Very stony soil.

[dtw]

It was made by Dick Strawbridge of 'It's not easy being green' fame.
I can't find the article anywhere, but it's pretty basic.

[dtw]

[norfolklass]

from their "it's not easy being green" website:

How does the greenhouse heat sink work?
Some details are in the book, but half the adventure is working out for yourself and thereby customising you solution. All you need is:

The heat sink is imploded glass - we used bottles from various parties and the local pub - imploded courtesy of Krysteline www.krysteline.net
Big hole - we lined it with polystyrene that came as packaging
Pipes were standard waste pipes for under sinks etc
Fan was a small 12V computer fan from a market £1.20 - in a box made of scrap wood
A 12V deep cycle leisure battery
Small 11w solar panel and charge controller
Old chimney cowl for the air to be pushed out the glass

Air sucked down from the apex heats the broken glass under the floor. Fan runs 24 hours a day. As the greenhouse cools the cooler air is still pumped through the glass, but when it comes out through the cowl, at floor level, the glass has warmed it so a constant supply of warm air is now being pumped through.

[sand]

We dug loads of stones out of our patch and transported them to the tip. 

Now I'm always on the look out for big enough stones to hold down fleece and netting ::)

So keep a stone pile!

Sand