Methods to determine soil water potential.pptx

This ppt briefs about different methods to determine the soil water potential and their advantages and disadvantages.

1. Methods to determine soil water potential Presented By Rajat Kumar (F-24-55-M) To Dr. Nitin sharma (Assistant professor)

2. TENSIOMETER The invention of the tensiometer for measurement for soil water matric potential is commonly attributed to Willard Gardner . Tensiometers are the only instruments that can make a direct measurement of "soil suction" - the force that plants have to overcome in order to acquire needed water, and the force that determines the moisture distribution and transport within the soil. The tensiometer indicator is graduated from 0 to 100 where each unit represents the 100 th of the atmosphere . The unit measure is centibar or kilo pascals values close to zero indicate saturated soils .

3. 1 Elastic cover closing the refilling cup. 2. O-ring, which ensures air-tight connection between vacuum gauge and the tensiometer. 3. Connection point for the vacuum gauge with the possibility of different inclination for easy readings. 4. Mark on the tensiometer body for positioning of the tensiometer into the correct depth. 5. Thick-walled Plexiglas tube creating the body of the tensiometer; resistant to solar radiation and water and soil effects. 6. Replaceable ceramic tip (cup) with high value of hydraulic conductivity. 7. Push-button of the injection mechanism used for water refilling, it also takes off the collected air from the tensiometer. Water reservoir with a volume of water sufficient for several months of use. 9. Vacuum gauge for soil suction measurement. 10. Graduated scale with different units of pressure (i.e. kPa, cb , ..) 11. Elastic outer shell protecting the vacuum gauge against the influence of the weather.

4. PRINCIPLE A tensiometer consists of a tube with a porous ceramic tip on the bottom, a vacuum gauge near the top, and a sealing cap. When it is filled with deaerated water and inserted into the soil, water can move in and out of the tensiometer through the connecting pores in the tip . As the soil dries and water flows out from the tensiometer, it creates a vacuum inside the tensiometer which is indicated on the gauge. At the moment when the created vacuum corresponds to the "soil suction", the water outflow from the tensiometer is stopped. The dial gauge reading is then a direct measure of the force required to remove water from the soil. If the soil dries further, additional water flows out until a higher vacuum level is reached. When moisture is added to the soil, the reverse process takes place. Moisture from the soil moves back into the tensiometer through the porous tip until the vacuum level is reduced to a value corresponding with the lower soil suction value. Then, the water flow stops. If enough water is added to the soil until complete saturation, the gauge reading on the tensiometer will drop to zero.

5. ADVANTAGES Tensiometers indirectly measure soil moisture tension. Tensiometers provide readings at the same location over an extended period of time. Tensiometer readings are easily interpreted and indicate the soil water conditions experienced by the plants’ roots. Tensiometers are relatively inexpensive and reusable. Tensiometers are simple to operate. Tensiometers require some maintenance in the field, which involves adding water and using a hand vacuum pump to remove air from the tube.

6. THERMOCOUPLE PSYCHROMETERS Thermocouple psychrometer measure the water potential and its components by determining the vapour pressure in a sealed atmosphere in equilibrium with the sample containing liquid water. The liquid does not need to be continuous, making the method more versatile than others requiring liquid continuity. The sample is enclosed in a small, air-tight chamber and allowed to equilibrate. It requires 0.001 c temperature resolution It is based on the fact that the vapour pressure of the water is lowered as its water potential is reduced . It is first used by John Boyer and coworkers.

7. PRINCIPLE The amount of evaporative cooling on the psychrometer’s wet bulb is proportional to the amount of moisture in the air. Drier air absorbs more moisture from the bulb, which causes it to cool down faster. The temperature does not change as much because the warmer air cannot retain as much water from the bulb. In other words, the more humid the air is, the greater the change in the wet bulb temperature. Much information about atmospheric conditions, such as humidity, air temperature, surface temperature, and dew point, can be shown on a digital screen at once. However, these devices must be calibrated to local conditions for 20 to 30 minutes before use, and the wet bulb sensor will dry out if not used for a prolonged period of time. Psychrometer measures the water vapour pressure of a solution or plant sample on the basis of the principle that evaporation of water from a surface cool the surface .

8. ADVANTAGES Compact size for measuring microhabitats Rapid response time Ability to measure soil/plant water potential Temperature control of reference junctions for improved accuracy

9. SOIL MOISTURE METER A soil moisture meter also known as soil moisture sensor is a device that measures the amount of water in the soil . Soil moisture sensors can be stationary or portable an can be categorized by the technology they use like volumetric water content These meters are inserted into the ground to measure the amount of water in the soil the meter provides a number that indicates the soil moisture level. It works by measuring the property of the soil , such as its electrical resistance , dielectric constant or interaction with neutrons to indirectly determine the soil volumetric water content .

10. PRINCIPLE This sensor mainly utilizes capacitance to gauge the water content of the soil (dielectric permittivity). The working of this sensor can be done by inserting this sensor into the earth and the status of the water content in the soil can be reported in the form of a percent. The required voltage for working is 5V The required current for working is <20mA Type of interface is analog The required working temperature of this sensor is 10°C~30°C This sensor makes it perfect to execute experiments within science courses like environmental science, agricultural science, biology, soil science, botany, and horticulture.

11. ADVANTAGE Better equipped to maximize yields while maintaining high sustainability standards. Cost-effective and easy to use. Can help detect water scarcity and provide early warnings of potential drought conditions. Can aid in the management of landfills, as they can detect leaks and prevent contamination of groundwater sources. Aid good irrigation management, which gives better crops, uses fewer inputs, and increases profitability.

12. NEUTRON PROBE METHOD The neutron probe method is a non-destructive, in situ and relatively easy method for soil water measurement. The principal components of a neutron probe are a source of fast neutrons, a detector of slow neutrons, a scaler to count the number of slow neutrons detected and a means of positioning the source and detector at the required depth. Depth neutron probes need an access tube installed in the soil, down which the probe can be lowered to the required depths of measurement. For harvesting, the top part of the access tube is removed and a cap or rubber bung placed into it. There are two aspects to calibration, namely instrument and soil. Cosmic-Ray Soil Moisture Observing System (COSMOS) relies on neutron thermalisation to provide measurements of soil water content in the upper 0.1-0.7m of soil.

13. PRINCIPLE 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.

14. ADVANTAGES Soil moisture can be measured regardless of its physical state Thanks to its fantastic depth, a neutron probe can read an average moisture level Such probes can be interfaced with automatic downloading of stored data Soil moisture changes can be logged and measured to give accurate information Rapid changes of moisture level can be detected easily Measurements can be made and profiled in a non-destructive and reliable manner

15. GYPSUM OR ELECTRICAL RESISTANCE BLOCKS 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. Electrical resistance blocks are often referred to as gypsum blocks and sometimes just moisture blocks. The electrodes are connected to insulated lead wires that extend upward to the soil surface. Because of the pore size of the material used in most electrical resistance blocks, particularly those made of gypsum, the water content and thus the electrical resistance of the block does not change dramatically at suctions less than 0.5 bar (50 cb ). Therefore, 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.

16. PRINCIPLE 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. To make a soil water reading, the lead wires are connected to a resistance meter containing a voltage source. The meter normally reads from 0 to 100 or 0 to 200. High readings on the scale (corresponding to low electrical resistance) indicate high levels of soil-water, whereas low meter readings indicate low levels

17. ADVANTAGES Very low cost . Gypsum sensors can be made easily by unskilled labour . In soils with good hydraulic conductivity where water can flow freely , sensors will equilibrate with a large volume of soil and be unaffected by small stones , cavaties or plant root adjacent to the sensor . Resistance sensors can be automatically read and readings recorded by the equipment dedicated to this use . Gypsum sensors only work from the refill point to approximately six bars much less than that of the wilting point suction of most plants . Changes in soil water tension in wetter or drier ranges produced no change in the resistance of the sensor.

18. GRAVIMETRIC METHOD This is the simplest and most widely used method for determining the soil  moisture.This consists of obtaining a moist sample, drying it in an oven at 105°C until it losses no more weight and then determining the percentage of moisture. For soil samples containing significant amounts of organic matter, drying at the standard temperature for this method can result in organic matter decomposition, which can contribute to error in the calculation of soil moisture. For such soils, oven drying at 60 °C to constant mass, which can take up to 24 hours, should be considered. Structural water (crystallized water retained between soil minerals), which is not released below 105 °C, is outside the scope of this method. This structural water may be released during combustion analysis .

19. PRINCIPLE The gravimetric method allows the quantification of the soil moisture content based on the loss of weight (mass) due to the loss of water by heating the soil to a temperature of 105 °C ± 5 °C until constant mass is achieved. Soil moisture is expressed as a percentage of the mass of water in a given soil mass (e.g. grams of water per 100 grams of oven-dry soil) Before analyzing air-dry moisture content, the sample must have been pre-treated by drying to a constant mass, either at room temperature, or in an oven at a maximum temperature of 40 °C.

20. ADVANTAGES Accurate and precise – Gravimetric analysis is potentially more accurate and more precise than volumetric analysis . Possible sources of errors can be checked : Gravimetric analysis avoid problems with temperature fluctuations , calibration errors and other problems associated with the volumetric analysis. It is an absolute method . Relatively inexpensive .