[PureHarvest]

I'm always adding pieces too Dutch. Thanks for the kind words.
I am blessed in that I write nutrient mangament plans for farmers as part of my job.
I acknowledge that crop fertility is not an exact science but we have to go with some premises and guidelines.
I think the best thing someone can do to figure out the question of "how much fertilizer and what kind" can be answered with some relatively simple math. This can be done using organic materials like manure, organic ferts like alfalfa meal, or inorganic fert like 10-10-10 or 34-0-0.
The assumption that is made for the soil type in my area is that a tomato crop will need 120 lbs of nitrogen per crop, 0-100 lbs of phosphorus (amount needed is based on a soil test), and 0-300 lbs of potassium (also based on a soil test). The test at the lab I use reports results in plant available nutrients. Some people question soil tests and that the extraction methods don't mimic real soil solutions, but I think they come as close as we need them to in order to make sound decisions. Maybe some day we can get even closer, but for now it gives us a benchmark. Farmers can then build a history and track nutrient applications with yield.
All 3 elements are based on pounds of actual available elements per ACRE, so you have to scale it down to your growing area. Then you figure out how much actual elements are in your material and go from there.
If anyone gives me some numbers I can run through an example to show how you get there.
It might not be perfect, as many variables come into play, but it at least gives you a starting point with some reason behind it based on decades of trials. Most universities have a commercial crop recommendation guide that shows what each crop needs per year.
Just type in "Cornell commercial vegetable recommendation guide" into google, if you live in NY for example. Rutgers has one as does Penn State. I've looked at VA tech and Miss state too. Westerners would definitely find one from UC Davis I bet. You get the point on regionallity.
Finally, when you understand how the form of your "fertilizer" works, when it is available, and what load it carries, you can make a more responsible application to the land. This applies to both organic and inorganic ferts. They both can be leached, volatilized, run-off, and over applied. Organic doesn't imply safer and inorganic isn't automatically dangerous. It comes down to the 4 r's: right rate, right time, right placement, right form.