Nutrient Analysis (P,K,N)

Nmin in agricultural soils – In agricultural soils, critical limits for total N or available N (the mineral N content), related to specific soil functions, are difficult to define.
Organic layer of forest soils – Based on the relationship between N leaching and the C/N ratio in the organic layer of forests, C/N ratios of around 25 (between 20 and 30) are considered critical, with a very high N retention fraction and thus limited leaching risk at a C/N ratio above 30, while the N retention fraction is low and the leaching risk is high at a C/N ratio below 20; in between there is strong variation. A C/N ratio below a value of 25 is often suggested as a threshold value for increased leaching. (Table 3.2., EEA report 2023).

N Concentrations in air and water:

• NH3 in air: 1-3 mg NH3/m3 (Cape et al, 2009)
• N in soil solution: leakage from forests: 1mg N/l (De Vries et al, 2007)
• N in soil solution: impacts on forests: 1-5 mg N/l (De Vries et al, 2007)
• NO3 in groundwater: 50 mg NO3/l- (WHO, 2011)
• N in surface water: 1.0-2.5mg N/ (Camargo and Alonso, 2006)

P in agricultural soils:

• above a target level below which crop yield is limited (Mallarino and Blackmer 1992);
• below a critical level above which P leaching and run-off is significantly enhanced (e.g., Li et al., 2011). (See Bai et al, 2013 figure from EEA report 2023.)

Build up and maintenance approach (Li et al, 2011)

• not be made in soils with available soil P levels above the change point (threshold) for P leaching.
• equal the P withdrawal in harvested crops, if:
  • available soil P > target level for crop yield.
  • available soil P < critical level for P leaching.
• equal the P withdrawal in harvested crop plus an additional amount of P fertiliser, to build up available soil P to the required level, if:
  • available soil P < critical level for crop yield (Li et al., 2011). Critical Limits for dissolved P and Soil P saturation in agricultural soils
• The critical P saturation index (PSI) is mostly around 0.15, i.e. 15% (12.5-17.5%) of the concentration of (Al+Fe)ox, based on data for the Netherlands (Schoumans and Chardon, 2015) and Canada (Beauchemin and Simard, 1999).
• The critical value is expressed as 25-35% of the P sorption capacity, which in turn is calculated as 0.5×(Al+Fe)ox for sandy soils and non calcareous clay soils. Critical limits for N/P ratio in organic layer of forest soils
• N/P ratio in organic layer >18 (coniferous forests) and N/P ratio in organic layer >25 (deciduous forests).