Studies to determine the optimum rates of nitrogen and potassium fertilization of Early Sweet grapevine under open field and plastic cover conditions

Abstract: To determine the vines' optimum nitrogen and potassium fertilization rates of Early Sweet grape cultivar under open field and plastic cover conditions, the study was undertaken in the two periods of 2019 and 2020 with a preliminary season in 2018. Six treatments were applied in this experiment, three in an open field and three under a transparent plastic cover with different rates of (60, 45 and 30 units fed^-1) for nitrogen and (100, 75 and 50 units fed^-1) for potassium. Results showed that the vines receiving rates of N 45 units + K 75 units fed^-1 under plastic cover recorded a higher percentage of bud break followed by N 60 units + K 100 units fed^-1 treatment under plastic cover. In addition to an earlier sprout than those in the open field, they gave the highest values in term of yield and all physical and chemical parameters for reproductive growth, except for the vegetative growth where the best treatment was obtained from N 60 units + K 100 units fed^-1 under plastic cover followed by N 60 units + K 100 units fed^-1 in the open field. This result suggests that the plastic cover has a beneficial effect on decreasing the rates of fertilization due to the change in vine canopy microclimate through enhancing vegetative and reproductive growth and creating a balance between them.