Field instruments

Things to know about conductivity

Key parameters in soil and soil hydrology include the (field) saturated hydraulic conductivity or the infiltration rate. The determination of the pneumatic conductivity is important for the description of the gas transport in soils. This may also be used for deriving the hydraulic conductivity.

With innovative measurement techniques, It is now also possible to determine chemical parameters such as  the carbonate content, the electrical conductivity, and the pH-value easily and under real boundary conditions, which previously could only be determined in laboratory tests.

The saturated hydraulic conductivity indicates the water volume, which flows through a specific area at a specific slope per unit of time. The pneumatic conductivity is the equivalent value for gases. Both are soil-physical parameters which give information about the microstructure, pore connectivity and tortuosity of a soil. In addition to the potential hydraulic gradient, the conductivity of the soil is the decisive factor in the determination of the flux density (DARCY law).

The field-saturated conductivity and the maximum infiltration capacity of a soil are estimated using infiltrometer tests. In experiments of this kind, the water volume infiltrating per unit of time is determined by the temporal observation of the change in water level (of the ponding  water in the ring infiltrometer or in the storage vessel of a Mariott bottle). The infiltration rate, initially dominated by capillary forces, decreases with time asymptotically against a constant value. In this phase the gravitational force has the dominant influence on the infiltration rate. Taking into account the experimental setup (double-ring infiltrometer or single-ring infiltrometer), it is possible to determine the (field) saturated conductivity from the data basis by adapting suitable mathematical models.

The measurement of the pneumatic conductivity is comparatively associated with less effort. Furthermore, it is possible to determine the pneumatic conductivity directly in the field at different soil water contents without great effort. Various research has so far dealt with the derivation of the unsaturated hydraulic conductivity on the basis of the pneumatic conductivity of a soil at different water contents.

The pH-value of the soil is the negative decadic logarithm of the ozonium ion activity (H3O+) in the soil solution. The pH is one of the most important parameters for soils as it has a significant influence on different processes. These include, among other things, weathering and decomposition processes in the soil, but also the availability of various chemical elements and substances in the soil is influenced by the pH-value. For example, the mobility of some heavy metals can be increased at low pH-values and the availability of plant nutrients can be adversely affected.

The carbonate content, on the other hand, provides information about the buffering capacity of a soil. In soils with a high buffering capacity, the pH-value is kept constant even at a high proton load. Only when the buffer capacity is reached, the acid load will have a negative impact.