The primary nutrients are Nitrogen, Phosphorous, and Potassium. NPK or the 3 numbers you see on a fertilizer bag. For example 12-12-12.

Calcium, magnesium, and sulfur are secondary nutrients.

A lack of any of these nutrients will result in yellow color, decreased disease resistance, decreased density, smaller roots, less drought tolerance, and many more problems.

Right behind improper mowing. This is the next most neglected aspect of proper lawn care.

For more info on fertilization click here.