Cuticular penetration of calcium salts: effects of humidity, anions, and adjuvants

Physical laws of cuticular penetration of calcium salts have been studied u sing 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. Pe netration required dissolution of the salts and this is determined by their point of deliquescence (POD). POD corresponds to the humidity over a satur ated 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. CaCl2 and Ca(NO3)(2) have POD's of 32 and 5 5%, respectively, while POD's of organic calcium salts (acetate, lactate, p ropionate) 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 penetratio n are low due to low solubility even at 100% humidity. CaCl2 and Ca(NO3)(2) penetrate even at low humidity and solubility is much higher. When humidit y was above the POD, rate constants of penetration increased with increasin g humidity by about a factor of three and maximum rates were measured at 10 0%. Temperature did not affect rate constants of penetration from which it can be concluded that penetration is most rapid during the night when humid ity is high. All salts should be used with an effective wetter as with an a lkyl polyglucoside half time of penetration was decreased from 204 to 17 h. All other adjuvants tested (protein hydrolysates, EDTA, gum guar) decrease d rate constants of penetration by factors of 3 to 9. This finding is discu ssed in relation to mixing foliar nutrients with fungizides.