Comparative effects of foliar applied organic compounds on yield, quality, and nutritional traits in soilless grape production

The soilless culture technique, widely used for vegetables and ornamentals, has also proven advantageous for grapevine cultivation under protected conditions. This study investigated the effects of foliar applications of different organic compounds diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA), amino acids, humic acid, and fulvic acid on yield, fruit quality, and plant nutrition in soilless table grape production under greenhouse conditions in Çukurova, a warm Mediterranean region in 2021 and 2022 years. Four-year-old cv. Black Magic ( Vitis vinifera L . ) grapevines were grown under plastic cover in a pumice:cocopeat (1:1, v/v) substrate and irrigated with modified Hoagland nutrient solution. Organic substances were applied foliar at a concentration of 0.2% at three phenological stages (pre-flowering, fruit set, and veraison). The application concentration and phenological timing were chosen to coincide with critical stages of grapevine development and to maximize foliar uptake efficiency without inducing phytotoxicity. Control vines were sprayed with water only. Humic acid application increased yield by approximately 10–18% compared to the control in two growing seasons and also improved cluster weight, length, and size. Based on mean values, humic acid application increased berry weight by approximately 3–7% compared to the control across the two growing seasons. Significant treatment effects on must quality were observed mainly in the first growing season, in which amino acid and humic acid applications improved total soluble solids and maturity index compared to the control. Humic and fulvic acids enhanced leaf macro and micro nutrient concentrations by 15–35%, several nutrients into sufficient range during veraison. Humic acid and EDTA increased stomatal length by up to 45% and reduced stomatal density by approximately 15% compared to the control. Humic acid increased ascorbic acid content by approximately 150–170% and enhanced DPPH antioxidant activity by up to 8%. Fulvic acid increased total phenolic content by approximately 10–20% compared to the control.