I suggest that you contact your local county extension office. They
will give you free, good advice without being tainted with the pressure to
sell you something.
In my area, it is best to fertilize in the fall and maybe a little in
the spring but nothing during the summer. But each area is different. The
lawn products people want you to fertilize early and often, often causing
more damage than good for your lawn. Too much fertilizer or fertilizer at
the wrong time is a bad ideal and hard on a lawn, even though it may make it
nice an green and healthy looking for a week or two.
You should also have your soil tested as that will provide the
information the country extension agents can use to advise you. They will
be able to help you out with that as well.
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