Foliar Fertilization with Zinc: From Field Trials to Elemental Transport

Zinc deficiency is a widespread and important deficiency in horticultural crops worldwide and foliar application of Zn has become commonplace in many cropping systems. Information on the effectiveness and longevity of benefit from foliar Zn applications is scant and highly varied and evidence suggests that species, formulation, environment, crop phenology and spray methodology all have significant effects crop response. Results of a limited number of studies utilizing stable isotopes of Zn demonstrate the importance of the leaf cuticle on Zn penetration and suggest that Zn is phloem mobile if entrance into the leaf apoplast occurs. To better characterize the biological and physicochemical determinants of foliar Zn efficacy we have conducted a series of experiments in commercial fields, in controlled growth conditions, in model systems and in isolated tissues utilizing a range of formulations and methodologies. Stable Zn isotopes studies in field and model systems and high resolution micro Xray florescence of in-vivo Zn transport demonstrate that Zn enters the phloem and is transported to ‘sink’ tissues including fruit and roots. In general Zn formulations of low molecular weight, high solubility and high point of deliquescence are most effective at penetrating the leaf surface, increasing tissue Zn and enhancing transport of Zn to non-treated tissues. There is also a clear relationship between efficacy of the Zn foliar spray and propensity to cause phytotoxicity.