What are the key points of autumn field management for napa cabbage?

Napa cabbage, a biennial herbaceous plant, 40-60 cm tall, hairless throughout, sometimes with a few bristles on the midvein on the underside of the leaves. Basal leaves numerous, large, obovate-oblong to broadly obovate, 30-60 cm long, less than half as wide as long, apex rounded and obtuse, margin wrinkled, wavy, sometimes with indistinct teeth, midvein white, very wide, with numerous stout lateral veins; petiole white, flat, 5-9 cm long, 2-8 cm wide, margin with wide thin wings that are incised; upper stem leaves oblong-ovate, oblong-lanceolate to long-lanceolate, 2.5-7 cm long, apex rounded-obtuse to shortly acute, entire or with lobed teeth, petiolate or clasping, with a powdery bloom. Flowers bright yellow, 1.2-1.5 cm in diameter; pedicels 4-6 mm long; sepals oblong or ovate-lanceolate, 4-5 mm long, erect, pale green to yellow; petals obovate, 7-8 mm long, base gradually narrowed into a claw. Silique relatively thick and short, 3-6 cm long, about 3 mm wide, flattened on both sides, erect, beak 4-10 mm long, about 1 mm wide, apex rounded; fruiting pedicels spreading or ascending, 2.5-3 cm long, relatively thick. Seeds spherical, 1-1.5 mm in diameter, brown. Flowering period May, fruiting period June. Napa cabbage, of the genus Brassica in the family Brassicaceae, is native to northern China and is a traditional Chinese vegetable. Napa cabbage was first called "song," but song cabbage is not entirely the same as today's napa cabbage. Modern napa cabbage is generally divided into large napa cabbage and bok choy; northerners generally call large napa cabbage "cabbage" and call bok choy "rapeseed."  Autumn is the key period for planting napa cabbage, and field management of autumn napa cabbage must not be underestimated; scientific and standardized management must be carried out well, so from which aspects should autumn field management of napa cabbage begin?  1. Timely topdressing. Napa cabbage uses leaves as its product and is very sensitive to nitrogen, phosphorus and potassium requirements. In production, not only should base fertilizer be applied well, but topdressing should also be carried out according to different growth stages. Topdressing during the growth period is mostly based on quick-acting nitrogen fertilizers, such as ammonium sulfate, ammonium bicarbonate, ammonium phosphate, urea, etc. The timing of topdressing must be well controlled, generally applied in 4 rounds at the seedling stage, rosette stage, early heading stage and mid-heading stage. At the seedling stage, apply 5-10 kg of ammonium sulfate per mu; at the rosette stage, apply 1000-1250 kg of decomposed farmyard manure, 15 kg of ammonium bicarbonate or 10 kg of ammonium sulfate per mu; at the early heading stage, apply 25 kg of ammonium bicarbonate or 15-20 kg of ammonium sulfate per mu; at the mid-heading stage, apply 1000-1250 kg of dilute manure liquid, 10-15 kg of ammonium carbonate or a 1% urea solution per mu.

2. Combine pest and disease control. Vegetable growers attach great importance to the control of the three major diseases of napa cabbage, but often neglect pest control. In fact, the occurrence and spread of many diseases are inextricably linked to pests. Aphids are the chief culprit in spreading viral diseases and can cause napa cabbage to shrink and die; soft rot pathogens are mainly transmitted through insects, rainwater and irrigation water, infecting napa cabbage through wounds. After pests cause damage, they create wounds, which facilitate pathogen invasion and disease. Controlling cabbage caterpillars, striped flea beetles and underground pests well and reducing pest wounds can effectively reduce the occurrence of soft rot in napa cabbage. Therefore, pest control is also an important measure for disease control.

3. Delay tying. Tying can prevent napa cabbage from frost damage, promote solid heads, and also facilitate harvest. However, tying must not be done too early, because if tied too early, the photosynthetic efficiency of the outer leaves is greatly reduced, hindering the normal growth of napa cabbage and the filling of the head. Generally, 10-15 days before harvest, stop watering, lift the rosette leaves, wrap them around the head, and then use soaked sweet potato vines or straw to tie the leaves at 3-5 cm from the top of the head, so that the head becomes firmer and continues to grow.

Fertilizer and water management techniques for autumn napa cabbage:

Topdressing. Napa cabbage has high yield and high fertilizer demand. On the basis of applying sufficient base fertilizer, topdressing should be done in a timely manner. Topdressing should be determined according to different growth stages and seedling conditions. Generally no topdressing is applied at the seedling stage. If the base fertilizer is insufficient, the first application can be made at the 3-4 true leaf stage, applying 10 kg of ammonium sulfate per mu, broadcast on both sides of the seedlings, and watered immediately, called "seedling-promoting fertilizer"; the second application after final thinning or after transplanting seedlings, applying 15-20 kg of ammonium sulfate per mu, placed in furrows opened on both sides of the ridge, called "plant-developing fertilizer"; the third application at the rosette stage, applying 25-30 kg of ammonium sulfate and 10-15 kg of superphosphate per mu, applying the fertilizer into furrows or holes, then slightly earthing up and supporting the ridge, then watering, called "heavy topdressing"; the fourth application at the mid-heading stage, applying 15-20 kg of ammonium sulfate per mu, which can be flushed with water, called "head-filling fertilizer."

Watering. From the cluster stage to the rosette stage, the temperature gradually drops and the weather is mild; during this period, watering can be done appropriately. At the late rosette stage, water can be controlled appropriately for several days, and watering should resume after the third topdressing. After napa cabbage enters the heading stage, it needs the most water; therefore, as soon as the seedling-stopping period ends, water thoroughly once. Then after 2-3 days, water a second time. This time is very important; if the soil cracks at this time, lateral roots will break and fine roots will die, affecting heading. After that, generally water once every 5-6 days to keep the soil moist.