First of all, you should assess the soil of your yard which plays central role in choosing the right fertiliser. You should take a kit from any garden supply store for testing the soil pH of your yard.
You need to dig a hole in your yard for getting a sample of the soil. Make sure that the hole should not be between 1 to 2 feet deep which will help you to get a good sample of soil.
Now take a sterile container and put a small amount of soil in this container from the hole that you have already dug in your yard. After putting the soil sample in the container, send it to the laboratory.
After getting the report of your soil sample, you should evaluate the report of soil pH test which will help you to determine which type of fertiliser your lawn or yard requires.
Usually fertilisers comprise on ammonium sulphate, sulphur, calcium and magnesium. You need to know the benefits of all types of fertilisers in order to choose the right one for your yard.
You need to buy only those fertilisers that contain high nitrogen if you want to use it in your yard that is old or established one. Nitrogen is usually cheaper than phosphorus and you can use it without facing any extra burden on your pocket.
If your yard is new and you are using a fertiliser for the first time, then you should use only those fertilisers that contain a high amount of phosphorus because it helps in establishing the roots of the grass and also supports in fast growth of the grass.
If winter season has damaged your lawn or yard then you need to use only those fertilisers that contain high ratio of potassium that gives strength to the cell walls of the grass and increases the resistance of grass against diseases.