Selected Calcium Salt Formulations: Interactions between Spray Deposit Characteristics and Ca Penetration with Consequences for Rain-Induced Wash-Off

The effect of two ethoxylated rapeseed oils (RSO) as surfactants for foliar-applied calcium chloride CaCl2 and calcium acetate Ca(CH3COO)2 on deposit characteristics, penetration, and calcium (Ca) residues after rainfall was investigated in columnar apple trees (CAT, Malus domestica Borkh.) and apple seedlings. Irrespective of salt and apple variety, both surfactants enhanced droplet spreading and led to a greater area covered by calcium within the droplet residues. The addition of the rather lipophilic surfactant RSO 5 and the rather hydrophilic RSO 60 increased penetration of Ca from both salts through isolated apple leaf cuticular membranes. Our data demonstrate that penetration is better reflected by area covered with calcium than by droplet spreading area. Considering the point of deliquescence (POD) from the salts and results of cuticular Ca penetration, we postulate that in case of CaCl2 reduction of wash-off was due to enhanced penetration, whereas for Ca(CH3COO)2 wash-off was decreased due to distinct enhancement of rainfastness.