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Nutrient Analysis (P,K,N)

  • Soil Health

  • Chemical


Understanding the nitrogen supply stored in the SOM is fundamental to understanding how well the soil can support the microbial populations crucial for sustaining vital soil functions, including nutrient cycling, structural stability, water infiltration and storage, as well as the breakdown of organic residues, among other critical processes (Merrington 2006; Creamer et al., 2016).

Phosphorus, as an essential macronutrient, is required by all living organisms. Phosphorus is a crucial component of ATP (adenosine triphosphate), which is the primary energy carrier in living cells. This energy transfer is essential for various biochemical processes in plants, microbes, and other organisms in the soil (Merrington 2006).
Healthy phosphorus levels enhance the soil’s ability to sustain diverse plant and microbial communities. On the other hand, unhealthy phosphorus levels may identify an environmental hazard (Allen et al., 2011).

Potassium plays a significant role in influencing plant growth, species composition, and overall ecosystem function (Sardans & Peñuelas, 2021).

Methodology summary

Nutrient analysis should be carried out by a laboratory as it requires a certain level of expertise.

The following methods are the most commonly used for measuring nitrogen:

The Olsen P method is a widely used technique for measuring available phosphorus (P) in soil.

Potassium can be extracted from soil using ammonium nitrate.

Detailed information on soil sampling such as where to take the samples from, how many, the best time to sample, and depth of sampling, can be found at the Farm Carbon Toolkit.

Metric threshold or direction of change

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).

Technological innovations

The development of smart soil sensors using IoT (Internet-of-Things)-based systems for analysing and monitoring soil nutrients in agriculture is a promising tool for monitoring soil health.

These systems utilize a network of digital sensors that can provide real-time measurements. They are capable of quickly determining the nutrient content of the soil, including nitrogen, phosphorus, and potassium levels. The network of sensors can provide real-time measurements of other soil properties such as moisture, pH, electrical conductivity, and other plant properties at the same time as providing a NPK (nitrogen, phosphorus, and potassium) content in the soil (Pyingkodi et al., 2022; Ramson et al., 2021; Soetedjo & Hendriarianti, 2023).

Ion selective field effect transistor (ISFET) – ISFET is a modification of the normal field effect transistor used in many amplifier circuits. In the ISFET, the metal gate, which is normally used as input, is replaced by an ion-sensitive membrane, the measured solution, and a reference electrode Thus, an ISFET combines in one device a sensing surface and a signal amplifier which produces a high current, low impedance output and allows the use of connecting cables without excessive shielding (Kumar et al, 2015).

Conductimetric pH sensor – A standard conductimetric sensor consists of two identical electrodes, between which a sensing layer is deposited (Kumar et al, 2015). Also utilises polymers to help gathering of pH data.


  • Agricultural
  • Forest
  • Grassland
  • Peatland
  • Saltmarsh
  • Wetland


  • Community


  • Medium


  • Tier 1

Technical expertise

  • Low

Standardised methodology

  • Partial