Grapes have a higher demand for fertilizers compared to most other fruit trees, especially for potassium, which surpasses the need for nitrogen and phosphorus. This makes them known as "potassium plants." However, grapes are sensitive to chloride ions, so it's important to avoid using fertilizers containing chlorides. Calcium fertilizers are often applied during the post-harvest period to enhance fruit quality and extend storage life.
The uptake of nitrogen starts from germination and peaks during leaf development and flowering. Phosphorus is most actively absorbed during vigorous shoot growth and berry expansion, with a significant transfer to fruits during ripening. After harvest, phosphorus accumulates in stems, leaves, and roots. Potassium is continuously absorbed throughout the growing season, but its uptake increases significantly as berries expand. Iron absorption by grape leaves is slow, and foliar sprays of ferrous sulfate are usually ineffective. Boron demand is highest before and after flowering.
Fertilization should be tailored based on grape nutrient requirements and local soil fertility.
1. **Base Fertilization**: Typically applied after autumn harvest. Based on experience, 2 kg of organic fertilizer is recommended per kg of fruit produced. The application method involves surface or furrow application, depending on the trellis system. For surface application, topsoil is removed in a 15-30 cm layer, with shallow digging near the plant and deeper outward, ensuring minimal root damage. Fertilizer is evenly spread and covered with soil. For furrow application, trenches are dug 50-100 cm from the roots, with widths and depths of 30-40 cm, alternating each year to avoid root damage.
2. **Soil Topdressing**: Usually done 2-4 times annually. Pre-bud fertilization helps promote bud germination and reduce bleeding, mainly using nitrogen. Before flowering, when shoots and inflorescences develop, high phosphorus is needed. During berry growth, a mix of phosphorus and potassium is essential. In the coloring stage, phosphorus and potassium remain key, with limited nitrogen. If organic fertilizer is sufficient, 1-3 kg of superphosphate is mixed per 100 kg of organic fertilizer and applied deep into the soil with base fertilizer. Additional fast-acting fertilizers are applied at 1-3 kg per 100 kg of fruit. Topdressing is typically done in shallow rings or holes 40-50 cm from the plant, with 10 cm depth, followed by irrigation.
3. **Foliar Spraying**: Used when deficiencies occur. For example, if leaves turn yellow due to excessive fruiting, spraying 0.2%-0.3% urea and 0.1%-0.2% potassium dihydrogen phosphate can help restore green color. Magnesium sulfate (0.1%-0.2%) is effective for chlorosis. Borax (0.1%-0.3%) before flowering improves fruit set, while zinc sulfate (0.1%) increases fruit weight and yield. Calcium sprays before harvest improve quality and storage life. Foliar applications are best done in the early morning or late afternoon when temperatures are lower, reducing evaporation and increasing absorption efficiency. Avoid spraying during hot, dry midday or cloudy days to prevent leaf burn.
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