Physical laws of cuticular penetration of calcium salts have been studied using astomatous isolated pear (Pyrus communis L.) leaf cuticular membranes (CM). Penetration followed first order kinetics and was greatly affected by humidity, hygroscopicity, solubility of salts, and nature of adjuvants. Penetration required dissolution of the salts and this is determined by their point of deliquescence (POD). POD corresponds to the humidity over a saturated salt solution containing undissolved salt. When humidity was above POD the salt residue on the cuticle dissolved, while below a solid residue was formed and penetration ceased. CaCl 2 and Ca(NO 3 ) 2 have POD's of 32 and 55%, respectively, while POD's of organic calcium salts (acetate, lactate, propionate) were between 95 and 100%. Furthermore, aqueous solubility of the inorganic calcium salts is one to two orders of magnitude higher than that of the organic salts. Thus, organic calcium salts are not well suited for foliar nutrition since POD's are very high and driving forces of penetration are low due to low solubility even at 100% humidity. CaCl 2 and Ca(NO 3 ) 2 penetrate even at low humidity and solubility is much higher. When humidity was above the POD, rate constants of penetration increased with increasing humidity by about a factor of three and maximum rates were measured at 100%. Temperature did not affect rate constants of penetration from which it can be concluded that penetration is most rapid during the night when humidity is high. All salts should be used with an effective wetter as with an alkyl polyglucoside half time of penetration was decreased from 204 to 17 h. All other adjuvants tested (protein hydrolysates, EDTA, gum guar) decreased rate constants of penetration by factors of 3 to 9. This finding is discussed in relation to mixing foliar nutrients with fungizides.