Nitrogen losses from the soil

When designing a nitrogen fertilisation programme, it is imperative to take into account the nitrogen mobility factor of the soil. For example, sandy soils can lose nitrogen through leaching, while heavy, poorly drained soils can lose nitrogen through denitrification.

Leaching is the loss of soluble nitrate as it moves with the soil water below the root zone. Thus, nitrate that moves below the root zone can end up in groundwater or surface water. In contrast to the biological transformations described earlier, the loss of nitrate during leaching is a physical problem.

Soils with a coarse texture (sandy soils) have a lower water-holding capacity and therefore a higher potential for nitrate loss through leaching than soils with a fine texture (silt or clay soils). In general, it should be remembered that nitrate can leach from any soil if rain or irrigation moves water through the root zone. Denitrification can be the main pathway for nitrate loss when the soil is saturated with water for two to three days.

Significant nitrogen losses can also occur through the process of evaporation. In this case, nitrogen is lost in the form of gaseous ammonia (NH3). Fertilisers containing urea can lose nitrogen in this way. This is because ammonia (NH3) is an intermediate form of nitrogen in the process of converting urea to ammonium (NH4+). The loss of nitrogen through evaporation is greatest when: the soil pH is above 7.3; there is a high air temperature; the soil surface is moist and there are many crop residues on the soil.

Nitrogen can also be lost through soil erosion and runoff. The addition of manure and fertiliser can help protect against nitrogen loss through erosion or run-off. Where soils are highly susceptible to erosion, conventional tillage can reduce soil erosion and runoff, resulting in less surface nitrogen loss.