A critical assessment of the suitability of phosphite as a source of phosphorus

Marketing of phosphite‐containing preparations for foliar application, together with recent reports of positive yield responses, has revived the question as to whether phosphite (HPO $ _3^- $ ) is a suitable P source for plants. Two experiments using zucchini ( Cucurbita pepo L. convar. giromontina ) have been conducted to evaluate the P‐nutritional effect of phosphite either provided via the substrate or as a foliar spray. Plants grown in a P‐deficient substrate were severely damaged when phosphite was applied as foliar fertiliser and more drastically when provided via the substrate. Growth of P‐deficient plants receiving phosphite as a foliar spray was impaired in a dose‐dependent manner after foliar P application (concentrations 0.0, 0.9, 2.7, and 4.5 g P L –1 ), while foliar provision of phosphate improved plant growth and yield. In the youngest leaves of phosphite‐treated plants, which had developed after foliar spray, phosphite accumulated to considerable extent, reaching a similar concentration as phosphate at tissue level. These results confirm that P‐deficient plants are very sensitive to phosphite, which represents a nutritionally ineffective form of P. It should thus not be considered as a form of P suitable for fertiliser manufacture.