The nitrogen cycle dictates the form and movement of nitrogen in the soil and in plants. Given adequate time and temperature, nitrogen in the soil will convert to the nitrate form, which is susceptible to loss in two pathways: denitrification, which happens in saturated soils that lack oxygen, and leaching, which happens when water moves through the soil, taking nitrate with it.
Think of the soil and the different forms of nitrogen as the charged ends of a magnet. The nitrate form of nitrogen is negatively charged, so it is not attracted to the negatively charged clay particles in the soil. This means it does not adsorb to clay particles, leaving it “loose” in the soil and subject to move with water. Though we can’t determine the exact extent of nitrate movement through leaching, we can estimate it.
Six Inches Movement per Inch of Water Drained
Research measuring drainage water at the Southern Research and Outreach Center in Waseca and elsewhere shows that nitrate will move about six inches for every inch of water drained. We have also seen nitrate move as much as twelve inches in coarse textured soils for every inch of water drained. Factors like compaction and underperforming drainage systems can slow movement down, while water flow through macropores (root channels, worm holes, cracks, etc.) can speed movement. These factors vary widely across different landscapes, which is why we can only estimate the extent of leaching.
University of Minnesota Extension Educator, Brad Carlson, believes in general, if you know a field is close to saturation, or at “field capacity,” you can assume more water will either run off or drain. At this point, you can estimate that an extra inch of rain percolating through the soil will likely move nitrate down six inches in the soil profile.
Estimating based on drainage system
You can also estimate based on your drainage system. If a system is designed with a half-inch drainage coefficient (commonly found in South Central Minnesota), it means a half-inch of water will move in a 24-hour period when the system is flowing at capacity. Of course, systems do not flow at full capacity very often, so estimate based on the approximate percentage of capacity and how long water is flowing.
Assuming field tile is 3.5 feet deep and you applied N at six inches deep, nitrate needs to move three feet before it is completely lost. This requires 6 inches of drainage, which is a very large amount, but not unheard of in extreme years. Looking at it another way, field tile would have to flow at full capacity for 12 days to achieve this. Most artificial drainage systems only flow at full capacity for a day or two each year.
Remember that downward movement of nitrate does not necessarily mean nitrate loss until it has moved below the rooting depth of corn. Keep this in mind if you plan to take soil samples to determine nitrogen availability. If you only sample one foot deep, but nitrate has moved to between one and two feet, it is still there and available for your crop, but will not show up on the soil test. Account for this difference to avoid over-fertilization, loss to the environment and unnecessary expenses.