In summer, the combination of high temperatures, heavy rainfall, and ample sunlight creates ideal conditions for photosynthesis in crops, making it the peak period for growth efficiency and nutrient uptake. During this time, plants produce a significant amount of carbohydrates, which require nitrogen to form amino acids, proteins, and nucleic acids. Applying fertilizers at this stage allows crops to absorb nutrients more effectively. However, some farmers tend to apply nitrogen fertilizer as a base fertilizer during sowing, leading to slow early growth and inefficient nutrient use. As the season progresses, when crops need more nutrients, much of the initial fertilizer has already been lost, resulting in nutrient shortages that can harm crop development and reduce yield and quality.
Therefore, proper top-dressing is essential for maximizing yields. Unfortunately, many farmers still apply nitrogen fertilizer carelessly during summer, believing it will quickly dissolve into the soil. This often leads to excessive chemical use and environmental pollution. To achieve better results, top-dressing should be done scientifically.
First, it's important to tailor fertilizer application based on soil conditions. Each soil type has different properties, so conducting tests before top-dressing helps determine the right fertilizer type. For example, acidic soils benefit from alkaline or neutral fertilizers like urea, while low-alkali soils may need acidic options like ammonium sulfate. In sandy soils with poor water retention, less volatile fertilizers such as urea are preferred. Additionally, managing the amount of fertilizer applied is crucial—applying smaller quantities more frequently reduces the risk of burning crops and minimizes nutrient loss through evaporation.
Second, the type of fertilizer matters. Most chemical fertilizers are salts, and their effectiveness depends on their composition. For instance, ammonium chloride and urea can be mixed in paddy fields to prevent nitrification, but they should not be used in saline soils. Potassium-based fertilizers must be avoided in crops like watermelons and potatoes, which are sensitive to chlorine. Urea, on the other hand, is versatile but needs time to break down in the soil, so it should be applied several days before other nitrogen sources and buried deeply to improve absorption.
Third, different crops have varying nutrient requirements. Field crops typically need more nitrogen, while economic crops like vegetables and grapes also benefit from potassium. For example, applying half of the urea as basal fertilizer and the rest as top-dressing in late July can significantly increase nitrogen utilization. Specific amounts vary per crop—rice might need 230–370 kg of ammonium sulfate per hectare, while soybeans require 130–170 kg of urea to meet their nitrogen needs.
Fourth, timing is critical. The best time to top-dress is during the crop’s critical growth periods, when nutrient demand is highest. For wheat, this includes the tillering and heading stages, while corn requires nitrogen after jointing. Soybeans benefit from nitrogen application during early flowering, and rice should be fertilized during green-up, jointing, and heading phases.
Finally, balancing the amount of fertilizer is key. Over-application can lead to weak plants prone to disease and lodging, especially in rainy summers. Under-application, on the other hand, fails to boost yields. A well-balanced approach ensures that all necessary nutrients are provided without waste, ultimately achieving higher productivity with fewer inputs.
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