PHOSPHATE FOLIAR FERTILISATION AS A SOURCE OF PHOSPHITE RESIDUES

In particular environments foliar fertilisation with phosphate is a common practice to overcome transient deficiency status. Phosphate liquid fertilisers are also used as acidifiers when hard water is available for pesticide treatments. Phosphites, a common by-product present in phosphate fertilisers, are among the metabolites of pesticides whose residues in foods are regulated by the law. Therefore, experiments were designed to evaluate the chances of finding phosphite residues in fruits and buds of apple trees treated with phosphate fertilisers. The study was carry out on two year-old apple (cultivar ‘Golden Delicious’/M9) trees grown in pots and in an open field trial on 8 year-old apples trees (cultivar ‘Mongerduft’/M26). Trees were treated either with a single phosphate fertilisation (SP) provided to soil at the beginning of the trials or with a foliar phosphate fertiliser (FP) (five treatments, from May 11 every 7 days). Control plants were not treated with any phosphate fertiliser. Phosphorous acid concentration in foliar fertilisers was 172 ppm. Bark and bud samples were collected only from the field trial in March and in December. Fruits were collected starting from May 18 till harvest, on a monthly base. Phosphite concentration was determined by GC. At the end of the season we found a two-fold increase in phosphorous acid content in FP treated trees as compared to untreated and SP treated trees. Phosphorous acid was detectable only in fruits treated with FP. It is concluded that phosphorous acid traces present in foliar phosphate fertilisers can enter the plant and be found in different organs (fruit and bark). The amount found in fruits at harvest was in both trials over the legal limit for this compound suggesting a possible interference of normal foliar fertilisation practices with pesticide treatments.