On Facebook there is a post about the correct fertilizer for dahlias. People seem to think you go into the garden store and there are packages of fertilizer for every type of plant: Tomato fertilizer, rose fertilizer, and where the hell is the dahlia fertilizer?
The acronym NPK must be fertilizer for NPK plants whatever they are. 
At Walmart, I found a "Dahlia Plant Food" fertilizer that is mislabeled as "Rose Plant Food". 
$1.49/lb is a pretty good price for a 2-3 month time-release with micros. I usually pay that for a 50 lb quantity of the equivalent. Contains sulfur so maybe not for PNW gardens. I use it in potted plants.
I found the label for the Expert Rose fertilizer. It has three sources of nitrogen and only one is time release and the other nutrients are not time release. Ammonium Sulfate is one source of nitrogen and it is very acidic. For our potted flowers we use Haifa 14-14-16 and all of the fertilizer is time release. It costs us $80.00 for a 50 pound bag and that is about $1.60 per pound. Yes that is bout the same price per pound but the Haifa has more fertilizer and longer time release period and more minor nutrients.
Teddahlia Ammonium Sulfate is one source of nitrogen and it is very acidic.
Yes, that is why I mentioned it might not be ideal for PNW gardens. For my pH 7 field, ammonium sulfate is my preferred source of nitrogen. For container plants Haifa 14-14-16 is superior. Unfortunately, most of the best time-release fertilizers are not sold in small quantities and those that are (Osmocote) are very expensive. Dahlia societies sometimes buy fertilizer/soils in bulk and sell smaller amounts to members
Our society sold some Haifa Multicote 14-14-16 a couple of years go. It is too expensive to use on our dahlias except the few that are gown in 15 gallon pots.
While searching for a scientific article on a breeding subject, I ran across a recent paper concerning the optimal amount of nitrogen fertilizer for dahlia cut flower production. The study determined the most economical amount of nitrogen when considering the cost of the fertilizer to the yield of cut flowers. While "more is better" the study determined that beyond a specific amount, the increase in fertilizer was not cost effective. Unfortunately, the amount was stated in kilograms of nitrogen per hectare.
Actually, they also stated the results in pounds of nitrogen per acre also. Since we all of grow at least an acre of dahlias the amount is easily calculated(joke). I am signing off for now to leave you all on the edge of your seats to do the math and double check the results.
We are lucky to have the internet:
google search: pounds per hectare converted to ounces per square feet
here's the conversion:
Mass / Area
1000 Pound / Hectare = 0.148645 Ounce / Square footFormula
for an approximate result, divide the mass / area value by 6727
so for any recommended pounds of fertilizer(x), for one hectare
use x/6727 to get ounces pure nitrogen per square foot
Then depending on the percentage of nitrogen in your fertilizer, there's another calculation to get equivalent ounces of pure nitrogen.
I found this recommendation from the Maryland ext.
https://extension.umd.edu/sites/extension.umd.edu/files/2021-02/Nutrient%20Mngt%20Cut%20flowers.pdf
1.5-2 pounds per 1000 sq/ft.
My well water contains 20ppm nitrate nitrogen which is the value that I try to keep my fish tank water under. Jacks fertilizer recommends 50-200 ppm nitrogen every 1-2 weeks. I can't do the math so I just compare the color of our dahlia leaves to the trees in the woods. If the dahlia leaves are darker no fertilizer is needed. If lighter (never happens) I'd give them a boost.
https://meridian.allenpress.com/jeh/article/42/1/14/499409
Nobody found the scholarly article. Note they recommend 150 pounds of nitrogen per acre. Bessie will gladly translate that to nitrogen per 1,000 square feet AND then recommend how much Urea (46%) that would be per 1,000 square feet and if you use a product that is 16% nitrogen how much of that per 1,000 square feet.
In my mind , I have guesstimated and decided that I have been using about this much but with Bessie's mathematical skills, she will verify that proposition.
On one of my plots (1/20th of an acre in size) I inject about 15 lbs of actual nitrogen in a 5 month growing period. That works out to about 225 lbs of N per acre. Considering my longer growing season, that is close to their recommendation. I think.
I knew that Steve would be one of the first to read the scholarly article.
I used my meager spreadsheet skills to determine how much nitrogen. My guess was about right but since I have poor records and apply "by feel" I could be off.
Takeaway: The people who say dahlias need low nitrogen fertilizer are mistaken. That is a polite word for just plain wrong.
"A recommended nitrogen application rate for potatoes is typically between 200 and 250 pounds per acre." So dahlias can be fertilized the same as potatoes as far as nitrogen although a bit less at 150 pounds per acre is good for dahlias and 200 is ok too but the 250 may be a bit much.
Another quote form the paper:
"....the consequences of both reliance on and overapplication of compost-based amendments, particularly when applied based on intuition over soil test reports. Most growers indicated annual use of composts or manures, which are associated with elevated soil test P and K,...."
One can tell that the authors are from a farmer background where you must rely on soil tests to assist you to determine proper fertilizer for your crop. The quote intimates that many flower growers do not have a "farmer" background and they assume that products like compost and manure fertilizer are "good for flowers" and do not do soil tests. The phosphates in manure and sometime compost are a negative issue and the flower farmers applied too much.
