Measuring Soil Water Content Through Volume/Mass Replacement Using a Constant Volume Container

Geoderma 271 (2016) 42–49

Contents lists available at ScienceDirect

Geoderma

journal homepage: www.elsevier.com/locate/geoderma

Measuring soil water content through volume/mass replacement using a constant volume container

Yuying Ma a,LiqinQub, Wei Wang c,⁎, Xiusheng Yang d,e,TingwuLeia,e,⁎⁎ a College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, China b China Institute of Water Resources and Hydropower Research, Beijing 100048, China c College of Engineering, China Agricultural University, Beijing 100083, China d Department of Natural Resources and the Environment, University of Connecticut, Storrs CT 06269, USA e State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, CAS and MWR, Yangling 712100, China article info abstract

Article history: Accurate and rapid measurement of soil water content is important in research and applications, such as Received 29 September 2015 hydrological processes, farmland water management, and ecological issues. Soil particle density (SPD) and Received in revised form 29 January 2016 water content were measured using a proposed constant volume container (CVC) through volume/mass Accepted 3 February 2016 replacement (VMR). The CVC consisted of a vessel body and a lid to limit the water surface to a constant level, Available online 23 February 2016 with 3-mm-diameter inlet to fill up the container with water to the full level as well as an outlet orifice to drain excess water. The mathematical algorithms for partitioning water and soil particles of the soil sample Keywords: Constant volume container were detailed. Measurements and computational procedures were performed on two soils with varied Volume/mass replacement method water contents. The unique structure of the CVC effectively eliminated volume measurement errors caused by Soil particle density liquid surface tension, resulting in 50% lower measurement standard deviation of SPD compared with those Organic matter content obtained using a pycnometer or a volumetric flask. The SPD values of loam and clay loam soil were 2.64 and 2.66 Mg m−3, respectively. The soil water contents of the tested soils determined with the measured SPDs by using the CVC exhibited equivalent accuracies to those measured through oven-drying, with R2 =0.98. Moreover, soil water content (5–35%) measured with an assumed SPD of 2.65 through VMR using the CVC showed an equivalent accuracy to that determined through oven-drying. This finding indicated the feasibility of measuring soil water content through VMR using the SPD of 2.65 Mg m−3 in these two experimental soils. Theoretical analysis and computation showed that soil organic matter content (SOM) affected SPD, thereby indirectly influenced the accuracy of soil water content measurements through VMR. The VMR method with the CVC could achieve acceptable accuracy with SOM values lower than 2% and soil water content higher than 5%. This approach can be used as an alternative to oven-drying with appropriate accuracy and efficiency, and it is potentially useful for measuring soil water content or bulk density in the field. © 2016 Elsevier B.V. All rights reserved.

1. Introduction content also plays an important role in plant growth (Robinson et al., 2008), and it is related to irrigation management and exchange of Determination of soil water content is important in studies of mass and energy in the soil–plant–atmosphere continuum (Yin et al., heat, water and chemical transport and retention in soil. The physical 2013). The amount of water available to plants from the soil determines properties of soil should be characterized to understand agricultural when and how much irrigation water should be applied (Evans et al., and environmental processes in modern soil physics research and appli- 1996). Application of excessive water may decrease crop quality as a cations. Soil water content is an essential part of the air, liquid, and solid result of the adverse effects of waterlogged plant roots; by contrast, systems of soil, and it partly controls the separation of precipitation into treatment with insufficient water may lead to irreversible damage to infiltration, evaporation, and runoff, as well as considerably influences crops because of drought stress (Huisman et al., 2003). Therefore, soil erosion and river discharge (Huisman et al., 2003). Soil water accurate and rapid measurement of soil water content is important in water and fertilizer management (Gao et al., 2012), as well as in the management of various geostructures, such as pavements, foundations, ⁎ Correspondence to: W. Wang, PO Box 146, China Agricultural University, No. 17 and earthen dams (Vereecken et al., 2008). Overall, determination Qinghua Donglu, Haidian District, Beijing 100083, China. of soil water content is of paramount importance in engineering, ⁎⁎ Correspondence to: T. Lei, PO Box 151, China Agricultural University, No. 17 Qinghua Donglu, Haidian District, Beijing 100083, China. agronomy (Haines, 1923), geology, ecology, and biology (Campbell, E-mail addresses: [email protected] (W. Wang), [email protected] (T. Lei). 1974).

Thermal drying, particularly oven-drying, is the only available using a wide-mouthed pycnometer for determination of soil water con- method that directly measures soil water content, and can be used tent; this technique requires less processing time per sample but should to calibrate other indirect approaches (Schmugge et al., 1980; be carefully conducted to ensure its accuracy. In this method, soil water Terhoeven-Urselmans et al., 2008), such as neutron scattering content should be measured under a constant temperature and the (Amoozegar et al., 1989; Chanasyk and Naeth, 1996; Elder and measurement requires a shaker; thus, this pycnometer method is Rasmussen, 1994; Fityus et al., 2011; Jayawardane et al., 1984; Li et al., unsuitable for field measurement. In addition, the procedure for deter- 2003), and dielectric techniques, including time domain reflectometry, mining SPD remains unclear, despite the fact that this parameter must frequency domain reflectometry, and capacitance analysis (Eller and be included in their proposed equations. Denoth, 1996; Gardner et al., 1997; Jacobsen and Schjonning, 1993; Rapid and accurate determination of the volume of water and soil Kelleners et al., 2004; Rao and Singh, 2011; Robinson and Dean, 1993; particle mixture is a key factor that affects the feasibility of soil water Selig and Manusukhani, 1975; Topp et al., 1982; Topp and Reynolds, measurement through volume/mass replacement (VMR). To the best 1998; Topp et al., 2000; Whalley et al., 1992). These indirect measu- of our knowledge, a direct and accurate method for measuring solid rement techniques are widely used, but their accuracy depends on the volume with irregular shape has not been established, provided that users' experience, careful calibration and soil properties. Oven-drying this material presents shape of scattered granules and uncertain constit- shows high accuracy and reliability but requires a long time to deliver uents. Researchers have experimented with filling a graduated glass or a results. Furthermore, the measuring accuracy of this technique is measuring cylinder with a certain amount of water; in this method, the influenced by drying period, which varies from one soil to another as solid matter to be measured is placed in the water, and the increase in required by different standards, thereby produces systematic measure- water volume is read to determine the volume of the solid. Although ment errors. This technique also requires the use of an oven and is this method seems reasonable, potential measurement errors cannot therefore not feasible for field work. In some cases, the alcohol burning be neglected, considering that the measurer cannot accurately deter- method may be used to rapidly measure soil water content. However, mine the increase in height of the water surface because of liquid this method is unsuitable for soil with a high proportion of organic surface tension. The same limitation exists when measuring the volume matter. of pure liquid. Thus far, a measuring cylinder, or a graduated glass is Volume replacement can be used to measure soil water as long as commonly used for liquid volume measurements, but this technique the replacing substance is easy to handle or measure. The volume or yields a relatively low accuracy. In most cases, soil samples exist in the mass of water is easier to measure than that of air; hence, water can form of a mixture of water and soil particles or a mixture of water, soil be used to replace air in the soil in volume replacement method. particles, and air; and the volume of this multi-phase mixture is usually Papadakis (1941) described a rapid replacement method for soil water difficult to be accurately measured using a traditional graduated measurement using a flask; however, this method has not been widely cylinder or glass. accepted because of the difficulty in transferring the wet soil sample Particulate organic matter (POM) and light fraction (LF) organic mat- into the flask. Moreover, the errors induced by variation in water densi- ter are potentially labile (active) fractions of soil organic matter (SOM) ty and exact volume of the flask should be determined when using this and are used as indicators of short-term changes in soil management approach. Prihar and Sandhu (1967) resolved the limitation of time lag practices. These two fractions mainly consist of partially decomposed between soil sampling and availability of measurements by using the plant residues, microbial residues, seeds, and spores forming organo- replacement method. In their approach, accurate meniscus readings re- mineral complexes with soil mineral particles (Sequeira et al., 2011). main difficult because of liquid surface tension; foam floating on the liq- Gregorich and Janzen (1996) defined LF as the fraction recovered by uid surface is also difficult to remove, and may affect the accuracy of the density fractionation and POM as the fraction recovered by size frac- readout. Furthermore, the accuracy of this approach is influenced by the tionation alone or by the combination of size and density fractionation uncertainty of whether trapped air is completely expelled by shaking procedures. The density of SOM ranges from 1.1 Mg m−3 to such a long-necked flask. During the measurement, a pipette was 1.5 Mg m−3 (Ruhlmann et al., 2006), which is much lower than SPD. used, which is tedious to operate. They took the soil particle density When the proportion of organic matter in soil is high, the accuracy of (SPD) as 2.67 Mg m−3 and for soil with different SPD values, soil soil water measurement may be influenced; thus, the appropriate appli- water content could be obtained by adding or subtracting 0.5 for cation scope of the VMR method for determining soil water content every 0.02 Mg m−3 change in SPD; however, the basis for such adjust- needs to be defined. ments has not been explained. The proposed two-piece flask method In this study, a constant volume container (CVC) was proposed and also requires longer processing time per sample than oven-drying. designed to rapidly and accurately measure the volume of soil mixture Aggarwal and Tripathi (1975) also reported a replacement method for determining soil water content through VMR. The procedures for

Fig. 1. Structure of the constant volume container. 44 Y. Ma et al. / Geoderma 271 (2016) 42–49 measuring soil water content and SPD were illustrated in this study. Correlation analysis was also conducted to compare the VMR and oven-drying methods. Furthermore, the possible effect of SOM on measurement accuracy was theoretically analyzed.

2. Materials and methods

2.1. Structure of the CVC

In this study, accurate volume measurement of water and soil particle mixture is the basis of VMR. The proposed CVC is a key apparatus for volume measurement. Fig. 1 illustrates the structure of the CVC both photographically and schematically. The lid (2) accurately fits the vessel body (1) to ensure that air is not trapped between the lid and the vessel body. The lower Fig. 2. Relationship among Vc, Vr, Va, Vw,andVsp in the constant volume container (CVC). brim of the inlet (3) and any parts of the lid bottom are positioned lower than the upper brim of the overflow outlet (4). The upper brim of the inlet is higher than that of the outlet. The lid is positioned over 2.3. Determination of soil particle density the vessel, and the inlet and outlet are situated on opposite sides of the closed apparatus. As the water rises to a level higher than the Soil particle density can be determined by using the CVC. The dried lower edge of the overflow outlet, excessive water drains out of the con- soil of known mass is placed into the CVC before the container is filled tainer. Therefore, a constant volume of water or a mixture of soil parti- up with distilled water. The volume of the soil particles and their density cles and water can be attained inside the CVC. are calculated as: The accuracy of measuring the volume of water or the mixture of water and soil particles by using the CVC is determined by its unique − ¼ − Mt Msp ð Þ V sp V c ρ ðÞ 2 structure. The volume of the CVC is pre-determined during manufac- 1000 w T ture, and this structure can be fabricated into various sizes to satisfy measurement requirements. In this study, the CVC was made of alumi- ρ ¼ Msp ð Þ num alloy (Alloy grade 5083) and designed with a diameter of 74.9 mm, sp 3 1000V sp a height of 65.78 mm, and an inlet diameter of 3 mm. The alloy has a thermal expansion coefficient of 23.4 μmm−1 K−1, and does not tend 3 to deform when exposed to temperature variation. Changes in CVC vol- where Vsp is the volume of soil particles, m ; Mt is the mass of the water fi ume with temperature (Table 1) indicate that the maximum volume and soil particle mixture lling up the CVC, kg; Msp is the mass of the soil ρ −3 change is less than 1 cm3 when the measurement is conducted within particles, kg; and sp is the SPD, Mg m . a range from 5 to 40 °C, which is around the ambient temperature (20–25 °C). 2.4. Measurement of soil water content

2.2. Calibration of the CVC The partitioning principle is based on the model of soil three-phase composition, which consists of soil particles, water, and air. The initial The volume of the CVC is pre-determined during manufacturing and soil mass is determined by the mass of soil particles and water when calibrated by weighing the container filled with distilled water at ambi- the mass of air is neglected. The partitioning procedure is as follows. fi ent temperature (20–25 °C). The volume of the CVC is calculated with Aspeci c amount of wet soil of known mass is placed into the CVC. Eq. (1): The relationship among each volume fraction of mixture in the CVC is illustrated in Fig. 2. An appropriate amount of distilled water at ambient temperature is poured into the CVC to immerse the soil sample. The ðÞ – ¼ Mw T ð Þ mixture is stirred carefully at a constant speed for 2 3 min to expel V c ρ ðÞ 1 fi 1000 w T the trapped air before adding more distilled water to ll the CVC nearly full. The mixture is continuously stirred for another 2 min. The foam is carefully removed from the surface of the mixture, and the lid is fitted 3 where Vc is the constant volume of the CVC, m ; Mw(T) is the mass of to the vessel body. Additional distilled water is added drop by drop distilled water in the CVC at temperature of T(°C), kg; ρw(T)isthe into the closed vessel through the inlet by inserting the pinhead of the density of water at temperature T(°C), Mg m−3. injector below the surface of the mixture. Water is added until a drop The vessel body of CVC is filled with distilled water to nearly full, of water runs out through the overflow outlet. Eventually, the air is and then covered with the lid. Additional water is added by using an replaced by the supplied water, and a constant volume of water and injector. The pinhead of the injector is inserted below the water surface soil particle mixture inside the CVC is accurately determined. inside the vessel body through the inlet. Water is injected drop by drop The volume of the CVC is partitioned into the following components: until a drop of water runs out through the overflow outlet. Finally, the constant volume of water inside the vessel can be accurately deter- ¼ þ þ ð Þ mined by the CVC. V c Vsp V w V r 4

Table 1 Changes in constant volume container (CVC) volume with temperature.

Temperature (°C) 5 10 15 20 25 30 35 40 Volume change (%) 0.0351 0.0702 0.1053 0.1405 0.1756 0.2107 0.2459 0.2811 Y. Ma et al. / Geoderma 271 (2016) 42–49 45

Table 2 When Eqs. (9) and (10) are simultaneously solved, Vsp and Vw can Properties of the two tested soils. then be derived as: Soils Clay (%) Silt (%) Sand (%) CaCO Organic 3 ρ ðÞ− − (b0.002 mm) (0.002–0.05 mm) (0.05–1 mm) (%) matter (%) 1000 spV c V r M0 V w ¼ ð11Þ ρ −ρ ðÞ Loam 15 40 45 0.64 1.72 1000 sp w T Clay loam 29 48 23 7.68 0.73 − ρ ðÞðÞ− M0 1000w T Vc V r V sp ¼ : ð12Þ 3 1000 ρ −ρ ðÞT where Vsp is the volume of soil particles, m ; Vw is the volume of water sp w 3 originally held by the soil sample, m ; and Vr is the air-filled volume 3 replaced by supplementary water, m . Gravimetric soil water content (θ)iscalculatedas: The corresponding components of mass are given as: V ρ ðÞT θ ¼ w w 100%: ð13Þ M ¼ Msp þ Mw þ Mr ð5Þ ρ V sp sp where M is the total net mass of the mixture inside the CVC, kg; Msp is For measuring soil bulk density, the undisturbed soil sample is the mass of soil particles, kg; Mw is the mass of water originally residing obtained by a standard soil corer. Soil bulk density can be determined in the soil sample, kg; and Mr is the mass of water replacing the air-filled using the measured mass of soil particles (Msp) from Eq. (14) and the volume, kg. known volume of the standard soil corer: The mass (M0) of the wet soil sample is the mass of the soil particles and the original water in the soil and can be written as: ¼ ρ ð Þ Msp 1000Vsp sp 14 M ¼ M þ M : ð6Þ 0 sp w M ρ ¼ sp ð15Þ b 1000V After rewriting Eq. (5), the mass of the distilled water that fills up the cr rest of the CVC is: 3 where Vsp is the volume of soil particles determined by the CVC, m ; ρb is ÀÁ −3 the soil bulk density, Mg m ; and Vcr is the volume of the standard Mr ¼ M− Msp þ Mw : ð7Þ corer, m3. The volumetric soil water content θ can be determined as: The corresponding volume is determined as: v θ ¼ θρ : ð Þ v b 16 ¼ Mr : ð Þ V r ρ ðÞ 8 1000 w T

The volume of soil particles and initial soil water is derived as: 2.5. Soil samples

Soil samples were collected from two regions in China. The properties V sp þ V w ¼ V c−V r: ð9Þ of the two soils are listed in Table 2. Soil 1 (loam) was obtained from Jilin Eq. (6) can be rewritten as: City, Jilin province in northeastern China, and Soil 2 (clay loam) was M collected from Yangling, Shaanxi province in northwestern China. V ρ þ V ρ ðÞ¼T 0 ð10Þ sp sp w w 1000 Thirty-ﬁve soil samples with varied soil water contents were sampled from the ﬁeld by using standard soil corer with a volume of 100 cm3. −3 where ρsp is assumed as 2.65 Mg m in this study. These samples were weighed immediately after sampling and sealed

Fig. 3. Correlation between gravimetric soil water contents measured by volume/mass replacement (VMR) method and oven-drying method. 46 Y. Ma et al. / Geoderma 271 (2016) 42–49 with plastic bags for further soil water and bulk density measurements Table 3 through VMR and oven-drying. Soil particle density (SPD) measured by volume/mass replacement (VMR) for soils with various initial water contents. Samples directly obtained from the field may not satisfy varied soil water contents, which are evenly distributed in a range of 0 to 30%. Initial water content (%) Experimental soil was therefore oven dried and passed through a 0.10 10.20 15.42 20.38 25.17 29.96 Mean 2 mm sieve before processing to obtain designed soil water contents SPD of loam (Mg m−3) 2.65a 2.64a 2.65a 2.64a 2.64a 2.64a 2.64 of 10%, 15%, 20%, 25%, and 30%. In order to ensure the soil was fairly SPD of clay loam (Mg m−3) 2.66a 2.66a 2.66a 2.65a 2.66a 2.66a 2.66 dry, the soil was re-dried after sieving and stored in a sealed container Note: different lowercase letters within the same row indicate significant differences at before it was used for preparing the designed soil water content the p b 0.05 level. samples or for SPD measurement. Soil samples with designed water content were prepared according to Eq. (17). Dry soil was spread evenly on a tray, and water was sprayed 3. Results uniformly on the soil. Another layer of dry soil was spread on the wet soil, and water was sprayed again. This procedure was repeated until Soil water content measured through VMR and oven-drying showed the weighed water and soil were used up. The soil samples were stored a linear correlation, with R2 = 0.98 both in clay loam and loam. More- in the sealed container for 2 days prior to the measurements. over, the measured water contents of clay loam and loam obtained through VMR were 2.13% and 2.88% higher on average than those obtained through oven-drying, respectively (Fig. 3). m w ¼ w 100% ð17Þ Soil bulk density of the field samples measured through VMR and 0 m s oven-drying also exhibited a linear correlation, with R2 = 0.99 in clay loam and R2 = 0.98 in loam, respectively. Furthermore, pairs of measurements were all very close to the 1:1 reference line (Fig. 4). where w is the gravimetric soil water content, %; m is the mass of the 0 s Wet and dried soil samples were used for measuring SPD through dry soil particles, kg; and m is the mass of the water, kg. w VMR. The results indicated that the measurements were not significantly Experiments were conducted with loam and clay loam to verify the influenced by soil water content (Table 3). Dried soil samples were also conceptions and computational procedures of the VMR method for used to determine SPD through pycnometer and flask methods. These measurement of soil water and SPD. Soil samples of 80–90 g with varied methods significantly affected the SPD measurements. However, no re- soil water contents were tested, and the measurements of each soil markable differences were found between the pycnometer and VMR sample were repeated at least three times through VMR and oven- methods in loam, as well as the pycnometer and flask methods in clay drying. loam. The standard deviations of loam and clay loam measured through VMR were about 0.01 Mg m−3, which was about 50% lower than those 2.6. Measurements obtained through the pycnometer and flask methods (Table 4).

Soil samples and the mixture of soil particles and water were 4. Discussion weighed by an electronic balance, with an accuracy of 0.01 g. Water temperature was determined by a thermometer, with an accuracy of 4.1. Measurement of SPD 1°C. For determining soil water content and bulk density by the oven- Measurements of SPD through VMR using the proposed CVC show drying method, soil samples were oven dried at 105 °C for 72 h. SPD higher precision than those through the pycnometer and flask methods was also measured through the pycnometer method by using the proce- (Table 4). The larger measuring errors of the pycnometer and flask dure of ISO/TS 17892-3 (2004), and by the flask method following the methods may be due to splash and loss of soil particles during heating procedure on the website of the Globe Program (2014). or boiling to expel the trapped air; this phenomenon is particularly

Fig. 4. Correlation between soil bulk densities measured by volume/mass replacement (VMR) method and oven-drying method. Y. Ma et al. / Geoderma 271 (2016) 42–49 47

Table 4 Soil particle density (SPD) measurements with dried soil samples using the volume/mass replacement (VMR), pycnometer and ﬂask methods.

Loam Clay loam

VMR Pycnometer method Flask method VMR Pycnometer method Flask method

Mean (Mg m−3) 2.64a 2.65a 2.63b 2.66a 2.65b 2.64b Standard deviation (Mg m−3) 0.01 0.01 0.02 0.01 0.02 0.02

Note: each method has 15 replicates. Different lowercase letters within the row indicate significant differences at the p b 0.05 level. evident when using the pycnometer, which has a small bottle mouth the micro-pores of soil aggregates, resulting in larger measurements by (Ma et al., 2014). Furthermore, accurate meniscus readings are still the VMR method. Moreover, even though the soil samples are dried for difficult when measuring the volume of soil particle and water mixture a long period of time during oven-drying, a small portion of water still by using a volumetric flask because of liquid surface tension. Moreover, remain in the soil, thereby causing lowered measurements of soil foam floating on the surface of the mixture in the elongated bottleneck water content (Ma et al., 2013a, 2013b). of the flask is difficult to remove, thereby influencing the accuracy of For soils with high clay content, the mixture of water and soil parti- volume measurement. However, the unique structure of the CVC can ef- cles may need to be heated to further expel the trapped air. Due to the fectively eliminate volume measurement error caused by liquid surface larger open mouth of the CVC with the lid removed, it is more conve- tension at the inlet. Assuming the increase in height induced by liquid nient and easy to stir the mixture during heating process. surface tension is constant, the volume measurement error is linearly As shown in Table 3, the measured SPD of the two experimental soils related to the square of the inlet radius of the vessel. In this study, the was close to 2.65 Mg m−3. The linear correlation analysis between soil vessel body of the CVC had a diameter of 74.9 mm. The CVC with an water measurements using the measured SPD of loam and clay loam inlet diameter of 3 mm can reduce the volume measurement error to and those using 2.65 Mg m−3 is illustrated in Fig. 6. It indicates that 0.16% of that of 74.9 mm theoretically estimated by Eq. (18): the SPD of 2.65 Mg m−3 is feasible to use in VMR to determine the soil water content of the two experimental soils (Table 5). 2 ΔV 1 R ¼ 1 ð18Þ 4.3. Effect of SOM on soil water measurement ΔV 2 2 R2 Organic matter is one of the major solid components in agricultural where ΔV1 is the volume measurement error produced by liquid surface soil (Johnston et al., 2009; Loveland and Webb, 2003); organic matter 3 tension with radius of R1,m ; ΔV2 is the volume measurement error content affects SPD, thereby indirectly influencing the soil water 3 produced by liquid surface tension with radius of R2,m ; R1 and R2 are measurement accuracy in the VMR calculation process. Measurement the radii of the vessel, m. errors induced by SOM were theoretically analyzed through simulation,

assuming soil samples of Mts (kg) with gravimetric water contents of 5%, 4.2. Soil water measurement 10%, 15%, 20%, 25%, 30%, 35%, and 40%.

þ ¼ þ ¼ ð Þ The average SPD values of loam and clay loam were 2.64 and Mds Mw Mds pMds Mts 19 2.66 Mg m−3, respectively (Table 3). Soil water measurements through

VMR using these measured SPD values are shown in Fig. 5. A strong where p is the gravimetric water content, %; Mds is the mass of the dry relationship was found between VMR and oven-drying, with the soil particle component, kg; Mw is the mass of the water originally R2 = 0.98 in loam and clay loam. Measurements of the two soils through held by the soil sample, kg. VMR were about 2% higher on average than those determined through The density values of soil particles and SOM were assumed to be 2.65 oven-drying. This measuring error is probably due to the trapped air in and 1.3 Mg m−3, respectively. The density of water is 0.9978 Mg m−3

Fig. 5. Comparison between gravimetric soil water contents measured by constant volume container (CVC) using measured soil particle density (SPD) and thosebyoven-dryingmethod. 48 Y. Ma et al. / Geoderma 271 (2016) 42–49

Table 6 Errors in water content measurements by volume/mass replacement (VMR) method for soils with different proportions of soil organic matter (SOM) and soil water content.

Water Proportion of SOM (%) content (%) 1 1.5 2 2.5 3 1 1.5 2 2.5 3

Absolute error (%) Relative error (%)

5 0.66 1.00 1.33 1.67 2.01 13.25 19.94 26.67 33.45 40.27 10 0.69 1.04 1.40 1.75 2.11 6.94 10.45 13.97 17.52 21.09 15 0.73 1.09 1.46 1.83 2.21 4.84 7.28 9.74 12.21 14.70 20 0.76 1.14 1.52 1.91 2.30 3.79 5.70 7.62 9.56 11.51 25 0.79 1.19 1.59 1.99 2.40 3.16 4.75 6.35 7.96 9.59 30 0.82 1.23 1.65 2.07 2.49 2.73 4.12 5.50 6.90 8.31 35 0.85 1.28 1.71 2.15 2.59 2.43 3.66 4.90 6.14 7.40 40 0.88 1.33 1.78 2.23 2.68 2.21 3.32 4.45 5.57 6.71

Fig. 6. Linear correlation between gravimetric soil water contents calculated using the 1990; Topp and Davis, 1984; Zegelin et al., 1989). The precision of TDR measured soil particle density (SPD) and calculated using the SPD value of 2.65 Mg m−3. measurements is lower compared with that of oven-drying when the soil water content ranges from 0 to 5% and above 22%. The absolute at 22 °C. The total volume of soil particles and soil water (Vts)was errors are higher than 2%, and the relative errors are higher than 10% partitioned as follows: (Sun et al., 2014) because of air gaps created when inserting the probe into the soil (Reeves and Smith, 1992). It indicates that for wet soil ðÞ− pMds qMds 1 q Mds with water content higher than 5% and organic matter content lower þ þ ¼ V ts ð20Þ 997:8 1300 2650 than 2%, soil water content could be measured using the assumed SPD of 2.65 Mg m−3 through VMR with reasonable measurement accuracy where q is the mass proportion of organic matter, %. (Table 6). However, for soils with higher organic matter content or When the densities of these soil particles were assumed to be lower water content, measurement of the organic matter content or −3 2.65 Mg m , soil water content was calculated as: SPD is necessary in order to obtain acceptable measurement accuracy when using the VMR method. 2650 V −M V ¼ ts ts ð21Þ Using the CVC, a portable scale, and an injector, and following the w 2650−997:8 same measurement procedures as in the laboratory, measurement of soil water content or soil bulk density can also be conducted in Mts−997:8 V ts V ¼ ð22Þ ﬁ ds 2650−997:8 the eld.

997:8V θ0 ¼ w 100% ð23Þ 5. Conclusions 2650V ds VMR is a rapid method for partitioning water and soil particle frac- where V is the volume of soil water, m3; V is the volume of soil parti- w ds tions of soil mixture, and it can be used to measure SPD or soil water cles, m3;andθ' is the gravimetric water content, %. Subsequently, errors content as long as the volume of the mixture is accurately determined. (Δθ) in soil water measurements induced by soil organic content in A CVC with a unique structure was designed in this study to accurately VMR can be determined as: and rapidly determine the constant volume of water and soil particle 0 mixture. The use of this CVC can reduce the measuring error induced Δθ ¼ θ −p: ð24Þ by liquid surface tension remarkably. As analyzed during the establishment of partitioning model in VMR For example, in dry soil particles (M ) of 0.1 kg, the absolute errors ds method, SPD is another key factor that affects the accuracy of soil water in soil water measurements increase with increasing soil water content measurement. Measured soil water contents by VMR with an assumed and organic matter content, whereas the relative errors decrease with SPD value of 2.65 were correlated with those obtained through oven- increasing soil water content (Table 6). drying, yielding R2 = 0.98 in two experimental soils. The theoretical analysis showed that SOM affected SPD signifi- 4.4. Potential application of the VMR method cantly, thereby influencing the accuracy of partitioning of water and soil particles in VMR. The VMR method with the CVC can achieve Time domain reflectometry (TDR) is widely used for measurement acceptable accuracy of soil water measurement when soil water of soil water content in agriculture (Ledieu et al., 1986; Roth et al., content is larger than 5% and SOM content is less than 2%. The VMR method with the CVC can be used as an alternative approach to the Table 5 fi Soil water content measurements using either the measured soil particle density (SPD) or oven-drying method with appropriate accuracy and ef ciency. This a value of 2.65 Mg m−3. CVC can also be used for measurement of soil water content through VMR in the field. SPD Initial water content (%) −3 (Mg m ) 10.20 15.42 20.38 25.17 29.96 Acknowledgments 2.65 9.94 15.59 20.50 25.48 30.21 Loam 2.64 9.69 15.32 20.23 25.20 29.91 2.65 10.38 15.93 20.67 25.39 30.07 This work was supported by the Natural Science Foundation of China Clay loam 2.66 10.63 16.19 20.94 25.68 30.36 under Project No. 41230746, and “Agricultural water transformation Note: at the p b 0.05 level, there are no significant differences in the measurements using based on multi-process driving mechanism for improving water use either the measured SPD or a value of 2.65 Mg m−3. efficiency — China” (No. 51321001). Y. Ma et al. / Geoderma 271 (2016) 42–49 49

References Papadakis, J.S., 1941. A rapid method for determining soil moisture. Soil Sci. 51 (4), 279–282. Aggarwal, G.C., Tripathi, B.R., 1975. A simple and rapid method of soil water determina- Prihar, S.S., Sandhu, B.S., 1967. A rapid method of soil moisture determination. Soil Sci. – tion. J. Soil Sci. 26 (4), 437–439. 105 (3), 142 144. Amoozegar, A., Martin, K.C., Hoover, M.T., 1989. Effect of access hole properties on soil Rao, B.H., Singh, D.N., 2011. Moisture content determination by TDR and capacitance – water content determination by neutron thermalisation. Soil Sci. Soc. Am. J. 53 (2), techniques: a comparative study. Int. J. Earth Sci. Eng. 4 (6), 132 137. fl 330–335. Reeves, T.L., Smith, M.A., 1992. Time domain re ectometry for measuring soil water – Campbell, G.S., 1974. A simple method for determining unsaturated conductivity from content in range surveys. J. Range Manag. 45 (4), 412 414. moisture retention data. Soil Sci. Soc. Am. J. 117 (6), 311–314. Robinson, M., Dean, T.J., 1993. Measurement of near surface soil water content using a – Chanasyk, D.S., Naeth, M.A., 1996. Field measurement of soil moisture using neutron capacitance probe. Hydrol. Process. 7, 77 86. probes. Can. J. Soil Sci. 76 (3), 317–323. Robinson, D.A., Campbell, C.S., Hopmans, J.W., Hornbuckle, B.K., Jones, S.B., Knight, R., Elder, A.N., Rasmussen, T.C., 1994. Neutron probe calibration in unsaturated tuff. Soil Sci. Ogden, F., Selker, J., Wendroth, O., 2008. Soil moisture measurement for ecological Soc. Am. J. 58 (5), 1301–1307. and hydrological moistureshed-scale observatories: a review. Vadose Zone J. 7 (1), – Eller, H., Denoth, A., 1996. A capacitive soil moisture sensor. J. Hydrol. 185 (1–4), 358 389. fl 137–146. Roth, K., Schulin, R., Flühler, H., Attinger, W., 1990. Calibration of time domain re ectom- Evans, R., Cassel, D.K., Sneed, R.E., 1996. Measuring Soil Water for Irrigation Scheduling: etry for water content measurement using a composite dielectric approach. Water – Monitoring Methods and Devices. NCCES. Resour. Res. 26 (10), 2267 2273. Fityus, S., Wells, T., Huang, W., 2011. Moisture content measurement in expansive soils Ruhlmann, J., Korschens, M., Graefe, J., 2006. A new approach to calculate the particle using the neutron probe. Geotech. Test. J. 34 (3), 1–10. density of soils considering properties of the soil organic matter and the mineral – Gao, C., Zhao, P., Liu, Q., Zhang, H., Li, X., 2012. A rapid measurement method of soil water matrix. Geoderma 130 (3), 272 283. content in the field using RR-7730M soil hygrometer. Agric. Res. Arid Areas 30 (5), Schmugge, T.J., Jackson, T.J., Mckim, H.L., 1980. Survey of methods for soil moisture – 165–169. determination. Water Resour. Res. 16 (6), 961 979. Gardner, C., Dean, T., Cooper, J., 1997. Soil water content measurement with a high- Selig, E.T., Manusukhani, S., 1975. Relationship of soil moisture to the dielectric property. – frequency capacitance sensor. J. Agric. Eng. Res. 71 (2), 395–403. J. Geotech. Eng. Div. 101 (8), 755 770. Gregorich, E.G., Janzen, H.H., 1996. Storage of soil carbon in the light fraction and macro Sequeira, C.H., Alley, M.M., Jones, B.P., 2011. Evaluation of potentially labile soil organic – organic matter. In: Carter, M.R., Stewart, B.A. (Eds.), Advances in Soil Science. Struc- carbon and nitrogen fractionation procedures. Soil Biol. Biochem. 43 (2), 438 444. ture and Organic Matter Storage in Agricultural Soils. CRC Lewis, Boca Raton, Sun, L., Dong, X., Chen, M., 2014. A comparative study of soil moisture measurement – pp. 167–190. accuracy under different humidity using TDR. J. Anhui Agric. Sci. 42 (14), 4279 4281. Haines, W.B., 1923. The volume changes associated with variation of water content in Terhoeven-Urselmans, T., Schmidt, H., Joergensen, R.G., Ludwig, B., 2008. Usefulness of soils. J. Agric. Sci. 13 (3), 296–310. near-infrared spectroscopy to determine biological and chemical soil properties: – Huisman, J.A., Hubbard, S.S., Redman, J.D., Annan, A.P., 2003. Measuring soil wter content importance of sample pre-treatment. Soil Biol. Biochem. 40 (5), 1178 1188. with ground penetrating radar: a review. Vadose Zone J. 2, 476–491. The Globle Program, 2014. Soil Particle Density Lab Guide. http://www.globe.ee/ fi ISO/TS 17892-3, 2004. Geotechnical investigation and testing — laboratory testing of soil TeacherGuide/soil/protocols/ eldguides/soil_fg_partdensity.pdf. — part 3: determination of particle density. Pycnometer Method. Topp, G.C., Davis, J.L., 1984. Measurement of soil water content using time-domain fl fi – Jacobsen, O., Schjonning, P., 1993. A laboratory calibration of time domain reflectometry re ectrometry (TDR): a eld evaluation. Soil Sci. Soc. Am. J. 49 (1), 19 24. fl for soil water measurement including effects of bulk density and texture. J. Hydrol. Topp, G.C., Reynolds, W.D., 1998. Time domain re ectometry: a seminal technique for – – 151 (2–4), 147–157. measuring mass and energy in soil. Soil Tillage Res. 47 (1 2), 125 132. Jayawardane, N.S., Meyer, W.S., Barrs, H.D., 1984. Moisture measurement in a swelling Topp, G.C., Davis, J.L., Annan, A.P., 1982. Electromagnetic determination of soil moisture clay soil using neutron moisture meters. Aust. J. Soil Res. 22 (2), 109–117. content using TDR: applications to wetting fronts and steep gradients. Soil Sci. Soc. – Johnston, A.E., Poulton, P.R., Coleman, K., 2009. Chapter 1 soil organic matter: its impor- Am.J.46(4),672 678. tance in sustainable agriculture and carbon dioxide fluxes. Adv. Agron. 101, 1–57. Topp, G.C., Zegelin, S., White, I., 2000. Impact of real and imaginary components of relative fl Kelleners, T.J., Soppe, R.W.G., Ayars, J.E., Skaggs, T.H., 2004. Calibration of capacitance permittivity on time domain re ecometry meaqsurements in soils. Soil Sci. Soc. Am. J. – probe sensors in a saline silty clay soil. Soil Sci. Soc. Am. J. 68 (3), 770–778. 64 (4), 1244 1252. Ledieu, J., Ridder, P.D., Clerck, P.D., Dautrebande, S., 1986. A method of measuring soil Vereecken, H., Huisman, J.A., Bogena, H., Vanderborght, J., Vrugt, J.A., Hopmans, J.W., 2008. moisture by time-domain reflectometry. J. Hydrol. 88 (3–4), 319–328. On the value of soil moisture measurements in vadose zone hydrology: a review. – Li, J., Smith, D.W., Fityus, S.G., 2003. The effect of a gap between the access tube and the Water Resour. Res. 44 (4), 1 21. soil during neutron probe measurements. Aust. J. Soil Res. 41 (1), 151–164. Whalley, J.R., Dean, T.J., Izard, P.J., 1992. Evaluation of the capacitance technique as a – Loveland, P., Webb, J., 2003. Is there a critical level of organic matter in the agricultural method for dynamically measuring soil water content. J. Agric. Eng. Res. 52, 147 155. fl soils of temperate regions: a review. Soil Tillage Res. 70 (1), 1–18. Yin, Z., Lei, T., Yan, Q., Chen, Z., Dong, Y., 2013. A near-infrared re ectance sensor for soil – Ma, Y., Lei, T., Zhang, X., 2013a. Direct measurement method of soil moisture by volume surface moisture measurement. Comput. Electron. Agric. 99, 101 107. fi replacement. Trans. CSAM 44 (12), 148–153. Zegelin, S.J., White, I., Jenkins, D.R., 1989. Improved eld probes for soil water content and fl Ma, Y., Lei, T., Zhang, X., Chen, Y., 2013b. Volume replacement method for direct measure- electrical conductivity measurement using time domain re ectometry. Water Resour. – ment of soil moisture and bulk density. Trans. CSAE 29 (9), 86–93. Res. 25 (11), 2367 2376. Ma, Y., Lei, T., Zhuang, X., 2014. Volume replacement methods for measuring soil particle density. Trans. CSAE 30 (15), 130–139.