How to sort it out?
A good starting point is to have the lawn’s soil tested. Do-it-yourself, at-home tests are available to get a general read on the basic nutrients and pH (the soil’s acidity level). But some of the best results come from tests offered through state land-grant universities and their Extension services. Extension offices and many garden centers sell kits, usually for around $10.
Lawn-owners send samples of their soil to the state lab and then receive a report on the soil’s fertilizer needs as well as recommendations on which fertilizer to add and how much.
A second up-front consideration is to identify problems the lawn has been having, such as poor color or growth, disease, or rampant weeds.
Many lawn problems are related to soil nutrition and can give you a clue about specific fertilizer needs. Grass that’s light green or yellow-green rather than dark green is often a sign, for example, that the soil is lacking in nitrogen.
Diseases such as dollar spot, rust, and red thread tend to occur more in under-fertilized, nitrogen-poor lawns and weeds are often more prevalent in under-fertilized lawns due to weak grass growth.
On the other hand, lawns that are over-fertilized – especially with too much nitrogen – tend to be more prone to diseases such as summer patch, brown patch, and pythium blight as well as more likely to develop excess thatch.
Read more on lawn diseases
Read more on thatch problems in the lawn