The quantity of N that is taken up and utilised by the crop will also differ largely between dry and wet seasons.
For this reason it can be expected that more N will be applied during a wet season to maintain the quantity of N in the soil. It can also be expected that more N will be left over in the soil after a dry season which can effectively be utilised during the next season. The management of a threshold value for N in the soil for every season will effectively result in fertilisation according to obtained yield and N removal from the soil over seasons.
N-losses and N-toxicity effects will however very strongly be affected by an under or over supply of rain. Apart from soil N-measurements, choice of N-source and N-management practises can effectively be used to reduce these negative effects.
El Nino conditions also coincide with high temperatures resulting in volatilisation losses from ammonia forming products such as urea. Ammonia losses can result from surface applications as well as soil incorporated applications when the topsoil dries out. Ammonia released in close proximity of plant roots will be toxic under dry conditions. Urease inhibitors such as NBPT will effectively reduce or delay volatilisation and toxicity from urea but will not eliminate these effects. Almost no N will volatilise from LAN even at high temperatures. LAN will only be moderately toxic at high concentrations. The band placement of high concentrations ammonia forming N-sources at planting but even before plating should therefore be avoided.
La Nina conditions also coincides with heavy downpours over short periods resulting in N-leaching in well drain soils or water logging in poorly drained soils. Urea-N and nitrate-N are equally leachable but due to the fact that nitrate uptake is much quicker it will effectively leach much less than urea. Ammonium-N does not leach significantly and is also taken up much quicker than urea-N. LAN will therefore also leach much less than urea. Due to the possible risk of leaching pre-plant applications should rather be avoided and multiple post-plant topdressings considered.
N is not taken up effectively in soils that are waterlogged for prolonged periods.
Oxygen is required for N-uptake but also for the nitrification process. Consequently high levels of ammonium-N and nitrite-N, which are toxic, will accumulate. Nitrate-N dissolved in soil water near the soil surface will be converted to atmospheric N through the denitrification process and lost. Vertical or lateral drainage of soils should improve this condition.
