Worms or more importantly a lack of worms.

I suspect that @Jocko has a clay soil @pete.

Mine here is clay and would form a near-impenetrable 'soil pan' with cracks in a dry summer, before I improved it.

Here is some advice from the RHS on identifying your soil type - using hands/eyes rather than a test kit. And this organic gardening article has more good advice, including on soil pans.

Repeatedly rotavating/ digging isn't going to sort this one out I would say, the problem will keep recurring unless you improve the structure of the soil. As @JWK and I said above, apply a good thick mulch this autumn and let the winter weather do the work for you. No digging required - though stirring up the surface with a hand cultivator/ border fork/ hoe before the mulch goes down can only help.

The organic gardening article also suggests growing a green manure - haven't needed to try that myself.

 

I'll say no more, as someone with clay soil, it can be a long slog but digging/ rotavating at the right time helps a lot along with compost/ manure.

 

The ground is clay. I lifted some slabs from the drive area and the holes left were wet red clay. You could have built a Pueblo village from it. I dug the clay out to plant specimen shrubs and I took bucketsful of it down to the bottom of the garden and built a spoil heap. I treated it over the winter with Horticultural Gypsum and that broke up the top reasonably well. I sowed wildflower seeds on the top and they did well.
I can no longer rotovate as the entire area is now planted with shrubs, perennials, spring bulbs and currently annuals dotted in between. My intention is to put down manure. I even thought of digging some holes, about a foot deep, and filling them with manure to try and attract worms

 

Sounds like top dressing is your only option now then.

 

I am getting some pelletised Lime and I wondered whether to put that on first then the manure later. Being clay the ground is sour with a ph of about 4.5.

 

4.5 is pretty acid.

 

That is why I thought of liming it first.

 

Usual to let lime wash in over winter, but as you have plants growing its not that simple.
Probably better to raise the PH gradually over a couple of years or so.

 

So I will give it a light dressing of lime, let it wash in, and then topdress with manure?

 

As far as I know the two will work against one another, manure tends to be acidic.
Not sure how much lime you will need, it's not just slightly acidic at 4.5, I think it could be a long job.

 

It will definitely be a long job.
But keep at it. Keep adding manure or compost and by working it into the ground you will gradually improve your soil. They aim should be to add as much organic matter as possible so the worms will get some food.

 

Just a thought, mushroom compost is usually considered alkaline, I believe, might be better if you can get some.

 

Clay is usually neutral due to its incredible ability to lock up ions across it's crazy huge surface area, and being made up of largely inert rock particles.

Are you sure it pH 4.5? That would be enough to cause significant skin irritation if you were in contact with it too long.

 

Clay soil is generally acidic according to the booklet that came with the Soil test kit.
As to 4.5 causing "significant skin irritation" I Googled it and found this: "A normal vaginal pH level is between 3.8 and 4.5, which is moderately acidic.". You learn something every day.
This morning the boss pointed out that the Flannel Bush was wilting (no, it's not a euphemism) and it is in the most compacted part of the shrubbery. I punched foot-deep holes around it with an aligning bar and filled them with water then once it dried in I filled them with compost.
I tried breaking up the surface in the vicinity using a three-pronged cultivator but it barely scratched the surface. I, therefore, gave the area a good top dressing with manure and later in the day I gave the whole area a good soaking using the hose.

 

Shred as many good quality autumn leaves you can get your hands on and mulch away. Any self respecting worm loves a pile of decomposing leaves.