ISHS Acta Horticulturae 594: International Symposium on Foliar Nutrition of Perennial Fruit Plants
FOLIAR NUTRITON USING INORGANIC SALTS: LAWS OF CUTICULAR PENETRATION |
| Author: | J. Schönherr |
| Keywords: | Citrus aurantium, Populus canescens, Pyrus communis, aqueous pores, calcium, half times of penetration, humidity, hydration, permeability, potassium, rate constants of penetration, point of deliquescence |
| DOI: | 10.17660/ActaHortic.2002.594.5 |
| Abstract:
Laws of cuticular penetration have been elucidated using calcium and potassium salts. These salts have hydration shells and they penetrate cuticles by diffusing in aqueous pores of molecular dimensions. Cations and anions penetrate in equivalent amounts, because electrical neutrality must be maintained. Penetration is a first order process and salts deposited on the surface of the cuticles disappeared exponentially with time. Velocity of penetration can be best quantified using first order rate constants or half times of penetration. Rates of penetration were greatly affected by humidity over cuticles, and hygroscopicity of salts. Penetration requires dissolution of the salt. This is determined by the point of deliquescence (POD) of the salt and humidity over the salt residue. POD is defined as that humidity over a saturated solution containing solid salt. When humidity is above POD the salt residue on the cuticle dissolves, while below a solid residue is formed and penetration ceases. Hence, salts suitable for foliar nutrition should have a low POD. The following salts meet this criterion: CaCl2 (33%), MgCl2 (33%) K2CO3 (44%), Ca(NO3)2 (56%) and Mg(NO3)2 (56%). Salts having POD’s above 90% (for instance K2HPO4, KH2PO4, KNO3, Ca-acetate, Ca-lactate and Ca-propionate) are not suitable for foliar nutrition, as they penetrate only at a humidity close to 100%. When humidity is above POD rate constants of penetration increased with increasing humidity by about a factor of three and maximum rates were measured at 90 to 100%. This is attributed to swelling of cuticles. The driving force for cuticular penetration is the concentration difference across the CM. Ion concentrations in the apoplast are in the millimolar range while water solubility of inorganic salts can be very high. Fortunately, with the exception of MgCl2 all salts mentioned above having a low POD’s also have high aqueous solubility ranging from 1.25 to 6.60 kg per kg water which shows that driving forces are very large following spray application and evaporation of excess water. Plasticiser and temperature did not affect rates of penetration and spraying should be done in the evening to take advantage of high humidity during the night. The effect of wetting agents and other adjuvants on rates of penetration is discussed. | |
Download Adobe Acrobat Reader (free software to read PDF files) | |
