Thanks John, that definitely makes sense.

working through the logic…

Low nutrient dense produce can be about 95% water and high nutrient dense produce can be about 85% water. For easy numbers, we’ll pretend water content is 100% (I know that doesn’t make sense, but it won’t change the calcs substantially). Let’s say a leaf thickness is 1mm and foliar film of water is 0.1mm. We would need 10x the concentration of target sap nutrient level in the foliar solution, to bring a nutrient that is at 0ppm in the sap and tissue to 100% of target ppm.

For example, boron, if target sap is 10ppm, and you have 2ppm in the sap, then by this logic, you would want to supplement 8ppm and therefore spray at 80ppm. Then the trial would be a foliar of Boron 40ppm, 80ppm, and 160ppm. Well, that checks out with the literature!

Where these calcs could be far off is: how much of water content is in the sap and how much in the tissue? How much of each micronutrient is in the sap and how much in the tissue? Sap usually concentrates nutrients higher than the concentration that is available in the root zone. for example, We see about 100x more ppm of calcium and nitrate in the sap than we see in the root zone (soluble calcium). But then calcium is about same in tissue and sap. Nitrate will be 10x less in tissue than sap, but total N will be 10x higher in tissue than sap. Boron, may be .5ppm in soil, 5ppm in sap and 50ppm in tissue. (don’t grill me on exact numbers, I’m only demonstrating order of magnitude concepts). So there’s an order of magnitude jump from root zone to sap and sap to tissue for boron. But every nutrient is different.