NPK ratio by season for a bluegrass lawn in Toronto?

I have researched for the proper NPK ratio for fertilizing a bluegrass lawn here in Toronto, ON, CAN.
I am not sure if I should be too concerned about the theoretical variation
to the fertilizer's NPK ratio to use at different times due to the season's temperature change. I have not yet found any info on this adjustment.
All the info I can find is a 3-1-2 ratio (e.g. 21-6-12), probably because the sites I found for KBG are all in the US, where there are not substantial temp changes or seasons.
Do I need more specific info about how the NPK ratio changed depending on the time of the year?
I believe I have found the correct times of the year, at least, though (possibly off a little):
Late winter/early spring (April-Early May)
Late spring/early summer (Mid June)
Late summer (Early Sept)
Fall (Late Oct./Early Nov)
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I'm going to paste a note I made to someone about a year ago about fertilizing lawns. Its a long one but comes after many courses on the subject and 11 years of practical experience in the business. The instructions are for a bluegrass lawn in Ontario Canada, but you could probably use most of the information.
Peter H ====================================
How to Fertilize a Lawn With a Broadcast Spreader.
There is one initial step, which must be done and is very important. You must pace off your lawn and calculate the square footage. Once this is done the rest is easy.
Completely ignore the instructions on the bag. This goes for straight fertilizer and/or weed and feed.
For the purposes of applying the fertilizer the only information you need from the bag is the nitrogen content, which is the first of the 3 numbers in series. This number represents the percentage nitrogen by weight. For example if your fertilizer is 15-5-10 it is 15% nitrogen by weight and a 20 lb bag would contain 3 lbs of nitrogen.
You normally spread .75 to 1.5 lbs of nitrogen per 1,000 sq. ft. of lawn each time you fertilize and target 4.0 lbs per 1,000 sq. ft. for the entire season. If not very confident in your abilities you may wish to target 4 x 1 lb applications in a season. Just be careful in the summer as an application at this rate can burn if applied on a sunny/hot day, particularly if not spread evenly.
As an example if your lawn was 4,000 sq. ft and the fertilizer was 20-8-16 then you would evenly spread 20 lbs of fertilizer on that lawn to achieve 1 lb of nitrogen per 1,000 sq. ft. The 20 lbs of fertilizer only contains 4 lbs of nitrogen and spread evenly this would give you the 1 lb per 1,000 sq. ft.
The next step is to pace off a 1,000 sq. foot area, preferably not the front lawn, calculate how much fertilizer you need for that area and then weigh it out. To weigh it use a bathroom scale and a pail. Zero out the scale with the pail on it and then add the fertilizer until you have the desired amount. If you are not sure where to set the control on the spreader use the center mark and then adjust as required. Spread the fertilizer and see how it goes. Adjust as necessary. Once the flow rate is set you are ready to do the lawn.
There are a few other helpful tips you may want to consider.
- walk at a leisurely pace when setting up the spreader. If you walk briskly you will need to walk briskly when doing the entire lawn.
- Do not fertilize shady areas where the grass grows very thinly, or do so very moderately. The fertilizer will only stimulate powdery mildew and thin the grass even more.
- Careful on the corners. Chances are that the spreader will have only one drive wheel and if you turn a corner on the non-drive wheel the spreader will stop "broadcasting" the fertilizer leaving you with an over-fertilized patch.
- When finished do a calculation to see how you did. If you laid down more than 1.5 lbs of nitrogen per 1,000 sq. ft. you may want to get the hose going.
- Be careful along the sides of walls and buildings, as the fertilizer will bounce back off them causing you to over-fertilize these areas, some of which may already have powdery mildew problems.
- If using weed and feed don't go near your garden and be careful around trees and shrubs.
- The higher the percentage of nitrogen in the fertilizer the greater the chances are that you will burn your lawn.
- Slow-release fertilizers coat the nitrogen is sulfur, or something similar, to ensure that the fertilizer is released over time rather than immediately. The greater the percentage of slow-release nitrogen the less chance of burn and the longer the fertilizer will last.
- If you would like to be the first on the block with a green lawn in the spring fertilize very late in the fall, after you have made your last cut of the lawn and just prior to or at freeze-up.
- Try to always fertilize when the lawn is dry unless there is rain in the immediate forecast, otherwise the granules may stick to the blades of grass and increase the likelihood of burning.
- Try to keep the nitrogen down during the heat of the summer. Don't lay down 1 and pounds of nitrogen in early July. Your lawn will want to go dormant in the heat and you should let it do so. Stimulating it to grow when the temperatures are over 85 degrees F can cause disease.
- When your lawn is struggling, particularly in sunny areas, always suspect an insect unless you can unequivocally rule it out.
- Try to only cut 1/3 of the blade each time you are cutting. This may require more than one cut per week in the spring.
- There is a trend to higher cutting heights these days, but the longer the grass the fewer grass plants you will have per square foot. I still like 1 and " as a standard.
- Relax and enjoy your lawn. No matter what happens if you just keep dragging the mower over it and whack the weeds a couple of times a year as long as it's not being trampled to death you will always have a lawn. It will recover from anything that is thrown at it eventually.
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"Try to keep the nitrogen down during the heat of the summer. Don't lay down 1 and pounds of nitrogen in early July. Your lawn will want to go dormant in the heat and you should let it do so. Stimulating it to grow when the temperatures are over 85 degrees F can cause disease."
This is the type of info I want. I wish I had real NPK number examples of the typical mainstream products - I used to. Or a formula covering all three numbers for the four applications based on the typical 3-1-2. I personally never know what to do with partial info.
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