Nutrients can be obtained from multiple sources:
• Sludge (residue from urban sewage treatment) can be recycled and used to provide large amounts of crop nutrients. Due to different sources, processing methods, storage methods and treatment processes, the nutrients in sludge vary greatly in content and form. The crop nutrients and possible pollutant content in the sludge should be analyzed regularly.
• Biological nitrogen fixation (BNF) is the process of converting inert nitrogen in the atmosphere into active nitrogen that can be used by plants. Biological nitrogen fixation exists in many crop-bacteria symbiota. In the symbiotic system of legume crops (such as soybean, pea, alfalfa) and rhizobia, biological nitrogen fixation is the strongest. The amount of biological nitrogen fixation is usually 20-400 kg N ha-1yr-1, which varies according to crop varieties, growing season length and climatic conditions.
• Chemical fertilizers are produced by the fertilizer industry, and farmers can purchase a range of products that supply one or more essential mineral elements. Farmers around the world apply an average of approximately 180 million tons of fertilizer (in terms of pure nutrients) every year to supplement the source of farm nutrients, thereby achieving sustainable crop yield and quality goals.
Fertilizers containing only one large amount of elements are called elemental fertilizers, and fertilizers containing two or three large amounts of elements are called compound fertilizers. Compound fertilizers include compounds (all nutrients are in particles of the same chemical structure) and mixtures (physical mixing of different fertilizer particles). Each fertilizer product has its advantages and disadvantages, which usually depend on the local agro-ecological and economic conditions (see Reetz 2016 for details).
It is worth noting that crops can respond to nutrients from all sources, but they mainly absorb nutrients in an inorganic state. Organic nutrients must be mineralized (from organic to inorganic state) to be absorbed by crops. Different types of fertilizers provide different nutrients between different agricultural ecosystems and within a system. Sustainable crop nutrient management needs to be able to identify and utilize effective nutrients from all sources.
The average nutrient content of some major fertilizer varieties (nutrient content as a percentage
of total fertilizer products, %)