Nutrients can be obtained from multiple sources:
• Rock weathering is a slow process of releasing small amounts of nutrients year by year. This process is not enough to achieve the goal of middle to high yield.
• The fertilizer applied in the last season was not absorbed by the previous crops, nor was it lost to the environment or retained in the soil, and could be used by the subsequent crops. Some nutrients, such as nitrogen and sulfur, are prone to large losses in the humid environment when applied in the same year. There are also some nutrients such as phosphorus and potassium, which can stay in the soil for a long time, usually several years, but the specific retention time depends on the soil type, rainfall, and management measures.
• Atmospheric deposition is an important source of nutrients in some areas, especially nitrogen deposition and sulfur deposition. In response to management regulations to reduce sulfur emissions to control acid rain, sulfur deposition has gradually decreased, and sulfur has gradually become a limiting factor. At present, the application of sulfur fertilizer has become a common measure in developed countries. In emerging and developing economies, it has also become more and more important. More and more common.
• Irrigation water also contains nutrients that can be used by crops.
• When crop residues, such as leaves, stems, and roots, are left on the soil surface or soil, they will decompose and release the nutrients they contain. The crop residues are mainly rich in potassium. Therefore, returning straw to the field has been the main source of potassium in the soil over the years. However, the large amount of straw burning or use as livestock feed has gradually depleted the potassium retained in the soil. The nutrient content of crop residues varies greatly, and the amount of crop available nutrients released within a specific time period is also determined by local conditions.
• Compost (decomposed organic matter) can be applied to the soil to supply nutrients and act as a soil conditioner. The quality of compost varies depending on the raw materials used and the fermentation process.
• Livestock manure is an important source of nutrients. The nutrient content of manure varies greatly under different sources and management measures. It is generally believed that low-quality livestock feed will reduce the nutrient content of livestock manure. Therefore, the nutrient content of manure should be analyzed before application.
The general content (g/kg) of main nutrients in crop straws and livestock manure
(quote Barket et.al., 2000)