MEASUREMENT OF SOIL MOISTURE POTENTIAL-1.pptx

Measurement of soil moisture potential

1. MEASUREMENT OF SOIL MOISTURE POTENTIAL BY: Sohail Ahmed REGISTRATION NO:MSA-2024-1912 M.Sc. Soil Science, SKUAST-K 1 st year,1 st semester COURSE NO:SOIL-501 COURSE TITLE:SOIL PHYSICS

2. SOIL MOISTURE POTENTIAL As defined by International Society Of Soil Science, total soil water potential is defined as “the amount of work per unit quantity of pure water that must be done by external forces to transfer reversibly and isothermally an infinitesimal amount of water from the standard state to the soil at the point under consideration.” YT = Ym+Yo+Yg+Yp where, YT: Total Soil Moisture Potential Ym: Matrix Potential Yo: Osmotic Potential Yg: Gravitational Potential Yp: Pressure Potential Units of Soil Moisture potential: N/m2

3. FIGURE 1 :Typical SWCC showing distinct zones of desaturation, Fredlund et al 2012

4. MEASUREMENT OF SOIL MOISTURE POTENTIAL DIRECT METHODS : Direct methods involve directly measuring the moisture content in soil samples through gravimetric, volumetric, or alcohol methods. 1.Thermo-gravimetric method(Oven Drying) 2.Alcohol Burning Methods 3.Volumetric method Except for Oven drying method, other methods are seldom used. INDIRECT METHODS : Indirect methods measure some property of soil affected by water content and all these methods require an initial calibration. 1.Tensiometer 2.Gypsum block or Electrical Resistance Method 3.Neutron probe 4.Pressure plate and pressure membrane apparatus 5.Time Domain Reflectometer(TDR) 6.Microwave Remote Sensing of Soil Moisture

5. 1.THERMO-GRAVIMETRIC METHOD(OVEN DRYING) Soil sample is collected in a moisture can and wet weight of the sample is recorded. The soil sample is dried in hot air oven at 105⁰C until constant weight is obtained and dry weight of the sample is recorded. Moisture content (on weight basis) = Wet weight-Dry weight X 100/ Dry weight. FIGURE 2 :SOIL SAMPLING ScienceDirect

6. 2.ALCOHOL BURNING METHOD Soil sample is mixed with a known volume of methyl alcohol . Then measure the change in specific gravity of a solution with hydrometer. This is a shortcut procedure but is not in common use.

7. 3.VOLUMETRIC METHOD Soil sample is taken with a core sampler or with a tube auger whose volume is known. The amount of water present in soil sample is estimated by drying it in the oven and calculating by formula. Volumetric Moisture content =Moisture content (%) by weight x Bulk Density (%) by volume The method is though accurate and simple it is used mainly for experimental purpose. Figure 3 :SAMPLING EQUIPMENTS Wikipedia

8. LIMITATIONS OF GRAVIMETRIC METHODS Sampling, transporting & repeated weighing give errors. It is also laborious and time consuming. The errors of the gravimetric method can be reduced by increasing the size and number of samples. However, the sampling disturbs the experimental plots and hence many workers prefer indirect methods.

9. 1.TENSIOMETER Tensiometer is a sealed, airtight, water-filled tube (barrel) with a porous tip on one end and a vacuum gauge on the other. A tensiometer measures soil water suction (negative pressure), which is usually expressed as tension. This suction is equivalent to the force or energy that a plant must exert to extract water from the soil. The instrument must be installed properly so that the porous tip is in good contact with the soil, ensuring that the soil-water suction is in equilibrium with the water suction in the tip. The suction force in the porous tip is transmitted through the water column inside the tube and displayed as a tension reading on the vacuum gauge. Soil-water tension is commonly expressed in units of bars or centi bars . FIGURE 4:TENSIOMETER Wikipedia

10. The suction at the tip is transmitted to the vacuum gauge because of the cohesive forces between adjacent water molecules. As the suction approaches approximately 0.8 bar (80 cb ), the cohesive forces are exceeded by the suction and the water molecules separate. When this occurs, air can enter the tube through the porus tip and the tensiometer no longer functions correctly. This condition is referred to as breaking tension. Tensiometers work in the range from 0 to 0.8 bar . Tensiometers are not recommended for clayey and silty soils unless irrigation is to be scheduled before 50 percent depletion of the plant-available water, which is the normal practice for some vegetable crops such as tomatoes.

11. 2.GYPSUM BLOCK OR ELECTRICAL RESISTANCE BLOCK Electrical resistance blocks consist of two electrodes enclosed in a block of porous material. The block is often made of gypsum, although fiberglass or nylon is sometimes used. The electrodes are connected to insulated lead wires that extend upward to the soil surface. Resistance blocks work on the principle that water conducts electricity. When properly installed, the water suction of the porous block is in equilibrium with the soil-water suction of the surrounding soil. As the soil moisture changes, the water content of the porous block also changes. The electrical resistance between the two electrodes increases as the water content of the porous block decreases. The block's resistance can be related to the water content of the soil by a calibration curve.

12. Resistance blocks are best suited for use in fine textured soils such as silts and clays that retain at least 50 percent of their plant available water at suctions greater than 0.5 bar. Electrical resistance blocks are not reliable for determining when to irrigate sandy soils where over 50 percent of the plant-available water is usually depleted at suctions less than 0.5 bar. FIGURE 5:GYPSUM BLOCKS OR ELECTRICAL RESISTANCE METER MPDI

13. 3.NEUTRON MOISTURE METER Soil moisture can be estimated quickly and continuously with neutron moisture meter without disturbing the soil. Another advantage is that soil moisture can be estimated from large volume of soil. This meter scans the soil about 15 cm diameters around the neutron probe in wet soil and 50 cm in dry soil. It consists of a probe and a scalar or rate meter. This contains a fast neutron source which may be a mixture of radium and beryllium or americium and beryllium. Access tubes are aluminum tubes of 50-100 cm length and are placed in the field when the moisture has to be estimated. Neutron probe is lowered in to access tube to a desired depth. Fast neutrons are released from the probe which scatters in to soil. When the neutrons encounter nuclei of hydrogen atoms of water, their speed is reduced. The scalar or the rate meter counts of slow neutrons which are directly proportional to water molecule. Moisture content of the soil can be known from the calibration curve with count of slow neutrons. FIGURE 6:NEUTRON PROBE ScienceDirect

14. 4.PRESSURE MEMBRANE AND PRESSURE PLATE APPARATUS Pressure membrane and pressure plate apparatus is generally used to estimate field capacity, permanent wilting point and moisture content at different pressure. The apparatus consists of an air tight metallic chamber in which porous ceramic pressure plate is placed. The pressure plate and soil samples and saturated and are placed in the metallic chamber. The required pressure of 0.33 or 15 bar is applied through a compressor. The water from the outlet till equilibrium against applied pressure is achieved. After that, the soil samples are taken out and oven- dried for determining the moisture content. FIGURE 7:PRESSURE PLATE APPARATUS Researchgate

15. 5.TIME DOMAIN REFLECTOMETER(TDR) The time domain reflectometer (TDR) is a new device developed to measure soil-water content. Two parallel rods or stiff wires are inserted into the soil to the depth at which the average water content is desired. The rods are connected to an instrument that sends an electromagnetic pulse (or wave) of energy along the rods. The rate at which the wave of energy is conducted into the soil and reflected back to the soil surface is directly related to the average water content of the soil. One instrument can be used for hundreds of pairs of rods. This device, just becoming commercially available, is easy to use and reliable. FIGURE 8:TIME DOMAIN REFLECTOMETER Wikipedia

16. 6.MICROWAVE REMOTE SENSING OF SOIL MOISTURE This method enables us to estimate soil moisture content over a large region or area. The basic principle is that the microwave back scattering coefficient is greatly influenced by dielectric property of the soil which is mostly a function of soil water content. As the soil water content increases, the dielectric constant of the soil also increases which decreases the emissivity(e) of the wet soil. The decrease in emissivity is approximately linear with increase in soil water content.

17. Case Study 1: Impact of Sensor Placement in Soil Water Estimation: A Real-Case Study: Orouskhani, E.,Sahoo, S.,Agyeman, B et.al. (2023) Key Highlights : Objective : To analyze how the spatial positioning of soil moisture sensors affects the accuracy of water estimation in real-world agricultural settings. Methodology : Sensors Used : A network of 42 soil moisture sensors was deployed. Installation Design : Sensors were placed at varying depths and spatial locations to measure soil moisture and potential. Data Logging : Multiplexers connected the sensors to data loggers, capturing real-time measurements. Findings : Sensor placement near irregular irrigation sources or at varying soil compaction levels caused deviations in measurements. Data inconsistencies arose when sensors were too shallow or located in areas prone to surface runoff. Properly calibrated and well-placed sensors provided reliable and consistent soil moisture data.

18. Conclusions : Sensor placement plays a vital role in obtaining precise soil moisture estimates. It is recommended to: Avoid placing sensors in regions with high variability in soil properties. Perform pre-installation soil assessments to identify homogenous zones. Regularly validate sensor readings with physical soil samples. FIGURE 9 :Illustration of an agro-hydrological system (Agyeman et al. 2021)

19. FIGURE 10 :Estimation results using simulated data.  a – c  Trajectories of the actual pressure heads (red lines), estimated pressure heads in case 1 (blue lines), and estimated pressure heads in case 2 (green lines) at depth of  a  5 cm,  b  15 cm, and  c  30 cm, below the surface.  d  Evolution of the RMSE of the original state vector during the simulation time in case 1 (blue lines) and case 2 (green lines)  Orouskhani, E.,Sahoo, S.,Agyeman, B et.al. (2023)

20. CASE STUDY 2 : Robust Soil Water Potential Sensor to Optimize Irrigation in Agriculture: Menne, D.; Hübner, C.(2022),Germany Key Findings: Sensor Design and Functionality: The sensor utilizes white ceramic discs, which have demonstrated promising results comparable to the TEROS 21 sensor in determining plant-available water in the soil. In contrast, sensors with brown ceramic discs exhibited significant differences in measurements, potentially due to their narrow pore size distribution and small average pore size, which may hinder hydraulic equilibrium with varying soil particle sizes. Field Experiments: Conducted in the Rhineland Palatinate High Forest, Germany, the experiments involved installing sensors at depths of 30 cm and 60 cm. The sensor with white ceramic discs effectively registered precipitation events and tracked soil drying over time, providing valuable data for irrigation management. Comparison with Commercial Sensors: The novel sensor's measurements were consistent with those of the TEROS 21 and the SMT100 dielectric sensor, indicating its reliability and potential for practical applications.

21. Implications for Irrigation Optimization: The development of this robust soil water potential sensor offers several advantages for agricultural irrigation: Enhanced Accuracy: By accurately determining plant-available water, farmers can make informed decisions about irrigation scheduling, ensuring crops receive optimal hydration. Improved Resource Management: Precise soil moisture measurements contribute to water conservation efforts by preventing over-irrigation and reducing water waste. Adaptability: The sensor's performance across different soil compositions and environmental conditions underscores its versatility for diverse agricultural settings. In summary, this innovative sensor represents a significant advancement in soil moisture measurement technology, providing a reliable tool for optimizing irrigation practices and promoting sustainable agriculture

22. FIGURE 11 :Correlation of soil water potential data (18) and count values from the sensor with white ceramic discs recorded during drying experiments in model soil lawn base layer, as well as forest soil (Palatinate Forest: Merzalben and Rhineland Palatinate High Forest: Hermeskeil). Menne, D.; Hübner, C.(2022)