International Journal of Aquaculture, 2025, Vol.15, No.4, 208-220 http://www.aquapublisher.com/index.php/ija 212 4.2 Fertilizer and water management and green fertilization model Water chestnuts have a relatively concentrated demand for nutrients. According to research, each 1,000 kilograms of fresh water chestnuts produce approximately 2.5 kg~3 kg of nitrogen (N), 1.2 kg~1.5 kg of phosphorus pentoxide, and 3 kg~4 kg of potassium oxide. Reasonable fertilizer planning is crucial to improving the yield and quality of water chestnuts: base fertilizer is the main and top dressing is the auxiliary, and balanced fertilization and potassium fertilizer application is added. Field experiments show that compared with nitrogen alone, the combined application of nitrogen, phosphorus and potassium can significantly reduce the incidence of water chestnut plague and blight by about 30%~40%. Therefore, it is recommended to apply compound fertilizers (N:P:K≈1:1:1) in base fertilizer, and to increase potassium sources such as potassium sulfate or wood ash in the middle and late stages to improve bulb starch accumulation and epidermal toughness (Arisandy et al., 2018). In the rice water flow rotation system, the use of green manure (such as cymbidium) to plant green manure in winter can increase the source of soil organic matter and nitrogen and reduce the amount of fertilizer. In addition, promote commercial organic fertilizers and biological organic fertilizers to replace some chemical fertilizers, and improve soil microecology while ensuring yields (Zhang et al., 2021). Areas with conditions have begun to try micro-irrigation fertilization technology, and transport soluble fertilizers to the fields regularly and in batches with irrigation water. This technology has begun to show results in water chestnut seedling fields, which can save 30% fertilizer and 20% water, and reduce the humidity in the field, which is conducive to disease control (A’Ashri et al., 2022). 4.3 Plant distance density and photoperiod regulation The planting density of water chestnuts has a significant impact on the population structure and yield, and it needs to be reasonably determined based on the variety characteristics and cultivation season. Traditional experience believes that "sparse planting balls are large", but too sparse will reduce the total population output. Studies have shown that medium density (13 000 to 15 000 plants per mu) often produces the highest yield, taking into account both the size and total yield of single shepherds. In practice, early planting and high fertility fields can be appropriately increased in density (Gao et al., 2015), while late planting or thin soil can be reduced in density. Generally, the distance between the plants is about 40 cm × 30 cm, with 1~2 seeds of shepherd's species per acre. This way, about 12 000 seeds per mu, which is considered to be a more reasonable density configuration. Water chestnuts are short sunshine crops, and the sunshine length affects the induction of tubers' differentiation. It is reported that the critical day length of water chestnuts is about 12 hours. If sunlight is below this, it is conducive to bulb formation, and lengthening delays tuber development (Li et al., 2000). At the same time, under the facilities, research has been conducted using a light-shading network to manually shorten the sunlight to induce bulb differentiation. When the water chestnut seedlings are 30 cm tall, the treatment is started for 4 hours a day. As a result, the tuber formed about 10 days earlier than the natural light group. 4.3.1 Case: premature maturity caused by shortening of photoperiod in low latitude and high altitude areas in Yunnan In some water chestnut planting areas in Yunnan (such as Qujing, Honghe and other places, at an altitude of about 1 600 m), farmers found that water chestnuts often wither and mature early in early and mid-October, earlier than the expected harvest period. Survey of this phenomenon shows that the sunshine in these areas has dropped to about 11.5 hours in late August, and the night temperature has also dropped below 18 ℃. Water chestnut plants are stimulated by the "double signals" of shorter sunshine and low temperature, and enter reproductive growth early, causing the tubers to stop swelling prematurely. As a result, although water chestnuts were launched early, the single bulb was small and the yield was damaged. To this end, the local agricultural department has taken some countermeasures: improve varieties, cover cultivation, and change the sowing period. Practice has proved that after these measures are combined with the application, the maturation period of water chestnuts in high-altitude areas in Yunnan can be postponed by about 10 days, and the yield per unit increase by more than 15% (Huo and Sun, 2024). This case illustrates the importance of adjusting cultivation measures to regulate water chestnut breeding according to regional ecological characteristics, and also reminds breeders to pay attention to
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