Nutrient balance in agricultural systems involves continuous influx (gain from sources) and efflux (loss to sinks) of elements.

In this chapter, we exclude  the discussion of active nutrient fertilization as one of the possible sources for plants. Fertilization as a tool to manage the balance between sources and sinks will be treated later. However, depending on the fertilizer material used und the method chosen for fertilizer application, the processes introduced here apply to fertilizers-derived nutrients as well.

Understanding these processes is essential for effective fertilization and sustainable crop management.

The figure below shows all the relevant processes in the nitrogen cycle that determine potentially plant available Nitrogen in soil.