Thick, rough lawn with lower half of stems yellow.

I have an acre of lawn. It has been getting harder to cut it, each year I have to raise the mower as it is getting so thick and stalky. When I separate the grass to look at the stems, the bottom half of the grass is all yellow. There is quite alot of moss aswell. I plan on using sulphate of iron this year, would scarifying and then fertilizing help? I also use weedkiller on it every year for clover and daisys etc. It looks more like a field than a lawn at the moment. What would you advise to help get it nice looking again.