I recently received this inquiry:
"What is your approach to K fertility with regards to phosphite products? Even though most are not labeled as K fertilizer, it's there."
That's correct for products that include potassium phosphite. I'm not sure how all phosphite products are labeled. Here's how I answered:
"I would count the K in that type of phosphite product as a K fertilizer addition.
I would ignore the P from phosphite, not counting it as a P fertilizer addition."
For more about phosphite, I recommend Penn State's Understanding the phosphonate products.