[Salmo]

Doing a pH test at different levels is a good idea but while acidity underneath may explain poor long term growth it will not explain bad germination. Non digging over a number of years can sometimes result in an acid surface layer due to leaching and organic matter rotting on the surface.

You may well have soil pests that are not obvious. I would put my money on pests rather than any soil problem. I may be wrong.

Apart from the usual suspects such as wireworms, millepedes, leatherjackets and slugs, there are tiny invertibrates such as symphalids and springtails which sometimes feed on the root hairs of germinating seedlings. They usually live peacefully in the soil living on rotting vegetable matter.

I know a little about symphalids and less about springtails. Google images will show you what they look like

Symphalids are white and shiny, about 3mm long and can be mistaken for small millepedes. Most of the year they are deep in the soil but in the Spring, when the soil is warm and moist, they move into the top layers where they feed on young roots. In small numbers they do little harm. Where and when they appear is unpredictable but sometimes they are in hugh numbers and can decimate crops.

Take a shovel of soil and drop it into a bucket of water. Pests will usually float to the top.

Pests can be very fickle and straight lines can easily occur due to slight differences in moisture, pH or organic matter.

[ex-cavator]

What was there historically? How long has it been allotments? You say there's gravel beneath - has there been extraction in the area / if so has landfill been associated with it? Check out the historic landfill maps on the Environment Agency website, also check www.old-maps.co.uk for any clues 

[Wild Pony]

To me in my own personal opinion, it soundss like the self seeded inhabitants have drained the life of the soil.
A good 50/50 mix of cow and horse manure can help as can calcified seaweed to release micro nutrients.
Cow manure is a cold slow release manure and horse is a hot manure producing rapid effect.

Another manure that is very useful, if you can get it, is rabbit! So if you know of someone with loads of bunnies and they clean out onto a compost heap ask if you can ease their burden.....by alot. Rabbits pass them re-eat their droppings and so pass a dryer cast, and its very coarse and straw or hay based.

Sheep/goat manure is also very good.

But improving the actual structure of the soil so it can retain then release nutrients is what you're after.

Again this is only my humble opinion.

[Guzzik]

Discovered the reason this morning.............................let me just say that some loser was a wee loose with a spray bar....................................Catch my DRIFT?

But, once more, my deepest thanks for all your time and effort in helping me resolve this.  You are all stars.

[Wild Pony]

No worries...go head your "spraybar" his direction. Cor! some people......