What are the mistakes in applying fruit-swelling fertilizer to Shatang mandarin?

Applying the fruit-swelling fertilizer well from July to September is one of the key steps for high-quality and stable production of citrus, and it is also an effective measure to prevent the alternate bearing phenomenon in citrus. However, in production, fruit growers make many mistakes when applying fruit-swelling fertilizer to citrus, achieving the opposite result and affecting yield and fruit quality. Based on years of experience and lessons, the editor has summarized the following mistakes.

One-sided and excessive application of nitrogen fertilizer

In production, urea is used relatively often. According to surveys, in recent years, about 40% of mandarin orchards apply urea alone as the fruit-swelling fertilizer, and they apply it at will, often resulting in excessive dosage; for example, applying more than 500 grams of urea per tree is not uncommon. One-sided and excessive application of nitrogen fertilizer causes the peel to thicken and become coarser, forming inferior thick-skinned large fruit; coloring is delayed, and a large number of green fruit appear; the flavor becomes poorer and the juice rate decreases, especially making the taste sour and astringent, unpalatable. Phosphorus and potassium fertilizers have an obvious effect on promoting acid reduction and sugar increase in fruit, and can promote citrus fruit expansion. Therefore, phosphorus and potassium fertilizers should be appropriately increased, nitrogen fertilizer should be controlled, and medium or trace elements should be supplemented when needed, so as to achieve balanced fertilization. For bearing trees 6-10 years old, apply 200-300 grams of nitrogen-phosphorus-potassium compound fertilizer per tree in July, and apply 2 kilograms of cake fertilizer and 100 grams of urea per tree in September; for mature bearing trees over 10 years old, apply 300-400 grams of nitrogen-phosphorus-potassium compound fertilizer per tree in July, and apply 3-4 kilograms of cake fertilizer and 200 grams of urea per tree in September. Mandarin orchards on red soil should also pay attention to applying calcium fertilizer, about 150 kilograms per mu, which can increase yield by more than 10%.

The farmyard manure applied is not fully decomposed

After these undecomposed farmyard manures such as pig and cattle manure, cake fertilizer, and chicken manure are applied, they release a large amount of heat during fermentation and injure the roots, causing root rot, root death, and even tree death. At the same time, undecomposed pig and cattle manure contains a large number of parasite eggs, causing environmental pollution. In addition, excessive application of undecomposed farmyard manure can also cause microorganisms to compete for nitrogen. Therefore, manure fertilizers such as pig and cattle manure and chicken manure must be composted and fermented until fully decomposed before being applied to the mandarin orchard; cake fertilizer can be soaked in clean water for 7-10 days or mixed with human and animal urine, and used only after being fully decomposed.

Improper fertilization position

Some mandarin growers apply fertilizer under the “tree base”, which is actually the position of the tree plate. The roots under the tree plate are mainly skeletal roots, whose main function is not to absorb nutrients but to fix and support the plant. Fine roots are the main organs for absorbing nutrients, and they are distributed most abundantly in the area of the canopy drip line. Therefore, fertilizer applied in the tree plate cannot be well absorbed and utilized, and digging holes can easily injure roots; in particular, the chemical fertilizer applied can easily burn roots, causing lateral roots to burn and rot, or even die. Therefore, the fertilization position should be determined according to the size of the tree canopy. Generally, fertilizer should be applied outside the outer edge of the canopy drip line or slightly farther and deeper from the outer edge, using the method of digging ring-shaped trenches or holes. This not only facilitates fertilizer absorption and utilization and does not cause fertilizer damage, but can also induce roots to develop deeper.

Fertilization not combined with drought resistance

Drought often occurs from July to September. If fertilizer is applied only without combining it with drought resistance, the nutrients cannot be well absorbed and utilized, and it is also easy to cause fertilizer damage and volatilization of nitrogen in chemical fertilizers. Since fertilizers can only decompose, move, circulate, and be absorbed under moist soil conditions, fertilization under drought conditions should be combined with drought resistance: for flatland mandarin orchards, after fertilization, adopt the method of digging trenches and irrigating water for drought resistance; for mountain mandarin orchards, dig 8-12 holes outside the outer edge of the canopy drip line, with a hole width of 30 cm and a depth determined by not injuring roots, and apply quick-acting fertilizer liquid as a topdressing, combining drought resistance and fertilization together.

Improper timing and position of foliar fertilization

Many mandarin growers do not choose an appropriate fertilization time, often spraying foliar fertilizer after 10 a.m. and before 4 p.m. on sunny days, and they only spray the upper side of the leaves, not the underside. Because July-August has high temperatures and is dry, the water in the fertilizer liquid evaporates quickly, which easily causes fertilizer damage, and after the fertilizer liquid dries up, the mineral elements cannot be absorbed by the leaves. In view of this, choose cloudy days or spray before 10 a.m. and after 4 p.m. on sunny days. Because the underside of leaves has many stomata and a stronger ability to absorb nutrients than the upper side, spray both the upper and lower sides of the leaves, stopping before liquid drips.

Only by reasonably mastering the fertilization techniques for Shatang mandarin can yield be effectively increased; otherwise, the opposite result may occur. The fertilization time and fertilization location for Shatang mandarin both require attention, and the fertilizer needed under different circumstances is also different. Therefore, in the process of fertilization, more is not necessarily better.