to help nature along the way but as I cant find a definitive answer I think I shall just leave it or pay the additional money for the agricultural stuff if I can find a supply.

[mumofstig]

mushroom compost is quite limey, so I wouldn't have thought you needed more.

The soil will get less lumpy as you work it, anyway.

It may have been better (for the soil structure, not the weeds ) to have left your soil uncovered for the winter, so that the weather would have broken it down.

[aelf]

Do I add lime if the soil is already neutral to slightly alkaline? I thought as this was already the pH value of the soil I should add gypsum as it does the job of lime without altering pH values.

I really was just looking to help nature along the way but as I cant find a definitive answer I think I shall just leave it or pay the additional money for the agricultural stuff if I can find a supply.

I would say you do not need to add any lime or gypsum as your soil is already alkaline. Many plants prefer the soil to be neutral or even slightly acidic. As said before, you would be better adding humic material to the soil to help break it down.

[Pompey Spud]

I was refering to crop rotation plans. ie Brassicas like alkaline ie lime!!!!!!!!   

Depends whats going where.

Don't add anything else chemical! For other stuff apart from humous etc.

See...too technical and sweating about nothing.   

[JayG]

It's the presence of calcium ions which cause flocculation of clay particles and hence an improvement in the structure of such soils.

Gypsum does it without changing the soil pH.
Lime (hydrated or slaked lime) also does it but increases the soil pH quite quickly.
Ground limestone also does it but much more slowly, and will also slowly increase the pH.

Horses for courses, and don't expect it to work miracles on its own (humus is needed as well!) 

[Subversive_plot]

JayG, you are 100% correct. Plaster (Plaster of Paris) will not change the pH but will add calcium ions.

The stickyness of clay is made worse by sodium, but decreased by adding available calcium and magnesium. Gypsum (plaster) is perfect for this, and moderately soluble. Another sulfate that will also help is magnesium sulfate (Epsom salts, highly soluble).

[Deborah1]

To add my two pennorth... I think your Gypsum will do no harm, especially if your soil tends to acidic, or if you have a specific crop in mind that prefers a more alkaline soil.

Here I have a soil which loves Camellias, Rhodos, Blueberries. I grab everything limey that I can. Not for them!
Crushed oyster shells, household plaster etc. all goes on the garden for those that will need it next year. Hope this is useful.