Easily obtainable soil information: when to use which method?

This thesis showed that easily obtainable soil information can be used to assess soil functioning, in order to improve the environmental performance of agricultural land.

Instead of focussing on a single soil function, soil quality assessments ideally assess various soil functions at the same time, because of synergies and trade-offs between soil functions (Schröder et al., 2016a). For each soil function the question remains what the preferred assessment method is: soil series or VSE? Table 5.1 and 5.2 provide an overview of the overall challenges and opportunities of using soil series and VSE to assess soil functions, followed by an overview of the use of soil series and VSE for assessing each soil function.

Chapter 2 addressed the use of soil series to provide soil-specific information in nutrient balances, and the required spatial scale in nutrient balances, given any soil variation. Other soil functions that likely can be assessed with soil series are the soil functions ‘serving as a carbon pool’ and ‘serving as archive for geological and cultural heritage’ (Table 5.1, 5.2). To identify areas that have most capacity to store carbon, soil Table 5.1. Overview of strengths, challenges and opportunities of soil series and visual soil evaluation, in relation to the assessment of soil functions (Table 5.2).

Soil series Visual soil evaluation (VSE)

Strengths Insight in spatial variation of

a mapped area. when deploying various VSEs.

Identification of dynamic soil properties that reflect management impacts (Karlen et al., 2001).

Raising awareness of soil quality (Ball et al., 2018).

Challenges Availability of detailed soil

maps.

Availability of VSE and soil function data to develop region-specific scoring functions (Section 5.3.4).

Opportunities Combination of soil series and measured soil physical and hydrological properties (e.g. BOFEK and HYPRES:

Wösten et al., 2013, 1999).

VSE can be easily extended with soil profile descriptions and classification, if soil maps are not available.

Combination of soil maps and VSEs to assess soil functions.

Chapter 5

Table 5.2. Overview of the potential suitability of using soil series and visual soil evaluation to assess soil functions.

Soil function Soil series Visual soil evaluation (VSE)

Crop production

Maybe: indicator for

inherent fertility of soils (Smaling et al., 1997), but

Maybe: biomass and functional group diversity of earthworms as combination with soil texture (Wills et al., 2007), in combination disturbance, and soil profile descriptions (e.g. FAO, 2006).

series need to be supplemented with climate data, and land use data (e.g. Stoorvogel et al., 2017). For the soil function ‘serving as archive for geological and cultural heritage’ it is questionable how to quantify this function. Indicators derived from the soil map could be the layering of soil, drainage conditions (e.g. preservation of peat bodies requires anaerobe conditions) and the origin of soil (e.g. the manmade ‘enk’

earth soils, or ‘plaggic anthrosols’, are literally an archive of cultural heritage). These indicators provide details about the ‘protection potential’ of the soil, to serve as geological archive and to preserve archaeological artefacts (Van Beek, Pers. Comm. 11-03-2019). Soil series can furthermore be an indirect indicator for crop growth, depending on the climate region and intensity of farming systems, because soil series can provide information about inherent soil fertility (Smaling et al., 1997). As discussed in Chapter 4, however, crop production is preferably assessed based on a combination of soil physical and chemical properties, and management data.

Furthermore, soil series can be related to soil hydraulic parameters by implementing pedotransfer functions (e.g. BOFEK for the Netherlands Wösten et al., 2013, and HYPRES for Europe Wösten et al., 1999), which may improve the assessment of soil functions that are driven by soil hydrological properties. Overall, soil series may show most potential in the assessment of the soil functions ‘storing, filtering, and transforming nutrients and water’, ‘serving as a carbon pool’, and ‘serving as archive for geological and cultural heritage’ (Table 5.1, 5.2).

VSE is a more direct indicator than the use of soil series, to assess soil functioning.

Next to assessing crop production and storing, filtering, and transforming nutrients and water using VSE (Chapter 4), VSE has potential to assess the soil functions

‘maintaining biodiversity’, ‘serving as carbon pool’, and ‘serving as archive for geological and cultural heritage’ (Table 1). A visual indicator for biodiversity is the biomass and functional group diversity of earthworms (Pulleman et al., 2012;

Shepherd, 2009). More challenging, however, is to assess the soils’ potential to maintain biodiversity, because many soil processes interact with biodiversity. For the soil function ‘serving as archive for geological and cultural heritage’, currently VSE does not include indicators. Indicators that may be added to VSE are the number and description of found archaeological artefacts in a soil block (e.g. pieces of plastic, bones, charcoal, brick and clay pipes); and the degree of soil disturbance (e.g. mixing of soil layers and drainage of peat). Also, soil profile descriptions (e.g. FAO, 2006) could be used to assess the presence of soil layers that protect deeper soil layers containing archaeological artefacts (Van Beek, Pers. Comm. 11-03-2019). Next to the assessment of ‘crop production’ and ‘storing water’, VSE shows potential to assess the soil functions ‘serving as carbon pool’ and ‘serving as archive for geological and cultural heritage’.

Chapter 5

Soil series and VSE could be used separately to assess soil functions, but the combination of soil series and VSE is an opportunity for assessing soil functioning.

First of all, soil series provide insight into the spatial variation of soil, which can be used to determine the VSE sampling locations (as done in Chapter 4, and e.g.

Sonneveld et al., 2014). Secondly, soil maps provide the soil inherent characteristics that can be used to define the soils’ potential to function, and VSE reflect dynamic soil properties that can easily change after management impacts (e.g. Karlen et al., 2001).

The combination of soil series and VSE, therefore, provide the soil information that is needed to assess and evaluate soil functions in relation to the soils’ potential to function (Karlen et al., 2001). If soil maps are not available to complement VSE, visual soil profile descriptions and classifications can be performed using e.g. FAO (2006).

To conclude: soil series and VSE can be used to assess various soil functions, but the combination of the two methods is ideal.

In the following sections the main focus is on VSE, as there are still several challenges for the development of scoring functions to evaluate soil functions.