114 ence in the form of the relationship between ψS ψPD: in sandy loam, an exponential model function Furthermore, there was a highly significant differ- and gave a good fit, while in the lava substrate, the cor- relation was almost linear. Soil hydraulic phenomena are more likely to explain the differing model func- tions than the different seasons. The soil sensors were placed outside the root ball, thus measuring the water potential of the surrounding soil, while ψPD reflected the water potential within the rhizosphere, i.e., mainly the root ball. So, the linear model function for the lava treatment implies that the soil water potential of the surrounding substrate and that of the root ball both changed in an equal manner. The exponential model function for the loam treatment on the other hand suggests that the soil water potential of the root ball decreased more rapidly than that of the surround- ing soil. Additionally, ψPD erally reached lower values than in the lava treatment. This means that for the same range of ψS in the loam treatment gen- measured within the planting substrate, the root ball dried out faster and more severely when it was surrounded by loamy soil than by lava substrate. In a hydraulic continuum, the total potential drop equals the sum of all partial potential drops along all corresponding partial flux resistances (van den Honert 1948; Richter 1973; Matyssek et al. 2010). The water potential drop between the root ball and its surround- ing loamy soil can therefore be explained by a corre- sponding partial flux resistance within the hydraulic soil-plant-atmosphere continuum. In general, the hydraulic conductivity of soils decreases with their saturation. Its magnitude and dynamics of change are determined by the soil’s pore size distribution. Soils high in clay, as the one used in the loam treatment, are dominated by fine pores and therefore have a lower hydraulic conductivity than the root ball substrate (Matyssek et al. 2010; Amelung et al. 2018). This may have constrained water flux towards the root ball. This hypothesis is in line with reports of unfavorable hydraulic effects when combining different soil types and root ball substrates (Balder and Strauch 2000). Results indicate that with sensors installed in con- tact with but outside the root ball, a site-specific cali- bration is necessary to estimate the plant water status. However, even with calibrated model functions, users are faced with significant estimation errors. Further- more, combinations of different soil types or sub- strates can in general lead to adverse hydraulic effects impairing the water supply. ©2021 International Society of Arboriculture Hertzler and Rust: Soil Moisture as Predictor of Plant Water Status Further studies should investigate if sensors installed in root balls both bypass the necessity of site-specific calibration and reduce the estimation error. If so, roots growing out of the root ball into the surrounding soil raise further questions of how long the sensors installed in the root ball remain represen- tative for the plant water status. LITERATURE CITED Amelung W, Blume HP, Fleige H, Korn R, Kandeler E, Kögel- Knabner I, Kretzschmar R, Stahr K, Wilke BM. 2018. Scheffer / Schachtschabel Lehrbuch der Bodenkunde. 17th Ed. Berlin (Germany): Springer-Verlag. 583 p. https://doi.org/10.1007/978 -3-662-55871-3 Balder H, Strauch KH. 2000. Wenn Bäume trotz Wasser verdursten. Deutsche Baumschule. 52:37-40. Centeno A, Baeza P, Lissarrague JR. 2010. Relationship between soil and plant water status in wine grapes under various water deficit regimes. HortTechnology. 20(3):585-593. https://doi .org/10.21273/HORTTECH.20.3.585 Cochard H, Forestier S, Améglio T. 2001. A new validation of the Scholander pressure chamber technique based on stem diameter variations. Journal of Experimental Botany. 52(359): 1361-1365. https://doi.org/10.1093/jexbot/52.359.1361 Fereres E, Goldhamer DA, Parsons LR. 2003. Irrigation water management of horticultural crops. HortScience. 38(5):1036- 1042. https://doi.org/10.21273/HORTSCI.38.5.1036 FLL. 2010. Empfehlungen für Baumpflanzungen—Teil 2: Stan- dortvorbereitungen für Neupflanzungen; Pflanzgruben und Wurzelraumerweiterung, Bauweisen und Substrate. Bonn (Germany): Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (FLL) e. V. 64 p. Goldhamer DA, Fereres E, Mata M, Girona J, Cohen M. 1999. Sensitivity of continuous and discrete plant and soil water status monitoring in peach trees subjected to deficit irrigation. Journal of the American Society for Horticultural Science. 124(4):437-444. https://doi.org/10.21273/JASHS.124.4.437 Intrigliolo DS, Castel JR. 2004. Continuous measurement of plant and soil water status for irrigation scheduling in plum. Irrigation Science. 23:93-102. https://doi.org/10.1007/s00271 -004-0097-7 Intrigliolo DS, Castel JR. 2006. Performance of various water stress indicators for prediction of fruit size response to deficit irrigation in plum. Agricultural Water Management. 83(1-2): 173-180. https://doi.org/10.1016/j.agwat.2005.12.005 Irmak S, Payero JO, VanDeWalle B, Rees J, Zoubek G, Martin DL, Kranz WL, Eisenhauer DE, Leininger D. 2016. Principles and operational characteristics of watermark granular matrix sen- sor to measure soil water status and its practical applications for irrigation management in various soil textures. Lincoln (NE, USA): University of Nebraska-Lincoln. Extension Report EC783. 14 p. https://extensionpublications.unl.edu/ assets/pdf/ec783.pdf Jones HG. 2004. Irrigation scheduling: Advantages and pitfalls of plant-based methods. Journal of Experimental Botany. 55(407):2427-2436. https://doi.org/10.1093/jxb/erh213
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