Soil Moisture Measurement

Measuring the moisture level in soil is a complicated undertaking because so many different factors interfere with the results.  For example, the soil type, the soil structure, the density, especially of soil substrates, random air pockets, temperature and salinity all influence moisture measurement.

The amount of water in the soil or the water availability can be determined using various physical properties. These properties include electrical conductivity and capacity (dielectric), thermal conductivity, the reflection of radiation (for instance, infrared), simple weight determination and the suction pressure. However, not all methods of measurement are practical for use in the field and most are not in common use.

Measurement of the Suction Pressure (= Suction Tension)

The so-called suction pressure can be measured relatively simply using a tensiometer.  The porous, clay tip of the tensiometer transfers water from within to the drier outer surroundings by means of capillarity, thereby, creating a sub-pressure within the sealed tensiometer tube. This sub-pressure is a measure of the moisture level and can be determined as a value or used directly to activate an electrical switch. The customary unit of measurement is hPa (hectopascal): 1 hPa = 1 mbar = 1 cm water column.
The suction pressure is the force with which water is being held in the soil or is available for absorption. This is the force that must be produced by the plant roots in order for water to be absorbed. Fine pores and the corresponding capillaries in the soil are the critical factors affecting suction pressure. A tensiometer directly measures this, for plants, important soil characteristic.  Tensiometers do not need to be calibrated, giving them a special advantage compared to electrical instruments for pressure measurement.
The suction pressure increases as long as the surrounding area is drier and the substrate is capable of further transporting water und maintaining a moisture differential. If the moisture in the surrounding area increases, the process reverses itself. Very close soil contact is required for a quick tensiometer reaction and in order to get the typical tensiometer readings associated with certain types of soil and substrates.
However, a tensiometer also functions in dry air as long as evaporation can take place over the porous, clay chamber. Therefore, moisture levels can be measured even in coarse-grained or very loose substrate. The slight surface contact and the larger proportion of air pockets result in a specific suction pressure for this substrate.  The best experiences with suction pressure measurement have been made with mineral substrates such as „Seramis“.
Since the water is not 100% reabsorbed, the tensiometer uses a small amount of water during the process of measuring, especially in dry ranges. Measuring constantly in dry soil would lead eventually to a slow emptying of the tensiometer. Therefore, tensiometers are predestined for use primarily in moderately moist ranges where the maintenance is minimal.

Evaluating suction Pressures

The measuring range of a tensiometer is about 0 - 900 hPa and is limited by the capillarity of the porous, clay chamber and whether or not the system has any leaks. This range is sufficient for measuring the typical suction pressures for most moisture levels found in soil (see table).
Suction pressure measurements are largely independent of the salt concentration of the substrate or soil. Inaccuracies can occur, as with all pressure measurements, through large variation in temperature, especially when the tensiometer contains too much air.
In general, the tensiometer measurement is a point measurement. The absolute value measured is applicable only for the particular site where the measurement was taken. Measuring suction pressures over a large area is difficult due to a multitude of differences in the soil and the existence of a certain soil dynamic. However, a single measurement has relative little predictive value, due to the possible presence of random or deviant values.
The strength of measuring suction pressures lies in the comparison of many results and series of results over time in order to depict the course of moisture levels.  Also, controlling irrigation sequences through suction pressure allows for sufficient room for the deviation of single values.
Suction pressures do not give information as to the amount of stored water. The water content as a volume percentage must be determined for each individual soil type (see Soil Science Methods in the Literature Appendix).

Measuring Capacitance

The principle behind the electrical measurement of soil moisture levels is the capacity (loading capacity) of a condenser. Electrical fields are created during measuring; in this case, the soil functions as an insulator (dielectric) and, depending upon the moisture content, possesses a particular “dielectric constant”.
The amount of water molecules present in the soil is decisive for measuring, in other words; the water content is being measured, not the availability of water. In contrast, suction pressure measures the availability.
Salt content (ions), soil type and density as well as temperature all have a clear influence on the results. Therefore, measurements must be either adjusted through instrument calibration or by technically offsetting the results.
Measurements can be made without maintenance and the range measured is well above that of a tensiometer, this is particularly of interest when measuring in areas that are very dry or inaccessible.
Different methods are used to put this measurement principle into practice. For instance, particular frequencies can be measured (FDR-Procedure) or an impulse time measured (TDR-Procedure) or other related techniques can be employed. As a result, the instruments used all vary as to construction and design features. Externally, the designs also differ in the type of conductors used, for example, with either 2 or 3 steel rods (electrodes), ring-shaped electrodes within a larger rod or integrated, covered conductors. Also, the output signals vary, from the standard signal of 0 – 5V or      0 - 10V up to adjusted values for moisture content in percent of volume.
The TensioTech Sensor „CapTensio“ uses a reactive surface encasing a compact cylinder (covered conductors) to measure capacitance by impulse length; in addition, microprocessors measure and compensate for temperature and conductance. The measuring parameters are programmable and therefore, can be adjusted for soil type and sensitivity.

Other methods for measuring soil moisture levels

Another electrical method worth mentioning is measurement of the conductance value (conductivity or electrical resistance), whereby the soil is tested solely for its ability to transfer an electrical current. However, this method is dependent not only upon the water content of the substrate but also the concentration of salt; the latter of which has an even greater influence with this method than when measuring capacitance.
Therefore, a simple conduction sensor can function accurately only when the salt concentration remains constant or with minimal fluctuation (for instance, perhaps outdoors). Blocks of plaster of Paris also function by measuring conductivity, however, in this case, the plaster encasing the electrodes is meant to minimize or prevent the influence of salt.  “Watermark Sensors“ function similarly but use an alternative encasing to solid plaster. Both sensors are not exact enough for use in wet ranges.
Currently, there are no other practical devices on the market as an alternative for measuring soil moisture content, for instance, methods using thermal conductivity or reflected radiation in the infrared or other ranges.
The classic lab method, gravimetrical determination, determines the moisture content by measuring the weight of wet and dry samples using accepted lab methods. This is regarded as the standard method, however, for quick, practical use and for automation of irrigating systems is unsuitable.
Newly developed is the IRRIGAS®  System for controlling soil moisture levels by means of water saturation of a porous, clay vessel. The degree of saturation is directly dependent upon the moisture content of the surrounding soil and becomes impermeable to air under wet conditions and permeable by dryness. Various clay chambers with defined permeability are tested using air pressure to determine their current state of permeation. Irrigation can then be started when a particular level of soil dryness is reached.