How to tackle nitrogen losses in soil

This May and June brought almost record-breaking rainfall, which has led to questions about how much nitrogen was lost from the soil.
More to the point, the real question being asked is, “How much extra fertilizer should I apply to keep from losing yield?”
This question cannot be answered precisely since many variables enter the picture, but we at least can start to make some educated guesses. The following is from Keith Reid, soil fertility specialist with the Ontario Ministry of Agriculture and Food (OMAF):
Nitrogen losses can occur through either leaching or denitrification. It is important to remember that, for both of these processes, it is the nitrate form that is subject to loss.
Organic or ammonium nitrogen will stay put.
Fertilizer nitrogen can be in the ammonium form (e.g., urea, ammonium sulfate, anhydrous ammonia), the nitrate form (calcium nitrate), or a combination of both (ammonium nitrate, calcium ammonium nitrate, UAN solution).
Ammonium N is converted to nitrate in the soil, but this process is quickest in warm, well-aerated soils. Fertilizer that was applied within two-three days of the beginning of the wet period probably stayed as ammonium until the ground dried up again.
Fertilizer that had a week of dry weather before the rains started likely would be completely converted to nitrate.
Nitrogen leaching is a function of the movement of water down through the soil. It is of greatest concern on light textured soils.
The depth of nitrate movement depends on the amount of infiltration. An inch of water soaking into a sandy soil can move nitrate down about six inches, but water running off the surface of the soil won’t cause any nitrate leaching.
As well, any evaporation from the soil surface has the effect of reducing the net infiltration.
Any soil where the net infiltration (rainfall minus runoff minus evaporation) exceeds four inches after fertilizer was applied could have lost a significant part of the nitrate below the rooting zone.
Denitrification is a biological process that occurs in water-logged soils, as bacteria steal the oxygen from nitrate for their metabolism. Because it is biological, it will be affected by the availability of a food source (carbon compounds) and by temperature.
The relatively cool conditions in May and June will have reduced the losses from denitrification, although the length of the wet period may have more than made up for the slower rate of loss.
Some experts in the U.S. Mid-West have estimated all of the nitrate in the top six inches of soil could be denitrified if the soil was saturated for three-five days in warm conditions, but it would take twice as long in cool conditions.
The other side of the equation is nitrogen supply. If the fertilizer rate applied was generous relative to crop needs, then significant losses can occur before the crop suffers any yield impacts.
Obviously, the closer the rates are to the actual optimum rate for the field, the greater the concern about the size of the losses.
Aside from fertilizer, nitrogen for crop production comes from organic matter breakdown from the soil, from added manure, or from crop residues.
These sources have not been releasing nitrogen into the soil quickly because of the cool, wet conditions, but should kick-in once the weather warms up. Aerating the soil with a light cultivation can kick-start this process.
The final piece to the puzzle is crop demand. Flooded soils will suffer the greatest N loss, but these areas are unlikely to produce any crop this year anyway so demand will be low.
Of more concern are areas that were waterlogged for a couple of days, but otherwise have good yield potential.
A soil nitrate test can help to determine whether there is enough nitrogen in the soil to meet the crop needs for the season.

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