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3. Balanced Fertilization
Among the most critical agronomic practices that influence yield are planting density, nitrogen application rate, nitrogen fertilizer management, and chemical regulation techniques. Transgenic hybrid cotton exhibits a significantly higher biological yield compared to conventional varieties, which increases its overall nutrient demand throughout the growing season. Therefore, it is essential to implement rational fertilizer management strategies to ensure high-yield and stable production of insect-resistant hybrid cotton.
3.1 Fertilizer and Dry Matter Accumulation
Studies show that dry matter accumulation varies with different nitrogen rates, but the differences between treatments with higher nitrogen levels are relatively small. When biological yields are considered, applying NPK above 187.5 kg/hm² leads to diminishing returns in yield increase as nitrogen application increases.
3.2 Fertilizers and Individual and Population Development
As nitrogen application increases, both plant height and the number of large bolls per plant also rise, though the differences between treatments remain modest. Quality indicators across all groups improve with increased nitrogen, particularly the ratio of branches to shoots, with higher nitrogen rates showing more favorable results. The leaf area index develops more optimally at 187.5 kg/hm² of nitrogen, reaching its peak during the flowering stage. However, when nitrogen is either too low or too high, the peak leaf area index is delayed. Low nitrogen causes an early delay due to poor initial growth, while excessive nitrogen leads to late vegetative growth, shifting the peak later. These variations align with the development of the "three peaches" (early, middle, and late bolls), with higher nitrogen promoting a greater proportion of autumn bolls.
3.3 Fertilizer and Yield Formation
Experiments reveal highly significant differences in seed cotton and lint yields across various nitrogen application levels. Lint yield was significantly higher when 337.5 kg/hm² of pure nitrogen was used compared to 112.5 kg/hm² and 37.5 kg/hm². However, no significant difference was observed between 262.5 kg/hm² and 187.5 kg/hm² applications.
3.4 Fertilizer Management
Transgenic hybrid cotton has a much higher biological yield than conventional varieties, requiring consistent and adequate fertilizer input throughout the growing season. Compared to conventional cotton, transgenic hybrid cotton requires an additional 22.5–37.5 kg/hm² of pure nitrogen. Research suggests that approximately 300 kg/hm² of nitrogen is ideal for transgenic insect-resistant hybrid cotton, with adjustments based on transplanting density, local soil, climate, and vegetation conditions. The lower and upper limits are 270 kg/hm² and 330 kg/hm², respectively. If plastic mulch is used, nitrogen application should be increased by about 10%. A basal fertilizer ratio of around 35% is recommended. Phosphorus and potassium fertilizers should be applied appropriately, along with trace elements based on soil nutrient status to maximize the potential of transgenic cotton. Given the rapid growth of seedlings and buds, careful control of nitrogen application is crucial to balance vegetative and reproductive growth. The principle of “full, slow, and heavy†fertilizer management is key to achieving multiple boll formation. Sufficient basal fertilizer (about 30% of total nitrogen) ensures early growth and high yield, without causing overgrowth. Bud-phase fertilizer should be applied moderately, ideally 5–7 days after first flowering, and the second round of fertilizer should be delayed by about 7 days compared to conventional cotton. Late root spray should be applied 2–3 times. In Jiangsu Province, where boron deficiency is common, 4–12 kg/hm² of borax can be added as base fertilizer, which helps promote earlier maturation and improves flowering before frost.
3.5 Application of Different Fertilizers
The use of fertilizers in genetically modified, insect-resistant hybrid cotton fields depends on the specific crop intercropping system. Fertilizer application should be adjusted based on soil quality, fertility level, and cotton type.
3.5.1 Soybean (Maize) Cotton Intercropping
In high-fertility cotton fields with good soil structure and broad bean pods, apply 300 kg/hm² of cake fertilizer and 300 kg/hm² of phosphate fertilizer as base fertilizer. Top dressing includes 450 kg/hm² of urea as seedling and flower-bell fertilizer. Apply twice—once in early June and again from late June to early July. For fields with poor fertility, such as those left with wheat stubble, increase the fertilizer amount accordingly.
3.5.2 Garlic Cotton Intercropping
Garlic fields generally use 5000 kg/hm² of straw, 1500 kg/hm² of cake fertilizer, 1500 kg/hm² of phosphate fertilizer, and 75 kg/hm² of urea as base fertilizer. Three top dressings are applied at 750 kg/hm². For this type of cotton, only 450 kg/hm² of urea is needed as flower-bell fertilizer, eliminating the need for additional base fertilizer.
3.5.3 Watermelon Cotton Intercropping
Watermelon fields typically use 1125 kg/hm² of cake fertilizer, 750 kg/hm² of phosphate fertilizer, and 112.5 kg/hm² of urea as base fertilizer. Cotton planted in these fields does not require additional base fertilizer and uses 375 kg/hm² of urea as top dressing, divided into two light intermediate applications: 75 kg/hm² as seedling fertilizer, 225 kg/hm² as flower-bell fertilizer, and 75 kg/hm² as canopy fertilizer.
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