Soil Water Measurement

1. Soil Water Measurement

2. Soil Water Measurement • Soil water affects plant growth through its controlling effect on plant water status. • Two ways to assess soil water availability for plant growth: • by measuring the soil water content; and • by measuring how strongly that water is retained in the soil (soil water potential).

3. Soil Water Content • saturated soil: All soil voids (pore space) are filled with water. • Field Capacity (FC): All readily drainable water (by gravity) are vacated macro-pores, approximately 0.33 bar (330 cm or pF = 2.5). • Permanent Wilting Point (PWP): The soil moisture content at which the leaves of sunflower plants wilt permanently and do not recover if water is applied, approximately 15 bars (15,000 cm, pF = 4.2). Water is left only in micro-pores.

4. Available Water Capacity (AWC) • Volume of water that is kept in the soil between FC and PWP. • This water is “potentially” available to the plant and the value is generally used for determining frequency of irrigation and the depth of water that should be applied. AWC (mm m-1)=(FC by volume-PWP by volume)x(10)

5. Readily available water capacity (RAWC) • Not all the water held between FC and PWP is available at the same rate to the plants. • RAWC, kept at the lower tension (lower pF values), is considered a better indicator of soil moisture stress and should be used for irrigation scheduling. • Rule of thumb: 50% to 75% of AWC is considered as RAWC, varying based on crop physiology, rooting depth and volume, and moisture extraction pattern of each crop.

6. Measurement of FC and PWP • FC and PWP can be measured in the laboratory, using appropriately sized pressure plates and corresponding pressure membranes. • PWP measurement: Use of pressure plate (at -15 bars matric potential or pF4.2) is an accepted method. • Many question the validity of laboratory measurement of FC, and prefer field measurement.

7. AWC calculation • Soil moisture is determined on a weight basis. • Using Db values, MC on a weight basis is converted to MC on a volume basis: MC (% by volume v/v) = MC (% by weight w/w)x(Db) or, MC (% v/v) = (water weight/dry soil weight) x (weight of dry soil/total soil volume) Where, Db = Bulk density, and MC= Moisture content

8. Soil moisture characteristics curve • As water content in soil decreases, the matric potential decreases (becomes larger negative number). • The functional relationship between matric potential (the potential resulting from attractive forces between the soil matrix and the water) in the soil and changes in soil water content is named the soil moisture characteristics (retention) curve.

9. Moisture retention curve determination • Moisture content at saturation (water-content at pF = 0) is an indication of soil’s total pore-volume percentage. • Retention curve is produced for different soils by determining water content at different tensions between saturation and PWP. • Normal tensions applied (vacuum) are 0.05, 0.2, 0.33 (FC), 1.0, 3.0, &15 bars (PWP) that are equivalent to 1.7, 2.0, 2.5, 3.0, 3.5, and 4.2 pF values, respectively. • Moisture content of oven dry soil can be used as the equivalent tension of 9,800 bars (pF value of 7.0).

10. Laboratory Procedures for pF Curves • Saturate the soil cores until a film of water is formed on soil surface, letting water to be adsorbed from the bottom; • After weighing, place pre-saturated soils on top of the ceramic plate; • Make sure that there is a good contact between the soil cores and the ceramic plate;

11. Laboratory Procedures for pF Curves (cont.) • The outlet tube of the ceramic plate should then connected to the outflow tube of the pressure chamber; • The chamber should be pressurized to intended positive pressure; • The system should stay pressurized until equilibrium is reached with the applied water pressure. The equilibrium is reached when outflow of water has ceased which may even take three to four days;

12. Laboratory Procedures for pF Curves (cont.) • After reaching the equilibrium, the pressure should be released and the core samples should be weighed; • This procedure should be repeated for all intended matric potentials, until all measurements are completed; • After all measurements are completed, soil cores should be dried in forced air oven at 105oC.

13. Laboratory Procedures for pF Curves (cont.) • The volumetric water content for each matric potential will be calculated using: Volumetric water content (%)=Vol. of water (cm3)/Core volume (cm3) The volume of water at each matric potential (pF value) is then determined from: Vol. of water=(Mass of equilibrated soil–Mass of oven dried core)/DbH2O Where: DbH2O = 1 • The soil moisture characteristic curve is then produced by plotting the soil water matric potential (bar or pF value) against soil volumetric water content (%).

14. Soil water characteristics (retention) curves

15. Field measurement It is best to directly measure the degree of wetness (soil moisture content) or the matrix potential, rather than using calibration curves for estimating soil water content for irrigation scheduling, because of the effect of hysteresis caused by wetting and drying of soil samples.

16. Non-destructive water content measurementNeutron Probe • Neutron probe uses the property of scattering and slowing down neutrons (H+ ions). • Alpha particles emitted by the decay of the americium (241) collide with the light beryllium nuclei, producing “fast neutron”. • Fast neutrons, encountering hydrogen in the soil, lose their energy and are slowed down or thermalized. • The detection of “slow neutrons” returning to the probe allows estimation of the amount of H+ ions present. • Since most of the H+ ions in the soil is associated with soil water, it provide water content estimate.

17. Non-destructive water content measurementTime Domain Reflectometry (TDR) • TDR measures the spread of an electromagnetic wave through the soil. • The characteristics of this propagation depends on soil water content. • A good agreement exist between the TDR and neutron probe measurements. • The cost of neutron probe and TDR are prohibitive.

18. Non-destructive water potential measurementGypsum block/Granular Matrix Sensors • Exhibit a wide range relationship between their electric conductivity and soil water potential. • Somewhat unreliable in some soils caused by loss of contact with the soil due to dissolving of gypsum, inconsistence pore size distribution and soil salinity effects. • GMS works based on Gypsum block technology, but reduces the general inherent problems of gypsum blocks, using a granular matrix mostly supported in a metal or plastic screen.

19. Non-destructive water potential measurementTensiometers • Another type of instrument that measures the energy status (or potential) of soil water. • Tensiometers are extensively used for irrigation scheduling because they provide direct measurements of soil moisture status and are easy to manage. • Tensiometers are available at BoWRD.

20. Non-destructive water potential measurementTensiometers (Components) • A porous ceramic cup and a rigid body tube that is connected to a manometer or a vacuum gauge with all components filled with water, having an air-tied seal. • A Bourdon tube vacuum gauge is commonly used for water potential measurements.

22. Non-destructive water potential measurementTensiometers (Operation Principles) • Tensiometers are placed with ceramic cup firmly in contact with soil in plant root zone. • Since ceramic cup is porous, water moves through it to equilibrate with soil water, causing a hydraulic contact between water in the cup and soil water. • Water moving out of the cup develop a suction or negative pressure (partial vacuum) that causes a reading on the vacuum gauge. • Gauge reading, an indication of the attractive forces between water and soil particles, is a measure of the energy that would need to be exerted by the plant to extract water from the soil.

23. Non-destructive water potential measurementTensiometers (Operation Principles) • Tensiometer is able to follow changes in the matric potential as a result of soil drying out due to drainage, evaporation or plant uptake of water (transpiration). • When moisture is replenished by rain or irrigation, the matric potential will drop. • Tensiometer continuously records fluctuations in soil water potential under field conditions.

24. Non-destructive water potential measurementTensiometers (Operation Principles) • Accurate tensiometer response will occur only if air does not enter the water column. • Air expands and contracts with changes in pressure and temperature, thus causing inaccurate tensiometer readings. • Air leaks or dissolved air can enter through the ceramic cup during normal operation of the instrument. • If a significant amount of air enters the instrument, it must be expelled and the tensiometer refilled with water before it can reliably operate again.

25. Non-destructive water potential measurementTensiometers (Operation Range) • The useful range of a tensiometer is limited from 0 (saturation) to as high as 0.85 bar (85 cm head). • Above 0.85 bar the column of water in the tube will form water vapor bubbles (cavitate), causing instrument to stop functioning. • In many agricultural soils, the tensiometer range accounts for 50% of the soil water that is taken up by the plants (almost RAWC)

26. Non-destructive water potential measurementTensiometers (Site selection) • Tensiometers measure soil water tension in a small volume of soil immediately around the ceramic cup. • Should be placed within the root active zone(s) of the crop for which irrigation is scheduled. • Depending on crop type and its root distribution, one or more tensiometers of variable length may be required.

27. Non-destructive water potential measurementTensiometers (Placement in the field) • Site(s) selected for installation must be representative of the surrounding field conditions. • Tensiometers should be placed within the active root zone, in the plant canopy in positions, receiving typical amounts of rainfall and irrigation as the intended crop. • shallow-rooted crops (vegetables) need only one tensiometer, centered in the crop root zone, 10-15 cm below the surface. • Deep rooted crops (tree crops, most row crops) two tensiometer should be used at each site.

28. Non-destructive water potential measurementTensiometers (Installation) • Before field installation, each tensiometer should be tested to ensure it is working. • Fill tensiometers with clean water (deionized water) and keep vertically for at least 30 minutes to saturate the ceramic tip. • After fully wetting the ceramic tip, it can be refilled and capped.

29. Non-destructive water potential measurementTensiometers (Installation) • Tensiometer will not be serviceable immediately after filling because of air bubbles in the vacuum gauge. • small vacuum hand pump should be used to remove all air bubbles from the tube and vacuum gauge and test for air leaks. • After air bubbles are removed, tensiometers should be installed in previously cored holes to the appropriate depth in the field.

30. Non-destructive water potential measurementTensiometers (Installation) • Soil around tensiometer should be tamped at the surface. • After installation, several hours is required, before tensiometer can read the correct soil water potential value due to installation induced disturbance of the soil and the need for water to move through the ceramic cup before equilibrium is reached.

31. Non-destructive water potential measurementTensiometers (Installation) • Tensiometers must be periodically serviced in the field. • Under normal operation, air will be extracted from water under tension and becomes trapped within the tensiometer, reducing response time and its operability. • Tensiometer tube should be inspected each time the tensiometer is read. • If more than 0.5 cm of air is accumulated beneath the service cap, the trapped air should be removed and the tube refilled with deionized water.

32. Tensiometers (Automation) • Tensiometers can be instrumented to provide automatic control of irrigation systems. • Vacuum gauge is equipped with a magnet and a magnetic pick-up switch so that, when a desired (and preset) water tension occurs, the switch closes, starting the irrigation pump. • Pump operates for a preset period of time, lowering the tensiometer reading, after which the tensiometer is again monitored until the critical water tension again occurs.

35. I hope we all enjoyed the five days of office discussions. I certainly did and look forward to the upcoming field work. Let us make our hands dirty!