There are nutrients required by crops that are not accounted for in a simple nutrient balance accounting for kg applied and kg exported in grain. This is partially due to the following
- The remaining stubble contains high quantities of nutrients that are not translocated into the grain (especially in the case of K and S). Depending on the fate of the stubble, these nutrients may, or may not be, returned to the soil for use by future crops or pastures.
- The soil, organic matter and atmosphere can contribute differing amounts of some required nutrients (N, S and some trace elements)
- There may be a low plant availability of applied nutrients due to sorption by soil and organic matter particles (P and trace elements).
- Soil applied nutrients may be unavailable due to dry conditions.
- Nutrient concentrations in final product can vary greatly