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Here is an essay on ‘Soil-Water Potential’ for class 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Soil-Water Potential’ especially written for school and college students.
Essay on Soil-Water Potential
Essay Contents:
- Essay on the Meaning of Soil-Water Potential
- Essay on the Components of Soil-Water Potential
- Essay on the Methods of Measuring Soil-Water Potential
- Essay on the Units of Soil-Water Potential
Essay # 1. Meaning of Soil-Water Potential:
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Soil-water potential or soil moisture tension is a measure of the tenacity with which water is retained in the soil and shows the force per unit area that must be exerted to remove water from the soil. It is measured in terms of the potential energy of water in the soil measured, usually with respect to free water. It is, usually expressed in atmospheres, the average air pressure at sea level. Other pressure units like cm or mm of water or mercury are also used.
1 atmosphere = 1036 cm of water = 76.39 cm of mercury
1 bar = 106 dynes cm2 = 1036 of water
1 milli bar = 1/1000 bar
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The term capillary potential and pF are also often used to define the energy with which water is held by the soil. The pF function, analogous to acidity-alkalinity scale pH, is defined as the numerical value of the negative pressure of the soil moisture expressed in cm of water.
The energy with which water is held by soil is as important as that of the amount of water retained by the soil. This energy at any given temperature is measured with references to a flat surface of pure water at some specified elevation and at a standard pressure. Pure water in saturated soil at the same elevation, pressure and temperature as the reference has a total water potential of zero.
As the water content of soil is decreased, the remaining water held by the force fields emanating from the soil particle surfaces increases. Thus, the drier the soil becomes, the more tightly the remaining water is held. Since, energy must be added to this soil-water to restore it to the reference state, its potential energy is said to be negative.
Similarly, soil-water potential of a soil at lower elevation than the reference is negative. If it is higher than the reference level, its water potential can be negative. The same holds true for samples at different pressures than the reference. Solutes in soil-water also lower its potential energy. Soil-water moves constantly in the direction of decreasing potential energy.
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The rate of decrease of potential energy with distance is, in fact, the driving force causing flow. Therefore, it is not the absolute amount of potential energy contained in the water that is important, but rather the relative level of the energy in different regions within the soil.
As defined by the International Society of Soil Science, the total potential energy of soil-water is the amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a pool of pure water at a specified elevation of atmospheric pressure to the soil-water (as the water under consideration).
This total water potential, Ψ, can be divided into parts to distinguish between the actions of different force fields indicated above. The algebraic sum of these parts or component potentials must always equal the total water potential.
Essay # 2. Components of Soil-Water Potential:
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Four component soil-water potentials are usually recognized:
1. Matric or capillary potential (Ψm).
2. Gravitational potential (Ψz).
3. Pressure potential (Ψp).
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4. Osmotic potential (Ψo).
1. Matric or Capillary Potential (Ψm):
It is the amount of work that an unit quantity of water in an equilibrium soil-water or plant-water system is capable of doing when it moves to another equilibrium system identical in all respects except that there is no matrix present. It results from the interaction of soil particle surfaces with water.
2. Gravitational Potential (Ψz):
It is the amount of work that a unit quantity of water in an equilibrium soil-water or plant water system at an arbitrary level is capable of doing when it moves to another equilibrium system identical in all respects except that it is at a reference level. It results from elevation with respect to the reference level.
3. Pressure Potential (Ψp):
It is the amount of work that an unit quantity of water in an equilibrium soil-water or plant-water system is capable of doing when it moves to another equilibrium system identical in all respects except that it is at a reference pressure. It results from external pressure on the soil-water. In unsaturated soils, the pressure potential is, usually, considered zero and in saturated soils, the matric potential is considered zero.
If the quantity of water involved in the above definition of water potential is a mass, soil-water potential has units as Joules kg-1 or ergs g-1. The quantity of water can be expressed in volume, in which case the water potential has units such as ergs cm-3, which is dimensionally the same as pressure. Since 106 ergs cm-3 is numerically equal to a bar pressure, water potential can be expressed in bars, if desired.
To avoid use of negative quantities of expression of energy state of water, alternate terminologies are often used. One of these, the suction system, defines total suction as the negative gauge pressure, relative to the external gas pressure on soil-water, to which a pool of pure water must be subjected in order to be in equilibrium through a semi-permeable membrane with the soil-water at the same elevation.
4. Osmotic Potential (Ψ0):
It is also termed as solute potential. It is the amount of work that an unit quantity of water in an equilibrium soil-water system is capable of doing when it moves to another equilibrium system identical in all respects except that there are no solutions. It results from the solutes dissolved in soil-water.
Total suction is the sum of osmotic and matric suctions. Except for algebraic sign, these component suctions are identical to the same component water potentials defined above where unit volume of water chosen as a basis for computation. Suction can be expressed in any pressure units. However, atmospheres or bars or equivalent height of a hanging water column are the most popular.
When dealing with movement of liquid water in unsaturated soil, it is usually only necessary to consider the matric and gravitational components of the water potential. In this case, the term soil-water tension is used instead of matric suction.
Soil-water tension is defined identically to matric suction, being the negative gauge pressure relative to the external gas pressure on the soil water, to which a solution identical in composition with the soil-water must be subjected to in order to be in equilibrium through a porous permeable wall with the soil-water. Soil-water tension is that quantity measured by a tensiometer.
When a column of soil, covered to prevent evaporation, is immersed partly in water, the hydrostatic pressure that exists below the free water surface and the soil water tension that exist above are shown for equilibrium in Fig. 7.10.
Measured in terms of length of a water column, the values are equivalent to the elevation: positive if below the free water surface and negative if above. If the soil above the free water is either wetter or drier than should be the case at equilibrium, water will flow in the direction to restore equilibrium.
The size of the tension head leading towards restoration is determined by deducting the soil-water tension, which should exist at equilibrium from the existing tension at the point under consideration.
This is equivalent to adding matric potential to gravitational potential or the gravity head to the tension head (soil-water tension measured in the same units). Gravity head is the elevation of the point under consideration measured from an arbitrary reference level, in this case the level of free water surface.
If the tension head at point P1 is – 5 units and the gravity head + 2 units (Fig. 7.9), then the net head is -3 units (-5 + 2) and water movement will be upwards at this position. If the tension head at point P2 is – 3 units and the gravity head + 5, then the net head is + 2 units and water must flow downward at this position until equilibrium is restored.
Essay # 3. Methods of Measuring Soil-Water Potential:
Measurement of soil-water content is not sufficient to provide a description of state of soil-water. To obtain such description, evaluation of energy status of soil-water (potential or suction) is necessary. Measurement of soil-water content in terms of its potential energy is indirect method of soil-water content measurement.
The sum of matric and osmotic potentials (total water potential) is useful index for characterising the energy status of soil-water with respect to plant-water uptake. The sum of matric and gravitational heads (hydraulic head or hydraulic potential) is useful in evaluating the direction and intensity of the water moving forces in the soil profile.
The energy level or total water potential (Ψ) in soils can be expressed as indicated below:
Ψ = Ψp+ Ψm+ Ψs + Ψt + Ψf
Where Ψp = pressure potential
Ψm = matric potential
Ψs = solute or osmotic potential
Ψt = temperature potential
Ψf = potential due to external forces.
Tensiometer:
Matric potential (suction or tension) can be measured in situ with tensiometers in the tension range up to about 0.8 bars. Tensiometer consists of a ceramic porous up and a mercury manometer attached to the water filled cup through a water reservoir tube (Fig. 7.11).
The porous cup has high conductance, low response time and air entry pressure of about 1.0 bar. When a porous cup is placed in soil and equilibrated, water tends to move out of the cup under the suction exerted by soil. As a result, vacuum pressure develops in the cup and to make up this, mercury rises in the manometric tube attached to water reservoir tube. Vacuum in porous cup is actually the metric potential of soil- water.
Tensiometer indicates the tension with which water is held by the soil but not the actual water content. Relationship between soil moisture tension and available soil moisture (Fig. 7.12) has to be prepared for reading soil moisture percentage.
Useful limit of most tensiometers is about – 0.80 bar of maximum potential. Hence, they are more accurate in wet range of soil. Tensiometers are ideal for sandy soils as – 0.80 bars may occur about 80 per cent of the ASM. Matric potential (suction or tension) can be measured in situ with tensiometers in the tension range up to about 0.8 bars.
Merits:
1. Same location and depth is observed with time, giving consistency.
2. Good for irrigation scheduling of sensitive crops which require frequent irrigations.
3. Good for irrigation scheduling in coarse textured soils where majority of ASM is between 0 and – 80 kPa.
Drawbacks:
1. Maximum pressure potential which can be measured is about -0.8 bars (- 80 kPa).
2. Maximum depth of insertion is about 5 m, as at this depth the measurable potential is reduced to 0 to – 30 kPa due to elevation difference.
3. Water in tensiometer must be maintained at a constant height.
4. May require hours of equilibrium time after filling or initial instillation.
Pressure Membrane and Pressure Plate Technique:
Osmotic potential is usually measured by extracting solution from the soil and determining its total potential either by vapour pressure measurements or by freezing point depression. In a bulk solution, the matric potential is zero, so that the only remaining component of the potential is osmotic. Often the osmotic potential of a soil solution is inferred from its electrical conductivity (USSL 1954).
To overcome this difficulty Richards (1947) developed pressure membrane apparatus for increasing the pressure on soil rather than decreasing the pressure of water in the tensiometer cup. Laboratory measurements of soil moisture potential are usually made with pressure membrane and pressure (suction) plate equipment (Fig. 7.13).
It consists of ceramic pressure plates or membranes of high air entry values contained in air-tight metallic chambers strong enough to withstand high pressure (15 bars or more). The apparatus enables development of soil moisture characteristic curves in the higher range of matric potential (>1 bar), which is not possible on suction plates.
The porous plates are first saturated and then the soil samples are placed on these plates. Soil samples are saturated with water and transferred to the metallic chambers. The chamber is closed with special wrenches to tighten the nuts and bolts with required torque for sealing it. Pressure is applied from a compressor and maintained at desired level. It should be ensured that there is no leakage from the chamber.
Water starts to flow out from saturated soil samples trough outlet and continues to trickle till equilibrium against the applied pressure is achieved. Soil samples are taken out and oven dried to constant weight for determining moisture content on weight basis.
Moisture content is determined against pressure values varying from – 0.1 to – 15 bars. The pairs of pressure and moisture content data so obtained are used to construct soil moisture characteristic curve.
Thermocouple Psychromcter:
If the values of several component potentials are unknown, total water potential (Ψ) may be measured by measuring the resultant vapour pressure of water in the system. At equilibrium, the potential of soil moisture is equal to the potential of water vapour in the ambient temperature.
If thermal equilibrium is assured and the gravitational effect in neglected, the vapour pressure can be taken to be equal to the sum of the matric and osmotic potentials, since air acts as an ideal semi-permeable membrane in allowing only water molecules to pass.
Thermocouple psychrometer makes possible in situ measurement of soil moisture potential. It is a double junction of two dissimilar metals. If the two junctions are subjected to different temperatures, they will generate a voltage difference.
If on the other hand, an electromotive force (emf) is applied between the junctions, a difference in temperature will result; depending on which way a direct current is applied, one junction can be heated while the other is cooled and vice versa.
The soil psychrometer consists of a fine wire thermocouple, one junction of which is equilibrated with the soil atmosphere by placing it inside a hollow porous cup embedded in the soil, while the other junction is kept in an insulated medium to provide a temperature lag.
An emf is applied so that the junction exposed to soil atmosphere is cooled to a temperature below the due point of that atmosphere, at which point a droplet of water condenses on the junction, allowing it to become, in effect, a wet bulb thermometer.
The cooling is then stopped and as the water from the droplet re-evaporates the junction attains a wet bulb temperature, which remains nearly constant until the junction dries out, after which it returns to the ambient soil temperature.
While evaporation takes place, the difference in temperature between the wet bulb and insulated junction serving as a dry bulb generates an emf which is indicative of soil moisture potential.
The relative humidity (vapour pressure depression relative to that of pure free water) is related to soil-water potential according to:
Ψ = RT In Ps/P0
where, R = international gas constant,
T = absolute temperature of the system,
Ps = actual vapour pressure of the system
P0 = vapour pressure of pure water at atmospheric pressure and temperature T.
The units of Ψ(ergs g-1, ergs cm-3 or bars) will depend on the units used for R.
Essay # 4. Units of Soil-Water Potential:
Soil-Water potential may be expressed in volume, mass and weight basis.
If E is the potential energy of a volume V of soil solution, water potential on volume (Ψ), mass (ϕ) and weight (H) basis may be expressed along with their dimensional units as:
Ψ = Energy/Volume = E/V = Force/Area = M/LT2
ϕ = Energy/Volume = E/ρV = L2/T2
H = Energy/Volume = E/ρgV = L
From the dimensional units, it is clear that water potential on volume basis has the same unit is that of pressure. In CGS system, commonly used units for ϕ are ergs g-1, Joule kg-1 etc. The unit of H is expressed as cm or m.
1 bar = 100 kPa
= 0.99 atm
= 14.5 psi
= 33.5 feet head
= 10.2 meters head
1 psi = 2.31 feet head
= 6.90 kPa
1 kPa = 1 kJ m-3 = 1 J kg-1
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