## 4 Discussion
### FDR Response to Soil Properties  
It is well known that the FDR response is affected by soil bulk density, clay and organic matter content, voids and also salts (electrical conductivity). Therefore, we believe it is best that each sensor is calibrated in situ for the specific soil horizon it is located in, since the unique combination of all these factors in each horizon is hard to simulate in the lab, let alone their interactions. Soil bulk density (\(\rho_{\text{b}}\)) is one of the most spatially variable soil properties, especially in the topsoil of forest soils [33]. Moreover, if \(\rho_{\text{b}}\) is not determined from the same sample as the mass basis water content, there will be an error in the calculation of \(\theta_{\text{v}}\)[11] and this may seriously affect the quality of any calibration or validation dataset. Bulk density in forest soils is strongly and inversely related with organic matter content, which is its best predictor. The more organic the soil, the lower \(\rho_{\text{b}}\), the higher the meso- and macroporosity and the more water can be stored in the soil. In fine textured soils, such as clays, their volume will change as they dry, so their \(\rho_{\text{b}}\) may not be a constant and the relationship between FDR response and \(\rho_{\text{b}}\) may not be constant as well. Besides, if the soils shrink, this might change the contact of the rods with the soil which will affect measurement accuracy. The response of the CS616 to changing water content has been shown to change for some soils when bulk density exceeds 1500 kg m\({}^{-3}\), which is seldom the case for the studied soils. This response to changing water content is still well behaved, but the slope will decrease with increasing bulk density [20]. Most laboratory calibrations in small or larger containers try to mimic the average soil \(\rho_{\text{b}}\) of the field. Since \(\rho_{\text{b}}\) is strongly determining the RMSPE as we found in our study, lab conditions will presumably underestimate real intrinsic variation in field conditions and thus underestimate real RMSPE. A big advantage of the CS616 is its larger measurement volume--about 3740 cm\({}^{3}\) compared to other EM sensors mostly below 1000 cm\({}^{3}\)[10]--which results in less spatial variability than probes with a smaller measurement volume [3]. We expect effects from \(\rho_{\text{b}}\) and other soil properties to be averaged over the 30 cm long rods. The amount of organic matter and clay in a soil can alter the response of dielectric-dependent methods to changes in water content. This is apparent when mechanistic models are used to describe this measurement methodology, but less clear by using empirical calibration models [11]. Standard calibration coefficients are given for the CS616 for mineral soils with clay content less than 30% [20]. Consequently, higher clay contents require specific calibration but this calibration is also influenced by conductivity, compaction (\(\rho_{\text{b}}\)) and temperature. Therefore [10] suggested that manufacturers supply calibration relationships for a general group of mineral soils (covering a wide range of textures) and specifically for sandy and clayey soils. [PERSON] et al. [15] suggested that future research should focus on the effects of clay mineralogy on FDR response. We found that fitting calibration functions for topsoil layers and especially for OFH layers as part of the forest floor is quite complex and difficult. This is partly due to the high intrinsic heterogeneity of the organo-mineral toposils in forests both for the organic matter content and the resulting \(\rho_{\text{b}}\). Since most studies on FDR are conducted in arable soils, we found no papers dealing with FDR measurements in forest floor layers. For TDR, [PERSON] et al. [34] developed an adequate linear calibration function for forest floor media.