Soil type and rainfall
Gelderman says medium- to fine-textured soils can hold 2-3 in. of moisture/foot; while coarse soils (sand, sandy loams and loamy sands) are in the range of retaining 1-1.5 in./foot.
"Previous to the rainfall, most soils were probably already close saturation. Therefore less water would be needed to move nitrate deeper," he says. "The other factor is intensity of rainfall. If it comes hard, a large part of the rainfall can runoff into lower areas or waterways etcetera. Runoff will be higher with tilled, fine textured soils."
According to past data and observations with rainfalls of this type, Gelderman does not believe recent rainfalls resulted in large nitrate losses on medium- and fine-textured soils. However, in fields with course-textured soils, 4-6 in. of rainfall would be enough to move nitrogen 2-3 ft.
"We usually have bigger leaching loss issues when rainfalls of this amount occur slowly – perhaps over a two-week period," Gelderman says. "Significant denitrification losses occur the longer saturated soil conditions exist and the warmer the soil temperature."
He points to Illinois and Nebraska data as an indicator of denitrification rates at about 3%/day for soil temps in the 60s.
Gelderman says saturation for long periods usually occurs in the lower areas of the landscape. However, he adds that significant denitrification in these areas is not the only problem.
"Limited oxygen to plant roots – up to seven to 10 days or more – can kill the plant or cause root changes that will severely limit yield potential," he says. "Therefore if these areas dry out, and the plants are living, it is probably not profitable to apply N to these areas."
He says denitrification is usually only a problem with surface soils because that is where most of the bacteria that break down organic matter are located.
"These are the same bacteria that use nitrate under low-oxygen conditions," Gelderman says. "In low organic matter soils, or subsoils, there are fewer of these microbes."