Nitrogen has a very important role in the growth and development of apple trees. For a long time, nitrogen has been considered as an important nutrient during the growth and development of apples. The effects of nitrogen on vegetative growth, flowering, fruit set, yield formation, quality, and postharvest physiology have been widely confirmed. Many experimental studies and production applications showed that the rational use of nitrogen fertilizer can effectively enhance the tree storage, increase the fruit setting rate, improve or enhance the quality of the fruit, and enhance the tree resistance. Excessive or improper application of nitrogen fertilizer can result in declining fruit quality and prolonged tree growth. The absorption, movement, utilization, and redistribution of nitrogen in the tree are also influenced by other factors such as hormones.
First, the way the apple tree absorbs nitrogen
The roots of apple trees absorb inorganic nitrogen and organic nitrogen from the soil. Absorption of nitrogen to a large extent depends on the appearance of the leaves. There are two main forms of inorganic nitrogen that the roots absorb from the soil, namely NO3-, NH4+. Although there is partial N2 in the soil, the roots of the fruit trees cannot be absorbed and utilized because they have no nitrogen fixing ability. The roots of apples can absorb organic nitrogen in the soil. Ozerol and Titus (1968) and Shim et al. (1973b) found that apple roots can readily absorb urea 14C and can be synthesized into sugars, organic acids, and amino acids. After applying urea to the roots, 14C was detected from amino acids and sugars within 2 hours (Shim et al. 1973b).
Topdressing nitrogen fertilizer has a more significant effect. Yang Xinghong et al. studied the uptake of 15N-urea in apple branches (Guangguang, Fuji, Citrus aurantium) and their absorption and translocation after germination in early spring. The results showed that the apple shoots before germination had a certain absorption capacity for 15N-urea; 15N-urea, which was absorbed by buds, leaves, and branches, was transported to new organs and had the characteristics of storage nutrition. Applying nitrogen fertilizer out of the apple root before germination can improve leaf quality. The results of other studies have shown that nitrogen nutrition outside the roots of fruit trees, including nitrogen absorbed by shoots, can improve leaf quality and increase photosynthetic intensity.
Wang Zhongying et al.'s study on the changes of nitrogen content in the apple tree of short-stocked anthropogenic tree suggested that the nitrogen content of the root system was lower in the early spring budding period of the short-stocked tree, thus limiting the requirement for nitrogen growth in the early growth stage of the root and shoot. Therefore, we should pay special attention to the extra-root nitrogen topdressing on the short-stocked apple trees to supplement the lack of root absorption.
In the study of the effect of applying nitrogen fertilizer on roots outside the flowering period, 0.3% urea was sprayed on the leaves, which had a good effect on raising the fruit setting rate.
Late autumn topdressing with high concentration of urea has a very beneficial effect on the growth and development of fruit trees in the following year. After the fruit is harvested, 4%-5% urea is sprayed on the leaves 4 to 2 weeks before the deciduous period. The effect on the growth and development of the tree in the following year is directly reflected in the increase in the amount of flowering, the number of fruit set and the area of ​​short shoots.
Second, the relationship between nitrogen and phosphorus, potassium, etc.
In the production management of apple orchards, the application of nitrogen fertilizer to apple trees is usually carried out simultaneously with the application of phosphorus and potassium fertilizers. However, depending on the growth period, the amount of nitrogen fertilizer required is not the same. Wang Jikui and others believe that apple fertilizer is superior to other organic fertilizers in promoting fruit set, increasing fruit weight, increasing yield, and improving quality.
The application of organic biological nitrogen and phosphorus fertilizers on apple trees can significantly improve tree vigor, increase fruit set, quality and yield. The application amount of nitrogen fertilizer can have different effects on the growth, development and fruit quality of apples. Zhang Shaoling’s study on nitrogen fertilization for different tree vigors was based on strong, medium, and weak tree vigor. Nitrogen application rate was 0.5 and 15 kg urea per 666.6 m2 and was applied on April 25th. The results showed that the difference was that the amount of nitrogen applied had a significant effect on the ratio of fruit sugar and green fruit. The more nitrogen fertilizer applied, the higher the percentage of green fruit and the worse the fruit quality. In determining the amount of fertilizer, according to the strength of the tree vigor, the ratio of nitrogen and potassium should be adjusted to achieve the purpose of controlling the growth of new shoots and alleviating tree vigor, which is both high yield and high quality.
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