The content of plant nutrients is an indicator that represents the supply of agricultural land in Slovenia with the main plant nutrients, i.e. phosphorus (P) and potassium (K), accompanied by universal indicators such as soil acidity (pH) and soil organic matter content (OMC). The nitrogen content is indicated by the organic matter content.

The indicator presents the plant nutrient content for the following categories: extensive orchard, field, olive grow, pasture, orchard, and meadow.

The indicator is a summary of statistical processing of analytical values of parameters measured in soil samples.

The values are expressed as median values.

Preservation of soil fertility of agricultural land is stipulated in the Agricultural Land Act, regulations, and other laws. Preservation of high-quality agricultural soils is also a constitutional category.

With each crop, we also 'take' plant nutrients out of the soil. In order to prevent nutrient depletion, acidification and thus reduced soil fertility (i.e. soil degradation), nutrients should be returned to the soil by fertilizing in such quantities and in an environmentally sound manner that the soil contains appropriate/optimal amounts as specified in Guidelines for professionally justified fertilization (Mihelič et al., 2010). However, the optimal and achievable contents may vary.

The P content in the soil samples of the 2015–2020 period is most frequently (too) low in the upper soil layers of extensive orchards (3.4), very low in the soils of olive groves (6.9), meadows (4.7) and pastures (3.5), below optimum in orchards (10.8) and vineyards (9.6), and suitable in fields (19.0 - all contents are expressed in mg/100g of air-dry soil). Some (significantly) higher average values (arithmetic means) indicate a smaller number of sites oversupplied with P.

For K, soils of olive groves (25.2), meadows (18.1) and pastures (20.2) are adequately stocked, while contents are elevated in fields (24.8) and vineyards (27.5). For K, some higher average values also indicate a smaller number of sites that are oversupplied with K.

Soil organic matter content (SOM) is suitable in soils of fields (4.2%), orchards (5.6%), as expected good in soils of grassland (5.8) and lower in olive groves (3.2%) and vineyards (2.9%) due to climatic zone and production method.

Soil acidity is lower in orchards (5.9) and highest in olive groves (7.5) due to carbonate soils.

The indicator shows the situation in the period 2015–2020 and does not yet indicate the direction of development. Trends will be identified at the next update of the indicator at the end of the next four-year period.

In the analysis, the sometimes-limited geographical scope and number of soil samples should be mentioned. Data from the laboratory of the Agricultural Institute of Slovenia are processed. Unfortunately, data on soil properties are not (yet) systematically collected and processed in Slovenia. Analytical data from different soil laboratories are not yet merged and processed in a uniform way. Sometimes the results of measurements from different laboratories are also difficult to compare, as the methods are not fully standardized. This prevents a comprehensive and exhaustive review of the nutrient content and quality of agricultural soils. Nevertheless, we estimate that the developed indicator provides an insight into the state of nutrients in agricultural soils in Slovenia.

It should be noted that soil fertility includes not only chemical but also physical properties of soil. We have selected four indicators from which it is possible to infer the state of some other soil parameters. For example, it is possible to infer from the organic matter content the content of more stable forms of nitrogen in the soil, soil structure, soil porosity and permeability, retention and purification capacity, drought resistance and other physical, chemical and biotic properties, as well as the ability to provide ecosystem services of the soil. The soil organic matter content in the Slovenian agricultural soils is comparable to values in other countries of similar climatic zones and (very) good compared to Mediterranean EU member states (ESDAC 2010).