Most places have about a foot of topsoil, from years of ploughing. I'm wondering if someone has stripped off the turf to get rid of the weeds at some time, but didn't think to keep it. (Best case scenario is you have a heap of rotted down turf somewhere on the plot.) Do you know what the land was before it was made over to allotments?
Clay makes really good soil, but it takes many, many years and there's not a lot can be done to speed up the process.
Most plant roots hate pure clay. They just can't get into it. Without humus in it, there's nothing for worms or bacteria to eat, so they don't go into it either. Physically breaking it up, as in double digging, helps with drainage, but sill leaves the clay lumps individually impenetrable. Usually I advise against adding manure/compost to subsoils clay, as it's usually too deep to benefit the plants, and the worms don't like going too deep either, but with your plot having so little topsoil, it won't be too far down at all. :? The compost manure has at least two actions on the clay, firstly, it physically stops the lumps from sticking back together, secondly it provides food for the worms, and their movements help mix up the humus and the subsoil on a more minute level.
Normally for double digging, you'd dig a trench the depth of your spade, but in your case that would mix up the subsoil with the topsoil, so just dig down to the subsoil, (put the topsoil in a barrow or on a tarp) shove a load of manure or compost, (or whatever you can get!) and fork it into the clay at the bottom of the trench. Then dig another trench alongside the first, and put the topsoil from that into the first. Put manure or whatever into the bootom of the new trench and so on, across the plot, finally filling in the last trench with the topsoil from the first.
The other thing you can do is to grow plants whose roots can penetrate the clay, buckwheat and field beans are both reputed to do this, so I would think that broad beans would too.
Buckwheat and field beans are green manures, so if you can't get manure or compost in sufficient quantity, this is definitly the way to go. HTH 
Hi Woodburner
Yes it does help and thanks very much.
Our plot has been an allotment for years, which has been split up into three plots now – to meet demand I guess. I think that maybe the other 2 plots have got our topsoil, as they are higher than us. - So sadly no heap of rotted down turf.
Thanks for the information about the clay and how it all works. The double digging method was the way I was going to go until I found out there was no topsoil and just as you have suggested, I have only dug to the clay and no further, mind it's not easy in places not to dig it up.
Because of this problem, I have phoned the council and explained our situation, in the hope they might have some spare topsoil – and they have agreed to deliver to the allotment 2 tons later this month. I just wasn't sure how best to tackle the weeds and clay before I put on the topsoil. Also got 3 bins full of homemade compost at home to add as well.
Thanks again for your help. AG.
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May 03, 2014, 22:02