The effect of chelating agents on the foliar sorption of zinc fertilizers

Abstract

This study investigated the sorption of chelated zinc (Zn) fertilizers applied to plant foliage. n-Octanol water partition coefficients showed that rhamnolipid formed lipophilic complexes with Zn. It was hypothesized that the lipophilic complex would be better absorbed by leaves than ZnSO4 because leaf cuticles are primarily hydrophobic. Cuticle/water partition coefficients, which were measured using enzymatically isolated Valencia orange (Citrus sinensis) cuticles, showed that a chelating polymer called polyethylenimine and rhamnolipid increased Zn sorption to adaxial cuticle surfaces by 2-fold and 5-fold, respectively. The chelate EDTA reduced cuticle sorption of Zn to 17% of the ZnSO4 control. The rate of Zn diffusion across isolated adaxial cuticles was measured by dialysis. In addition, Zn sorption by cotton (Gossypium hirsutum L.) foliage was measured by spraying chelated Zn solutions on the foliage before washing the leaves with a rainfall simulator at set intervals. The results showed that the rate of Zn diffusion across C. sinensis cuticles was not related to the cuticle/water partition coefficients of the chelate solution. Nor did the formation of lipophilic Zn-rhamnolipid complexes increase Zn diffusion across the primarily hydrophobic cuticle. Six hours after application to G. hirsutum leaves, rhamnolipid, EDTA and polyethylenimine had reduced the rate of Zn sorption by 71, 74 and 81% of the ZnSO4 control, respectively, which suggested that these chelates may increase the risk of Zn-fertilizer runoff during rainfall events. The reduction in Zn sorption with chelate application was probably due to the size selectivity of aqueous pores that are present within leaf cuticles and the efficiency with which aqueous pores transport ionic Zn2+.