Soil is a major source of nutrients needed by plants for growth. The three main nutrients in soils are nitrogen (N), phosphorus (P) and potassium (K). Together they make up the well-known NPK.
Nitrogen (N) is a key element in plant growth. Among all the essential nutrients, Nitrogen is required by plants in the largest quantity and is important for soil fertility and soil quality (Reeves, 1997). Nitrogen is the most frequent limiting factor in crop productivity (Smil, 1999). The spatial distribution of nitrogen in the soil is affected not only by natural ecological processes but also by intensive human activities (K. Wang et al., 2013). This is an important challenge for accurate predictive mapping at regional scales.
Phosphorus is mainly derived from the weathering of minerals in parent rock material. It is usually the second most limiting nutrient for terrestrial primary production (Cordell et al., 2009). In agricultural areas fertilisation can result in higher levels of P, especially in highly productive areas where high input of P fertilisers is reported (Tóth et al., 2014). Modern agriculture is highly dependent on P fertilisers, and P supply is strategically critical at global level.
Potassium has different functions for plant life; it is a constituent of enzymes and acts as a regulator of drought tolerance and water use (M. Wang et al., 2013). In the soil, the principal sources of potassium are feldspars and micas, which release K during weathering (Hillel, 2008). In the soil itself, potassium appears in three forms: in the circulating solution; as an exchangeable ion adsorbed to the surface of clay particles; and in organic matter. Given the wide distribution of K-containing minerals and the fact that it is prevented from leaching by cation exchange, its depletion from the soil is quite uncommon.
Ballabio, C., Lugato, E., Fernández-Ugalde, O., Orgiazzi, A., Jones, A., Borrelli, P., Montanarella, L. and Panagos, P., 2019. Mapping LUCAS topsoil chemical properties at European scale using Gaussian process regression. Geoderma, 355: 113912.