Grapes have a higher demand for fertilizers compared to most other fruit trees, with potassium being the most critical nutrient. They are often referred to as "potassium plants" due to their high potassium requirement. However, grapes are sensitive to chloride ions, so it's important to avoid fertilizers containing chloride. Calcium is also beneficial for improving post-harvest fruit quality and extending storage life.
The absorption of nitrogen begins from the time of bud break, with peak demand occurring during leaf development and flowering. Phosphorus uptake is highest during active shoot growth and berry expansion, and it moves significantly into the fruit during ripening. After harvest, phosphorus accumulates again in the stems, leaves, and root system. Potassium is absorbed throughout the growing season, but its uptake increases notably during berry swelling. Iron absorption by grape leaves is slow, making foliar sprays of ferrous sulfate less effective. Boron is most needed before and after flowering.
Fertilization should be tailored based on grape nutrient requirements and local soil fertility. Basal fertilizers are typically applied after autumn harvest. A common practice is to use 2 kg of organic fertilizer per kilogram of fruit. The application method usually involves surface or furrow application, depending on the planting system. For vines grown on trellises, surface application is common. Soil is removed from the top layer (15-30 cm), spread with fertilizer, and then replaced carefully to minimize root damage.
For furrow application, trenches are dug 50-100 cm from the roots, either wheel-shaped or radial. These are alternated each year, with depths and widths around 30-40 cm. Fertilizer is placed in the trench and covered with soil. Adjustments can be made based on root depth and spacing.
Soil topdressing is generally done 2-4 times a year. Pre-bud fertilization helps promote bud growth and reduce sap bleeding, using mainly nitrogen. Before flowering, when shoots and inflorescences are developing, phosphorus is essential. Topdressing should include nitrogen, phosphorus, and some potassium, avoiding excessive nitrogen alone.
During berry development, rapid growth of roots, shoots, and fruits requires large nutrients, mainly phosphorus and potassium. During the coloring phase, nutrient accumulation begins, so topdressing focuses on phosphorus and potassium with minimal nitrogen.
If sufficient organic fertilizer is used, the amounts of N, P, K should be adjusted accordingly. For example, 1-3 kg of superphosphate is mixed with every 100 kg of organic fertilizer and applied during autumn. Additional fast-acting fertilizers are applied at 1-3 kg per 100 kg of fruit. If organic fertilizer quality is good, topdressing can be limited to 1-2 kg per 100 kg of fruit. Poor quality may require increasing to 2-3 kg.
Soil topdressing is usually done by digging shallow trenches or holes about 40-50 cm from the plant, 10 cm deep, and applying fertilizer followed by irrigation.
Extra-root topdressing, such as foliar sprays, is recommended based on plant needs. In cases of early spring yellowing due to poor nutrient storage, spraying 0.2%-0.3% urea and 0.1%-0.2% potassium dihydrogen phosphate three times over 10-15 days can restore green foliage. Magnesium sulfate (0.1%-0.2%) helps with chlorosis, borax (0.1%-0.3%) improves fruit set, and zinc sulfate (0.1%) boosts fruit weight and yield. Calcium sprays before harvest enhance fruit quality and extend shelf life.
Foliar sprays are best done in the morning or late afternoon when temperatures are cooler, allowing better absorption. Spraying during hot, dry midday or cloudy days may cause leaf burn.
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