As a wastewater constituent, potassium usually has no health or environmental significance. It is however, an essential nutrient for vegetative growth and is not typically present in wastewaters in the optimum combination with nitrogen and phosphorus. If a land treatment system depends on crop uptake for nitrogen removal, it may be necessary to add supplemental potassium to maintain nitrogen removals at the optimum level. Equation (3.4) can be used to estimate the supplemental potassium that may be required where the in situ soils have a low level of natural potassium. This most commonly occurs in the northeastern part of the United States.

(3.4)

where <i class="emphasis">K</i><sub<i class="emphasis">S</i></sub> = annual supplemental potassium needed, kg/ha<i class="emphasis">      U</i> = estimated annual nitrogen uptake of crop, kg/ha<i class="emphasis">      K</i><sub<i class="emphasis">WW</i></sub> = potassium applied in wastewater, kg/ha

Most plants also require magnesium, calcium, and sulfur, and depending on soil characteristics, there may be deficiencies in some locations. Other micronutrients important for plant growth include iron, manganese, zinc, boron, copper, molybdenum, and sodium. Generally, there is a sufficient amount of these elements in municipal wastewaters, and in some cases an excess can lead to phytotoxicity problems, as discussed in the sections which follow.