Re the brown patches on my lawn - thanks to Bamboo for his response.
By Petermac
Invernesshire,
United Kingdom 
In answer to Bamboo - I don`t see any red threads and yes we have quite sandy soil in Nairn. I just recently bought a petrol cylinder mower and have just discovered the blade height wasn`t set correctly - could this cause damage to the grass?
22 Jul, 2009
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Ah. Well hopefully it's just because you've been scalping slightly, which means you've got slightly higher spots in the lawn and if the blades were set low, they'd cut too close in those areas, and the result is what you've got now, although that doesn't look to be the cause - usually with scalping, you can see soil and possibly lots of moss - this looks like dead areas of actual grass growth. Still, no red is good. The other thing that occurred to me is that you say you've been feeding regularly with granules - if you distributed these unevenly, that could cause this sort of damage. Keep an eye on the patches to see if they develop any reddish pink tinges or threads though - this will be significant.
Asyour soil is sandy though, the most likely explanation is patchy drought - that causes those sort of patches too.
And by the way, I realise I refer to myself as a "plantsman" but only because I hate the term "plantswoman"!
22 Jul, 2009