Managing the point of deliquescence (POD) of a foliar spray solution can tremendously increase the performance of the products applied, particularly in drier climates with low humidity, and when the products applied are ionic salts.1
It is also possible for plants to absorb insoluble non-ionic nutrients through the leaf surface through endocytosis. For non-ionic products, the point of deliquescence is less critical but still important and useful to manage.
The point of deliquescence is described as the humidity threshold at which an ionic salt material dries into a crystal on the leaf surface. When humidity is above the point of deliquescence, the salt residue on leaf cuticle dissolves and can be absorbed. When the humidity is below this point, a solid residue remains on the leaf surface and penetration into the leaf stops.
Because of this effect, foliar sprays should be applied in the evening to take advantage of the higher humidity at night.
In addition to the ingredients mentioned in the article, a useful tool to increase the point of deliquescence when applying non-ionic materials are ocean mineral solutions with a low sodium content, which are generally quite hydroscopic and keep the foliar solution liquid on the leaf surface for a longer period. Urea is also a useful material in this regard.
The reference below describes the POD and speed of foliar nutrient absorption across the cuticle for various ionic nutrient compounds.