After the Beginning of Autumn, cotton needs even more attention to foliar topdressing

Cotton foliar topdressing, commonly known as leaf spraying fertilizer, means spraying a fertilizer solution onto the stems and leaves of the cotton plant so that the stems and leaves absorb nutrients, thereby nourishing the whole plant (including the root system). It has the advantages of saving fertilizer, taking effect quickly, and not being limited by soil conditions. It is an important yield-increasing measure for quickly supplementing nutrients to the cotton plant in the late growth period, especially after the "Beginning of Autumn." Why is this said?

Many studies have proven that nutrients absorbed by the leaves, like nutrients absorbed by the roots, can be transported and assimilated within the plant body. Foliar topdressing is a way for plants to obtain nutrients, so late-stage leaf spraying fertilizer can also slow root senescence. Further research shows that there are many fine structures on the outer wall of leaf epidermal cells, such as in the pore canal cells, at the base and around leaf hairs, and on the upper and lower epidermal cells of leaf veins. These fibrous pores directly connect the cell protoplasm with the outside world and are the channels through which nutrients enter the cell protoplasm, and this is an active absorption process.

After the "Beginning of Autumn" solar term, cotton has entered the late flowering and boll-setting stage. The root system's ability to absorb nutrients is gradually weakening, while the upper cotton buds continue to grow and develop. In cotton fields with little fertilizer application and nutrient deficiency, or in cotton fields where the middle and lower parts have many bolls and the upper part is poorly nourished, this is precisely an important period for leaf spraying fertilizer. In cotton fields where potassium fertilizer has not been applied or where there was heavy rain in the earlier period, obvious potassium deficiency symptoms have now appeared on the lower old leaves. With the increase in boll setting and rainfall, potassium deficiency will further intensify. If quick-acting potassium is not supplemented in time, it will cause early disease and premature senescence, fewer and smaller bolls, poor boll opening, and reduced lint percentage, and in severe cases even death of the cotton plant. Therefore, late-stage leaf spraying of potassium is a necessary yield-increasing technique for most cotton fields.

1. For the potassium fertilizer variety to spray, high-quality potassium sulfate is most suitable. High-quality potassium sulfate is also called first-grade potassium sulfate, containing more than 50% potassium oxide (K2O). The formulation should preferably be a water-soluble powder or microcrystal. Any other potassium-containing foliar fertilizer cannot reach the potassium content of potassium sulfate. Therefore, potassium sulfate is the most economical and safest variety for leaf spraying fertilizer. For example, the "Aipu" potassium sulfate produced in Russia contains up to 52% K2O and is very easily soluble in water (at 30°C, 100 grams of water can dissolve 13 grams of potassium sulfate). Calculated on the basis of spraying 3 times per mu, 0.6 kg per mu each time, and a utilization rate of 70%, 0.66 kg of K2O per mu can be absorbed and utilized. This potassium can meet the need for increasing seed cotton yield by 15 kg per mu.

2. When spraying fertilizer, both the front and back sides of the cotton leaves should be sprayed wet. From the perspective of leaf structure, beneath the lower epidermis of the leaf is spongy tissue, which is relatively loose, with larger intercellular spaces and more pore canal cells, while the palisade tissue of the upper epidermis of the leaf is relatively dense and absorbs nutrients less and more slowly than the lower side of the leaf. Therefore, when spraying, strive to fully cover both the front and back sides of the leaves with the solution. Do not spray less than 30 kg of liquid per mu, including 0.6 kg of potassium sulfate (2% concentration), mainly sprayed on both sides of the leaves in the upper half of the cotton plant.

3. Windless weather in the evening is most suitable for spraying fertilizer. The longer the solution keeps the leaves moist, the better the fertilizer effect. Generally, the moist period is required to last 0.5-1 hour, so it is best to spray in windless weather in the evening or on cloudy days.