Traditionally, fall application of N is practiced in areas where soil temperatures usually remain below 50°F from late fall to spring. Lower soil temperatures reduce the activity of nitrifying soil bacteria that convert ammonium to nitrates. However, if soil temperatures rise above 50°F, N is at risk of loss through leaching or denitrification.

Mass flow is where nutrients in soil solution move toward the root as the plant takes up and transpires water through the crop canopy. Consider how dry weather affects the plant root system. Root growth will slow in dry soils, the situation we are now experiencing in the upper soil depths.

Drought conditions change the way farmers should fall-apply nitrogen. Soil retains more nitrogen during a drought, so there could be an opportunity for farmers to spread out or delay input costs to avoid sending excess N downstream.