• The water content of soil at which all the soil pores are filled with water is referred to as saturation or maximum water holding capacity.
• Energy status of water at saturation is zero.
• Water content of a soil at saturation is approximately double that of field capacity.
• Soil moisture held by the soil against gravitational force is called field capacity.
• Energy status of water at field capacity is -0.1 to -0.33 bars.
• Field capacity is considered as the upper limit of water availability to plants.
• As the thickness of water around soil particles decreases, more and more energy is required to remove the water.
• When the energy status of soil moisture reaches -15 bars, plants cannot absorb sufficient moisture and show wilting symptoms even under high humidity conditions. This is called permanent wilting point.
• Permanent wilting point (PWP) was proposed by Briggs and Shantz in 1912. They utilized dwarf sunflower as indicator plant.
• At PWP plants wilt but do not die and are able to absorb small quantity of water just sufficient for their survival and plants recover if water is supplied.
• Soil moisture at -60 bars is not available to plants and they die due to lack of water. Soil moisture content at which plants die is known as ultimate wilting point.
• Hygroscopic coefficient of water is defined as the amount of water that soil contains when brought to equilibrium with air at one standard atmosphere at 98% relative humidity at the room temperature.
• Moisture equivalent is the amount of water retained by a sample of initially saturated soil material after being subjected to a centrifugal force of 1000 times that of gravity for a definite period of time, usually half an hour.
• In medium textured soils, the values of field capacity and moisture equivalent are nearly equal.
• FC>ME in sandy soil.
FC<ME in very clayey soil
• Sandy soil has the lowest amount of available water (8 cm/m depth) while clay soil has the highest amount (23 cm/m depth).
• Amount of water present in the soil between field capacity and permanent wilting point is called available water.
• Veihmeyer and Hendrickson proposed that water is available to plants with equal ease throughout the available range.
• Richards and Wadleigh (1952) proposed that soil water availability to plants actually decreases with decreasing soil moisture.
• Recent evidence indicates that yield of several crops do not reduce if the soil moisture is depleted up to 25 % of available moisture, but further decrease in soil moisture decreases the yield.
• The main defect of the above three theories is that they relate plant growth with soil moisture only without considering climate.
• When a water molecule is kept in a hypothetical situation, wherein it is not subjected to the influence of any force, its potential to move in any direction is zero.
• When the water molecule is attached to any substance, its ability or potential to move is reduced.
• Water potential of soil is always less than zero and is expressed in negative values.
• The total potential 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 from a pool of pure water at a specified elevation at atmospheric pressure.
• The total potential of soil water is the sum of matric, osmotic and gravitational potentials.
• Matric potential is the portion of the total water potential that is attributable to the solid matrix of the soil or plant.
• Matric potential is the negative potential which results from the capillary and adsorptive forces emanating from the soil matrix.
• At saturated condition in soil matric potential is zero
• Osmotic potential is the portion of water potential that results from the solutes present in the soil.
• Gravitational potential is the potential attributable to gravitational force.
• Pressure potential at a point is a direct result of the overlying water.
• Pressure potential is always positive.
• PF means potenz meaning power at 10.
• PF value was first time introduced and defined by Schofield (1935).
• PF is the scale through which we measure the force with which water is retained in capillary or soil.
• PF is defined as the logarithm to the base 10 of the numerical value of the negative pressure of the soil moisture expressed in centimeter of water.
• PF at field capacity is 2.5.
• PF at permanent wilting point is 4.2.
• In direct methods soil moisture is estimated thermo-gravimetrically either by oven-drying or by volumetric method.
• Soil moisture is estimated indirectly by tensiometer, gypsum block, neutron probe, pressure plate and pressure membrane apparatus.
• Tensiometers are also called irrometers since they are used in irrigation scheduling.
• Length of tensiometer tube varies from 30 cm to 100 cm.
• Tensiometers are sensitive up to 0.9 bars of soil moisture.
• Tensiometers are suitable for sandy soils as most of the available water in the sandy soil is within one bar potential.
• Irrometers do not give any information on the amount of irrigation water to be applied at each irrigation.
• Gypsum blocks are not suitable for saline soils as salts in the soil increases conductivity.
• In gypsum block the resistance reading is about 400-600 ohms at field capacity and 50,000 – 75,000 ohms at wilting point.
• Soil moisture can be estimated quickly and continuously with neutron moisture meter without disturbing the soil.
• By neutron moisture meter soil moisture can be estimated from large volume of soil.
• Neutron moisture meter consists of a probe and a scalar or rate meter.
• Probe contains fast neutron source which may be a mixture of radium and beryllium or americium and beryllium.
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