Black Spots in Lawn

Would appreciate help with lawn black spots in attached photo. My lawn is 30 years old. Black spots started to appear about 6 years ago in one corner of lawn. Now they have spread all over the lawn. The blackness does not seem to be growing on the soil. It seems to be attached to grass stems & leaves. It is a matted thing, with spongy appearance, & slightly slimy/sticky. It is worse in wet weather.
About 3 years ago I put white lime on the blackness. It cured it for a while, but it came back again. March 2012 I sprayed bluestone on moss patches, & scarified the lawn a few weeks later in dry weather. The blackness is not dead moss.
The lawn is not low-lying. It is not boggy. It is on good soil. It has a nice slope for water run-off. It does not show pools of water.
Any ideas on a cure. Any danger it will eventually kill the grass.

 

in the book 'The Lawn Expert', it states the following

quote:

Green or black slime is sometimes found as a coating on turf. The slime is made up of countless microscopic plants (blue-green algae) and is a feature of a bare and sparsely grassed surface coupled with waterlogged conditions. It commonly occurs under the drip line of trees, but you will also find it in turf on heavy soil which has been overrolled, underfed and cut too closely. It also attacks newly seeded areas on badly drained soil

Avoiding and curing the problem
Removing algal growth is quite a simple job-just water the affected area with Bio Moss killer. Routine treatment with Velvas lawn sand will also kill the algae. Unfortunately, the slime will return unless you can correct the basic cause. This calls for spiking and top dressing the compacted turf in autumn

 

Did not think lawn was compacted. Only compaction, is me cutting it for 30 years, plus kids growing up, playing on it. Re closely shorne - I have always cut it at 350 mm. Hungry - maybe yes - I only give it one dose of 7-6-17 per year, is this OK/sufficient. Sure, it is continuously damp in current wet summer. Maybe takes a bit longer to dry out in dry weather, because it is sheltered by 2 meter hedge on all sides. Thought drainage was Ok - good soil - on slope- but higher ground behind me - behind me was open fields for 20 years, but in last 10 years there has been housing developement, - so maybe it has caused my site to be generally damper.
I will spray it with Sulph of Iron or Bio Moss Killer. If this does not work, I will spike it with fork, as you suggest. Hard work - I have .15 acres of lawn. Bigger photo attached.
Thanks again - any more comments appreciated