Water is an essential and constant component in soil. Because of its importance methods of measuring soil moisture have been around for a long time. What is new in soil moisture measurement is digitalization of instrument output and the ability to down load data into data loggers and computers. Another new component of portable instruments is the ability to be attached to a GPS (Global Positioning) unit.

All soils always have water in them. Driving down a sandy road the cloud of soil behind the car contains 1% to 2% moisture. Driving down a clay road the cloud of dust behind the car has perhaps 20% moisture in it. Water is a constant component in soil but it is always changing. The amount of water in soil determines how water will flow, the solubility of organic and inorganic compounds and ions and the movement of these components into and through the soil profile. It is also essential for microbial growth and thus for bioremediation processes. Any soil contaminant will be in contact with water and its water solubility will greatly affect its mobility and activity in soil.

The movement of soil water ultimately depends on how strongly it is held to the soil matrix. If soil is saturated with water and the source of water removed then some of the water will drain from soil. This is termed gravitational water, which is held with pressures of –0 to –30 kPa. As water drains from soil air replaces it. Draining a saturated soil thus results in it going from being anaerobic to aerobic. The next most available water is called plant available water. It is held with a pressure of between –30 and –1500 kPa. Water held at pressures greater than –1500 kPa is in capillaries, is not available to plants and moves slowly in soil. When soil is oven dry water is held with a pressure of greater than –10,000 kPa.

Water moves from areas where it is loosely held to areas where it is strongly held. Gravitational water is pulled by gravity through the soil. Water in excess of gravitational water will flow over the surface of soil and into surrounding bodies of water. Knowing the water content of soil will tell us where the water is moving, the direction it is moving and if there is enough water for plant growth and thus bioremediation.

Oven dry weight is the most common method of determining the water in soil. A soil sample is placed in a weighed metal drying cup and the total weight obtained. This would be the wet weight of soil. The cup is dried for 24 hours at 105 0C and reweighed. At this point we can determine the % of water in the original soil sample using the following equation.

Although % moisture on a dry weight bases is a handy thing to know it does not tell us about the status of water in soil. This is because it does not tell us the pressure holding it. It also does not give us information about the possible direction of water flow.

To measure the pressure holding water one of several other methods is used. To obtain this information tensiometers, pressure membrane systems, resistance blocks, neutron probes, thermocouple psychrometers and various instruments based on Time Domain Reflectometery (TDR) can be use. Tensiometers and resistance blocks are inserted in soil and left in place. Neutron probes require an access tube into which the instrument is lowered to obtain a measurement. TDC comes in various configurations some of which are intended to be buried while others are carried from place to place and inserted into the soil and a measurement made (Figure illustrates some of these measuring instruments)

Both tensiometers and pressure membrane systems work on the same principle. A porous ceramic cup or plate, which allows water but not air to pass is used as the functional part of the system. The tensiometer has a long tube filled with water attached to a porous cup. The tube and cup are inserted into the soil. As the soil dries water moves out of the cup creating a negative pressure. In older models the top of the tube had a gauge on it to measure the pressure. Newer tensiometers have digital meters or have a system, which allows a single digital meter to be used on several tensiometers.

This method is easy to use and is a measure of the water status in the field. However it is limited to water held between -0 and -85 kPa. Although this is a narrow range it is a measure of the water easily available to plants. It is also the water that moves most easily through soil. Thus it is important in following water movement and in bioremediation.

The pressure plate system has a ceramic plate and a pressure chamber. Wet soil samples are weighed and placed on the plate. The plate is placed in a pressure chamber, which is pressurized to the desired kPa and water is subsequently pressed out of the soil. After some time pressure is released and the soil sample reweighed. The moisture at that particular pressure is thus obtained. The amount of water held at various pressures tells us how much water of each type there is or can be in that soil. This method is very precise and relatively easy to use but is restricted to the laboratory and it takes several days to complete an analysis.

Resistance blocks have two electrodes buried in a block of gypsum. The electrical resistance of the blocks changes depending on their moisture content. A standard curve is used to relate the resistance of a block buried in a particular soil to its moisture content. Today resistance blocks are made form a variety of materials, which are less prone to disintegration than gypsum.

Neutrons pass easily through most solid objects. However, they are stopped and deflected by hydrogen particularly hydrogen in water and organic matter. Fast neutrons interact with water by giving up energy and slowing down. For soils with low or constant organic matter content a neutron probe can be used to measure soil water in the field. An access hole is drilled in the profile to be measured and a liner, usually aluminum, inserted. The neutron probe is a cylinder with a source of fast neutrons in the lower end and a slow neutron detector in the upper end. The probe is lowered into the access tube to the desired depth and the slow neutrons coming back to the probe counted. The number of slow neutrons is then related via a standard curve to the water content of the soil.

Thermocouple psychrometers us a thermocouple junction housed in a small porous ceramic cup buried in soil. The cup is electrically cooled to a point where a drop of water forms on the thermocouple. The cooling is stopped and the water on the thermocouple evaporates at a rate inversely related to the relative humidity of the soil. This in turn is related to the soil moisture potential. A voltage is generated during evaporation and is converted to a readout of water potential. This method is only useful in dry soils and is relatively imprecise having an uncertainty of 50 kPa.

TDR is a newer method for measuring soil moisture. It measures the dielectric constant of soil, which depends on its moisture content. Air has a low and water a high dielectric constant. Thus, the amount of water in soil is directly related to its water content. The instrument usually comes with two or more rods attached to a probe head. The rods are inserted into soil and a MHz pulse is applied to the rods. The pulse time of travel down the rods is related to the soils dielectric constant. The dissipation of the pulse is related to the soil’s salinity.

Often the output from TDR instruments is reported as volumetric soil water content. This would be volume of water associated with 1 m3 of soil or milliliters of water per 1000 mL of soil. As a rule of thumb most scientist consider 1 mL of water to weigh 1 g. This is not exactly correct but it is close enough for most work. Thus, if we know a soil’s bulk density, it is easy to relate this to % moisture on a dry weight basis or any other way of representing soil water that one desires.

Different instruments have different ranges over which they are useful. Tensiometers can only be used in the range of 0 to –85 kPa. Resistance blocks can be used from –50 to –1500 kPa.

Most of the above methods of measuring soil moisture have been around for a long time. What is new is the digitalization of their output. Also relatively new is the ability to down load data into data loggers and computers . Instruments designed to be portable may also have the ability to be attached to a GPS (Global Positioning) unit.

Today virtually all models and methods come in arrangements that allow collection of data in digital form. This allows data to be recorded and stored on a data logger. This is a storage device, which stores the data for later retrieval and use. They can record thousands of data points and date stamp them. Data loggers can be kept with or be part of the instrument or they can be hand held instruments, which are carried from instrument to instrument. Sometimes they come with short line of sight radios, which allow data from a large number of instruments to be transmitted to a central location.

A limited number of manufacturers have designed their instruments such that the data can be loaded directly into a computer. This allows one to manipulate, graph and convert data for other uses.

GPS capability is important for locating the exact place where measurements are taken. It also allows repeated measurements at the same location to be made without disturbing the location with a marker.

Most activities one wishes to carry out in soil are affected by the soil’s water content. Growing crops or plants for phytoremediation requires adequate soil water. Following natural water or a washing solution movement through soil is important if one is to know what is happening to pollutants. Thus, measuring and recording soil moisture is an essential activity.

 Soil moisture measuring instruments (not drawn to scale)

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