gypsum

Brendan says...
G'day rs,
Try to buy 'natural' gypsum, some isn't. But saying that, I've never had a problem with ANY gypsum. It's hard to overdo it, IMO.
All gypsum is clay-breaker.
For faster results, use liquid gypsum.
I'd use both :-)

john says...
If you don't understand the cation exchange mechanism of gypsum then you will not understand where gypsum is inappropriate and you will waste money. Ringelstrumpf , gypsum is of no use in the BM.

Violet_Cactus says...
Gardening Australia has a fact sheet on how to test your clay soil to see if gypsum will work on it.
http://www.abc.net.au/gardening/stories/s1676965.htm

Presenter: Angus Stewart, 01/07/2006

Angus learns how to test clay soil

Clay often gets bad press in gardening books. But clay soils can be the best in the garden, depending on the type of clay and how it’s managed. By understanding clay and what gives it its unique properties, it can be turned into a beautiful growing medium.

Soil scientist Simon Leake is passionate about clay. Not only can he explain its make up he can show some simple tests to improve clay soils in the garden.

Simon says: “Clay is a special mineral. It's like flat, little, platelets, with an enormous surface area. For example a gram of bentonite clay has the surface area of a football field. It also has a weak negative charge which is the key to its nutrient and water holding ability.
“Sands and silts don't have that negative charge and that huge surface area, which explains why their water and nutrient holding is so much poorer than clay.”

Apart from knowing all you would ever want to know about clay, Simon also runs his own soil testing lab, where he has spent over 20 years solving soil problems.


He says the first test on soil is called the aggregate stability test and you can do it at home.

“Dry any little crumbs of clay and make sure they are nice and dry. Then get a jar or petri dish of good quality water. Drop the crumbs into the water and watch their behaviour. A halo of clay or milky cloud occurring around the aggregate is called dispersion. They magnetically repel each other to form a milky cloud. And that's an indication of problem clay.

“The next behaviour and you've got to watch closely to see - is the way the little aggregates fall apart. That is called slaking. Under heavy rain it will all fall apart and form a crust, or slake. It's the second worst kind of clay behaviour.

“The particles are not repelling each other to the point where they go into solution but are just falling apart.
“The next test is a gypsum requirement test to determine how to stop that dispersion.

“To do this get some clean jars. Put some soil in clean water and shake it until it goes milky. Then stand it for about five or 10 minutes. If it doesn't clear in the water, chances are it is going to respond to gypsum.

“Do the test adding some gypsum. See whether the gypsum makes the clay flocculate, or clump together.”

“Where the soil falls apart you need to add organic matter. Red or your yellow clays are more likely to slake and fall apart, whereas lots of organic matter and dark, black coloured clay are more likely to stay together.” According to Simon, organic matter represents the glue.

When using gypsum on a new clay start with about 200 grams per square metre. That's about two cupfuls. Dig that in, and do the gypsum test again. “If it settles out in the jar within five or 10 minutes you don't need any more,” Simon says.

So….clay doesn't necessarily have to be a bad thing. With some simple testing and the right sort of treatment, it’s possible to turn a nasty, heavy clay into a beautiful crumbly top soil that will grow virtually anything. The secret is to throw away the green waste bin and put every bit of organic matter back into your soil.

ringelstrumpf says...
Thanks for all these answers quite interesting! The problem is two fold. dirst I really might get heavy metals in my soil second it might not work.
Yes we have BM clay, but that is mixed with fill- concrete rubble (grrr). Someone told me that they dumped fill up and down the mountains. I can't tell how many m³ of organic matter I worked in so far. Tomorrow I will make a break and do these tests!

ringelstrumpf says...
You can imagine what I do with each concrete lump I find.
Today I filled the car with pine needles, because I insist to grow blueberries, and the soil test said 8. This despite the cedar mulch I have put on, but that maybe acts very slow. Now I have a 20 cm pine needle mulch around the plants. Hopefully that helps. (Nice school holiday excursions)