How often should I fertilize my lawn? Should I follow a certain schedule? I have heard of people fertilizing on around holidays [US: Easter (sometime in April), Memorial day (last week of May), fourth of July, Labor day (first week of September)].
My decades ago hort 101 class said fertilizer is not really necessary if you use a mulching mower. If you bag the clippings or if you want thicker greener lawn you can apply fertilizer up to 3 times per year. The applications should be timed to catch the grass growing but not the weeds.
Here in central Iowa with bluegrass/ryegrass/fescue lawns the grasses go brown/dormant during the hot part of the summer (July, August) and grow most vigorously in the cooler spring and fall. The weeds go nuts in the spring (April, May, June) - we do not want to encourage that! So the fertilization schedule works out to once in the first 1-2 weeks in September, again 2-3 weeks after that and (very optionally!) once in the early spring just after the ground thaws (first 1-2 weeks in April).
Below are a few quotes from this answer I posted here on SE. Hopefully you find them of some benefit:
- Seeing as my lawnmower is a mulching mowing, I leave all the grass clippings on the lawn (free, natural fertilizer), except with first cut and last cut of the season. I collect up those cuttings and dispose of them via a community yard waste pile.
- Fertilize the lawn with corn gluten meal (by Bradfield because I can get it easily locally) in very early Spring, when I see the Forsythia shrub in flower in my area.
- For the first time this year I also decided to fertilize the lawn with corn gluten meal in mid June. Why? See "Table 4" here: Cool-season grasses: Application schedule for organic fertilizers
- Corn gluten meal isn't a "magic bullet" for controlling unwanted weeds (plants). I've read and been told that it can take at least 2 to 3 years (following recommended application rates on yearly bases) before seeing any results with this method.
- I make 5 gallons of compost tea each week (from late Spring to earlier Autumn "Fall") and apply the 5 gallon batch to the front garden one week, then the following week apply a new fresh 5 gallon batch to the back garden. I repeat that cycle for the period given previously. I have been doing this for 2 years now, and without question I have noticed a massive increase in worm activity eg lots of worm castings on the surface of the soil.
- I spread the 5 gallons of Compost Tea via a watering-can over approx 1800ft² (170m²). Lawn area is about the same front & back for me. So one week the front lawn gets treated, then the following week the back lawn gets treated, repeat, repeat...
- Around "Labor Day" (beginning of September, early Autumn "Fall") here in the US, I prepare any bare spots for reseeding. Reseed using an appropriate seed for my lawn type. Cover the whole lawn with ½ to 1 inch (12.5 to 25mm) thick layer of STA-certified compost (bought in bulk locally). Water as needed, ie Amount needed for good germination to take place.
- Fertilize the lawn with Ringer Lawn Restore (by Woodstream Corp) at the end of September, early October.
You may also find the actual answer I pointed to above of some help: