Hi Tim!

I don't see your soil test attached here, so am inferring that:

• Your lawn is quite large
• Phosphorous is high or excessive -- you are wisely trying to avoid adding any more (THANK YOU!)
• Potassium is low ... not sure how low....but it seems you want/need to add some

Potassium is persistent - doesn't wash away with rain...so I trust you won't add too much.  (Just like phosphorus: have to wait for the lawn to take up the excess.)   As you know:  nitrogen does wash away, so your soil test results recommend adding some nitrogen regularly.  

You're right:  North Country Organics Pro-Booster is a nice organic fertilizer when you just want to add nitrogen (and support a VT company)....but it is expensive for large areas.  Did you receive the "Interpreting Your UVM Soil Test Results" document with your soil report?  (By Vern Grubinger, UVM Extension's Vegetable &  Berry Specialist...revised on April 15, 2021.)   If so: the last page has two very helpful charts on fertilizers (one on organic and one on synthetic).  I have not had to invest in fertilizer for such a large area myself; so the cost factor has not been a critical one for me.  I know that Chilean nitrate (in the chart:  15-0-2) is one of the ingredients in Pro-Booster....but I'm not sure what that costs nor how easy it is to find & buy.  

Without knowing your personal decision-factors for your lawn: allow me to note that if you don't mind seeing clover in your lawn it's a great way to provide an ongoing supply of nitrogen. I added clover to my lawn (probably a smaller lawn that yours, but the seed wasn't terribly expensive at the time) and we actually enjoy it.  Another very low-cost way to help your lawn green up and thrive is to simply set your lawn mower height high enough -- at least 2.5 inches....3 is very good....and allow the cut grass to mulch into your lawn.  I scatter a bit of compost on my (small & favorite part of my lawn) each spring (compost is usually about 1-1-1...but full of microbes and organic matter)....just lightly so it isn't very visible. But my lawn has grown healthier every year...recent guests asked what my secret was and I realized how far the lawn has come since we bought the house. :-)

Finally:  without knowing what you are trying to accomplish nor the specifics of your soil report:  even if you add less-than-the-ideal amount of fertilizer....you can still help your lawn out.  If you can prioritize sections: you could add a bit of Pro-Booster each season to different areas.

If I'm not coming close to helping you here: you could reply with your soil test attached.  I can also request a copy...but that can take several days (partly due to the pandemic).

Hope this is helpful,

- Robilee Smith