Soils have many functions, such as producing crops, filtering water by binding nutrients and solutes, serving as a carbon pool and providing a habitat for all kinds of soil fauna. Soil quality can be assessed using easily obtainable soil information, such as soil maps and visual soil evaluation (VSE). The aim of this thesis is to evaluate whether easily obtainable soil information (soil maps and visual soil evaluation) can be used to assess soil functions. To reach the thesis aim and to bridge the knowledge gaps, three research questions were formulated: 1) to what extent does the required spatial scale in nutrient balances depend on the level of soil variation, 2) to what extent are quantitative visual soil observations reproducible, and do they correlate with standard field or laboratory measurements, and 3) can quantitative visual soil observations be used to assess soil functioning? This thesis thereby contributes to the assessment of soil quality in agricultural land, which can contribute to environmentally sustainable crop production.
After the general introduction (Chapter 1), Chapter 2 discusses the role of spatial scales in nutrient balances on dairy farms. On five Dutch dairy farms, field level nitrogen (N) and phosphorus (P) balances were associated with soil series that were obtained from a 1:50.000 soil map. It is concluded that nutrient balances at field level are more informative than nutrient balances at the crop or farm level, which is relevant for decision making aiming to reduce nutrient losses to the environment. Chapter 3 uses quantitative visual observations in VSE. The two aims of this Chapter were to assess the reproducibility of quantitative visual observations, and to evaluate the correlation of quantitative visual observations with standard field or laboratory measurements. The reproducibility study and the validation study show that quantitative visual soil observations are moderately reliable, given the high probability that systematic errors are made by observers, and given the fact that more than half of the quantitative visual soil observations could not be validated. Chapter 4 investigates the correlation between quantitative visual soil observations and indicators for crop production (crop yield gaps as result of water surplus or water limitation, oxygen and water stress, and plant available water) and the storage, filtering and transformation of water and nutrients (water storage capacity, nitrate and phosphate concentrations in drain or groundwater). A stepwise linear regression approach showed that, except for crop drought stress in a dry year, the soil function indicators significantly correlate with a set of quantitative visual soil observations. As it was found that the interaction with clay content improved the correlation between several quantitative visual observations and soil function indicators, it may be ideal to include soil texture effects in future VSEs. The synthesis (Chapter 5) presents the overall conclusions of the previous chapters, which is that easily obtainable soil information can be used to assess soil functioning. The use of soil series can improve environmental assessments such as nutrient balances and life cycle assessment. Besides, soil series can be used in the assessment of the soil functions ‘serving as a carbon pool’, and ‘serving as archive for geological and cultural heritage’. Next to the use of soil series to assess soil functions, VSE can be used to assess soil functions that are dependent on soil physical properties, such as crop production and storing, filtering and transforming nutrients and water. Also, VSE shows potential to assess the soil functions ‘serving as carbon pool’ and ‘serving as archive for geological and cultural heritage’. The combination of soil series and VSE is an opportunity for assessing soil functioning. Soil maps provide the soil inherent characteristics that can be used to define the soils’ potential to function, and VSE assesses dynamic soil properties that can easily change after management actions. The combination of soil series and VSE, therefore, provides the soil information that is needed to assess and evaluate soil functions in relation to the soils’ potential to function. This thesis shows that easily obtainable soil information (soil maps and visual soil evaluation) can be used to assess soil functions, which contributes to the assessment of soil quality in agricultural land, and which can contribute to environmentally sustainable crop production.