Is fixed ammonium an important part of nitrogen cycling?

Dan Kaiser

Ammonium and potassium ions are present in the soil solution (what corn take up), exchangeable sites (loosely attached to soil particles and ready to go into solution to maintain a constant concentration level), and fixed between clay layers (slowly available). Measurement of fixed ammonium and potassium are not routine soil analyses and are not used to help make fertilizer suggestions. However, as fixed ammonium and potassium are released in the soil, they can represent an important source of available nutrients for corn. The soil tends to maintain a chemical equilibrium. As corn take up nutrients from the soil solution, exchangeable and fixed ions in the soil can become a controlled released source of nutrients for corn. These two nutrients are chemically similar in size. For this reason, both ammonium and potassium can be fixed in the same clay inter layers and high concentrations of either cation could potentially replace the other, increasing the concentration in solution of on cation while reducing the concentration of the other. However, little is known about these dynamic processes in the soils in Minnesota. This project will assess how corn responds to nitrogen at various levels of potassium fertilization.

The projects will determine how much fixed ammonium is present in soils in Minnesota, how fixed ammonium could be impacted by fixed potassium and vice versa, and ultimately whether fixed ammonium relates to the amount of nitrate present in the soil. While this is a basic science project, a clear understanding of these processes is critical to inform applied management decisions, such as determining the rate of N or K fertilizer needed for different soils.