DOI: 10.5061/dryad.76hdr7svc
title: Soil water retention curves and soil physicochemical properties

description: To identify the contribution of soil organic and inorganic fractions to
 soil water retention, we compiled the data of soil water retention curves
 and soil physicochemical properties. Data includes site information (Table
 1), soil pH, soil C, cation exchange capacity (CEC), exchangeable basic
 cations, particle size distribution, soil texture class, contents of
 short-range-order minerals (Table 2), and soil water retention at
 different pressures, saturated hydraulic conductivity, and termite nest
 percent in soil profile (Table 3).
description: Data compilation Data was compiled from our study and
 the published data sources by searching “soil moisture characteristics
 curve” or “soil water retention curves” or “pF curves” in Google Scholar.
 The papers were selected when soil physicochemical properties related
 especially to soil organic component (organic matter or carbon content)
 and inorganic component (clay, silt, sand, and short-range-order (SRO)
 minerals) were provided. Physicochemical properties of soil
 samples Soil samples were air-dried to pass
 through a 2 mm sieve. Soil pH was measured using a soil-to-solution
 (H<sub>2</sub>O or 1 M KCl) ratio of 1:5 (w/v) after shaking
 for 1 h. Total C concentrations in soils were determined using a CN
 analyzer (Vario Max CN, Elementar Analysensystem GmbH). The exchangeable
 basic cation concentrations and cation exchange capacity (CEC) were
 determined using the ammonium acetate (1 M and pH 7.0) method.
 Exchangeable Ca and Mg were measured using atomic absorption spectroscopy
 (A-A-640-01, Shimadzu). Exchangeable K was measured by flame
 photometry. Particle size distribution [clay
 (&lt;0.002 mm); silt (0.002―0.05 mm); sand (0.050―2 mm)] was
 determined by the pipette method. The contents of short-range-ordered
 (SRO) Fe and Al (hydr) substances (Fe<sub>o</sub>,
 Al<sub>o</sub>, Si<sub>o</sub>) in soils were
 estimated by extraction in the dark with acidic (pH 3) 0.2 M ammonium
 oxalate (McKeague and Day, 1966). The contents of crystalline and SRO Fe
 and Al oxides and organic Fe and Al compounds (Fe<sub>d</sub>,
 Al<sub>d</sub>) were estimated by extraction with a
 citrate-bicarbonate mixed solution buffered at pH 7.3 with the addition of
 sodium dithionite (DCB) at 80°C (Mehra and Jackson, 1960). The contents of
 Fe and Al in organo-mineral complexes (Fe<sub>p</sub>,
 Al<sub>p</sub>) were estimated by extraction with 0.1 M
 pyrophosphate at pH 10 for 16 h (Schuppli et al., 1983). The Fe and Al
 concentrations in soil extracts were determined using an inductively
 coupled plasma atomic emission spectrometer (ICP-AES, SPS1500, Seiko
 Instruments Inc.). The contents of allophane in the
 soil samples were estimated by multiplying Si<sub>o</sub> with
 a factor (7.14), while those of ferrihydrite were estimated by multiplying
 Fe<sub>o</sub> with a factor (1.7) (Parfitt and Henmi, 1982;
 Parfitt and Child, 1988), written as follows: Allophane content (%) =
 Si<sub>o</sub> content (%) ×
 7.14                                        (1) Ferrihydrite content (%) =
 Fe<sub>o</sub> content (%) × 1.7                        
               (2) The contents of crystalline clay
 minerals were estimated by subtracting the contents of allophane and
 ferrihydrite from the clay content, which was recalculated on the basis of
 bulk soil (Parfitt and Henmi, 1982; Parfitt and Child, 1988).
 Crystalline mineral content (%) =
 Clay content (%) × {100 – Total soil C (%)×1.8}/100 – Allophane content
 (%) – Ferrihydrite content
 (%)                                                (3) Soil water retention curves and
 saturated hydraulic conductivity We measured bulk density by oven
 drying (105°C, 24 h) of 0.1 L core soil samples and saturated hydraulic
 conductivity [<em>K<sub>s</sub></em> (m
 day<sup>-1</sup>)] using constant head or falling-head methods
 (Klute, 1965). The soil water retention curves were experimentally
 obtained for undisturbed soils sampled by 0.1 L cores using a sand column
 method (DIK-3521; Daiki Rika Kogyo Co., Ltd., Saitama, Japan) for 0
 (saturated) to –3.1 kPa and pressure plate (DIK-3404, Daiki Rika Kogyo
 Co., Ltd.) for –9.8 to –98.0 kPa, centrifugation method for pF 4.2
 (Kokusan or Hitachi Himac centrifuge), air-drying for pF 5.5, and oven
 drying (105°C, 24 h) for pF 7.0. References 
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