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When raw materials such as pig manure and urine, as well as grass, are added to the biogas digester, after a period of fermentation, it is necessary to replace the contents. The residues and nutrient-rich liquid that result from this replacement are known as biogas digester fertilizer, or biogas fermentation residues. Since biogas fermentation involves anaerobic decomposition under sealed conditions, it reduces the loss of ammonium nitrogen; as a result, nutrients are preserved and the fertilizer efficiency is improved. After thorough fermentation, the fertilizer produced from biogas digesters contains a wealth of nutrients required by various crops, such as nitrogen, phosphorus, and potassium, as well as various bioactive substances like amino acids, trace elements, plant growth regulators, B vitamins, and certain antibiotics. It is therefore an effective and high-quality fertilizer. The methods and precautions for applying biogas digester fertilizer in fruit cultivation are briefly described as follows: I. Application methods 1. Apply as top dressing by pouring it on the plants. Use 1,500 to 2,500 kilograms of biogas slurry per mu; it can be applied directly when the fields are moist, but in dry conditions it needs to be mixed with water at a ratio of 1:1 to prevent seed rot or damage to the seedlings. 2. Apply as a base fertilizer at deep depth. The biogas digester residue can be used directly as a base fertilizer, or it can be mixed evenly with turf soil and superphosphate (or phosphate rock powder), and after being left to ferment for 1–1.5 months, it can then be used as a base fertilizer; the mixing ratio is 10:10:1. After application, cover with 5–10 cm of soil. 3. Use as a top-dressing by spraying on the leaves. Biogas digester liquid that has been producing gas normally for over a month can be used after being clarified and filtered through double-layer gauze; the filtration is intended to prevent clogging of the sprayer. The application concentration should be determined based on the concentration of the biogas slurry, the growth stage of the crops, the season, and the temperature. As a general rule, water should be added at a ratio of 1 to 2 times when the plants are in the seedling stage; when the temperature is high, water should be added at a ratio of 1 time, while when the temperature is low, no water needs to be added for the older leaves. Choose a sunny afternoon and apply the solution every 7–10 days, using 40 kilograms of biogas slurry per mu. When spraying, focus on the underside of the leaves to facilitate absorption. II. Precautions 1. Biogas slurry cannot be applied immediately after being taken out of the tank. Sludge has a high reactivity; if applied immediately after being taken out of the tank, it will compete with crops for oxygen in the soil, affecting seed germination and root development, and causing the crop leaves to turn yellow and wilt. Therefore, after being taken out of the pond, it should generally be stored in a manure storage tank for 5–7 days before use; applying it after mixing it with phosphate fertilizer in a 10:1 ratio and allowing it to ferment for 5–7 days yields even better results. 2. Biogas slurry cannot be applied directly without being diluted with water. When using biogas slurry as a fertilizer, especially for seedlings, it should generally be diluted with water first; otherwise, it may burn the plants. 3. Biogas slurry should not be applied as topsoil. When applying biogas slurry to upland crops, it is advisable to create holes or trenches, then cover them with soil; in paddy fields, it should be spread evenly over the surface before being plowed into the lower layers. 4. Do not apply in excess. The application amount should generally be less than that of ordinary pig manure; applying it in large quantities blindly can cause excessive growth of crops and lead to reduced yields. 5. It should not be applied together with alkaline fertilizers such as plant ash and lime, as this will reduce its effectiveness.