Too much of a good thing leaches away yields and profits

The western Canadian wheat crop is off to a good start and in most areas is just starting its rapid growth phase, the tillering stage. 


From now until near maturity, the crop will add an average of 100 pounds of dry matter per acre per day and will take up to 2.5 lb. per day of nitrogen before anthesis, after which the nitrogen uptake rate will slow.


This spring has not been a wet one across Western Canada, but rainfall has been unevenly distributed with some areas receiving 150 millimetres or more over the past month. There is now concern in wetter areas about how much nitrogen might have been lost.


Microbe activity increases when soil temperatures are in the 20s C. These microbes are responsible for mineralization and the release of plant-available nitrogen from soil organic matter. They also convert ammonium to nitrate. 


Nitrate nitrogen is susceptible to two major loss mechanisms: leaching and denitrification. 


Many soils are moist or even wet in mid-June, but the threat of nitrogen loss is far higher where water has stood, or is standing, than where water has not stood for more than one to two hours. 


When water stands long enough for the crop to begin to lose some of its green colour, typically three to four days at warm temperatures, that’s a signal that soil oxygen is becoming depleted. 


Lack of oxygen has two negative consequences: 


  • the start of denitrification, which is the conversion of nitrate to gaseous forms of nitrogen

  • the beginning of root damage, some of which may be permanent 


Most of the fields suffering from oxygen deprivation this year have recovered well.


It is likely that most fertilizer nitrogen is in the nitrate form by now, though some applied as top dressed urea-ammonium may still remain as ammonium. 


Having most of the nitrogen present as nitrate in mid-June is typical. 


However, it means that the nitrogen is subject to denitrification and, in lighter-textured soil, to leaching or moving out of the rooting zone.


Previous studies have shown that as much as eight percent of the nitrate present can be converted to gas when soil temperatures are in the mid 20s C and lost for each day that saturated conditions persist. 


Conversion rates may be lower than this if temperatures are lower deeper in the soil and at night, if some of the nitrogen is still in the ammonium form and if soils still have some oxygen present. 


There are indications that denitrification losses may be less than expected in some fields. However, loss of nitrogen may be a smaller problem than the loss of yield potential from plant damage if plants are badly damaged by saturated soil. 


It is rare that whole fields remain saturated for days, so the risk of nitrogen loss by leaching in most fields is greater than the risk of loss by denitrification. 


It’s reasonable to assume it’s not necessary to add more nitrogen to fields where all of the nitrogen has been applied, especially if some was applied in the spring as anhydrous ammonia, crop colour has remained or is returning to a healthy green and water is no longer standing.


In fields where all of the nitrogen has been applied but where water stood long enough for the crop to lose much of its green color, adding supplemental nitrogen will increase yields only if plants can grow enough new roots to take advantage of the added nitrogen. 


Chances of such recovery are much greater when the water comes early, like it did this spring, than when it comes later. 


Nitrogen should be added as soon as practical to fields where plants stood in water to the point of turning pale green but now seem to be recovering.


Applying nitrogen as late as the flag leaf stage will often provide a yield benefit if enough rainfall is available to carry it to the plant roots.


It’s good to apply supplemental nitrogen if needed but the yield cost of further delays depends on the nitrogen available to the plant now.


Observing canopy colour is the best way to know how much nitrogen is available to the crop now. The plant is not deficient or at least not deficient enough to reduce yields as long as leaves remain a reasonable shade of green and the final nitrogen supply is adequate.


In that case, some delay in applying nitrogen may not cost any yield.


Uptake will be delayed if weather turns dry after surface application of nitrogen.


Adding nitrogen stabilizers to urea or UAN when top dressing will re-duce the loss of nitrogen through volatilization.


Thom Weir is an agronomist with Farmer’s Edge. He can be reached by emailing [email protected]