Optimal Allocation and Application of Nitrogen Fertilizers
The fundamental objective of researching optimal nitrogen fertilizer application lies in reducing nitrogen fertilizer losses, enhancing nitrogen fertilizer utilization efficiency, and maximizing the yield-enhancing benefits of fertilizers. Due to three non-productive loss pathways in soil—ammonia volatilization, nitrate leaching, and denitrification of nitrate nitrogen—nitrogen fertilizer utilization efficiency remains relatively low. Statistics indicate that nitrogen fertilizer utilization rates in China range from 35% to 60% in paddy fields and 45% to 47% in upland fields, averaging 50%. This means approximately half is lost, resulting in both resource waste and environmental pollution. Therefore, rational nitrogen fertilizer application and improving its utilization rate represent an urgent issue requiring resolution in agricultural production.
The rational allocation of nitrogen fertilizers should be based on soil conditions, the nitrogen nutrition characteristics of crops, and the properties of the fertilizers themselves. Soil Conditions: Soil conditions are a necessary prerequisite for fertilizer zoning and allocation, and also serve as the basis for determining nitrogen fertilizer types and application techniques. Prioritize allocating nitrogen fertilizers to medium- and low-fertility areas. For alkaline soils, choose acidic or physiologically acidic fertilizers such as ammonium sulfate or ammonium chloride. Acidic soils require alkaline or physiologically alkaline fertilizers like sodium nitrate or calcium nitrate. Ammonium chloride should not be applied to saline-alkali soils, while urea is suitable for all soil types. Ammonium nitrogen fertilizers are best applied in rice-growing areas, incorporated deeply into the reduction layer. Nitrate nitrogen fertilizers are suitable for dryland cultivation but should be avoided in regions with high rainfall or rice-growing zones. For “early-maturing fields,” adhere to the principle of light application early and heavy application later, with multiple small doses to prevent nutrient deficiency in the crop's later stages. For “late-maturing fields,” focus on promoting early seedling emergence while avoiding excessive nitrogen fertilizer later on, which can cause excessive vegetative growth and lodging. On heavy clay soils, nitrogen fertilizer can be applied in a single large dose, while sandy soils benefit from multiple small applications.
Nutritional Characteristics: The nitrogen nutritional characteristics of crops are the intrinsic factors determining the rational allocation of nitrogen fertilizers. The primary consideration should be the type of crop, with nitrogen fertilizers prioritized for economic crops and grain crops. Second, consider different crops' requirements for nitrogen forms. Rice benefits from ammonium nitrogen fertilizers, particularly ammonium chloride and ammonia water. Potatoes perform best with ammonium sulfate. Hemp prefers nitrate nitrogen, while sugar beets thrive on sodium nitrate. Tomato seedlings benefit from ammonium nitrogen, while the fruiting stage favors nitrate nitrogen. Cereal crops generally tolerate both nitrate and ammonium forms, while leafy vegetables typically prefer nitrate nitrogen. The effectiveness of nitrogen fertilizer varies across different crop growth stages. While ensuring adequate nutrition during seedling development, corn typically requires heavy application of ear fertilizer. Early rice benefits from heavy tiller fertilizer, moderate ear fertilizer, and supplemental grain fertilizer. Fruit trees require substantial winter fertilizer. These practices represent economically efficient nitrogen fertilizer management strategies.
Fertilizer Characteristics: The inherent properties of fertilizers also play a crucial role in the rational allocation of nitrogen fertilizers. Ammonium nitrogen fertilizers applied on the surface are prone to volatilization and should be incorporated as basal fertilizer through deep application and soil covering. Nitrate nitrogen fertilizers exhibit high mobility and are unsuitable for basal application, especially in paddy fields. Ammonium carbonate, ammonia water, urea, and ammonium nitrate are generally unsuitable as seed fertilizers. Ammonium chloride should not be applied to saline-alkali soils or low-lying areas, nor to chloride-sensitive crops such as cotton, tobacco, sugarcane, potatoes, grapes, and sugar beets. In arid regions, nitrate nitrogen fertilizers should be applied; in rainy regions or during rainy seasons, ammonium nitrogen fertilizers are preferable.
