Soil Water Content and Soil Humidity
Soil water content is the physical parameter used to characterise the availability of water for plants in the soil. Besides the fundamental gravimetric method of measuring the water content of a soil sample, a wide range of other approaches is available. The aim of these alternative methods is either to provide a fast measurement value (soil conductivity) or to provide values for continuous measurements over longer periods (gypsum block conductivity, tensiometers, thermal conductivity). All approaches need calibration for individual soils if absolute values are actually needed, and they can have a restricted linearity at high or very low moisture ranges. Most alternative methods describe the physical properties of a very restricted solid volume, which then becomes the representative for a larger bulk volume. By contrast, weighing defined soil portions with a general, known dry weight provides an average humidity value for the entire container and enables easy calculation of water balances.
Soil water content is either calculated as an absolute value (g water /g soil), or relative to the water holding capacity (% of water holding capacity). In all cases, it must be considered that soil water content provides well-characterised values, but the actual biologically available water content for the plants may be significantly different. An extreme example: Biologically available water in a loamy soil is continuously reduced over time, however when lowering the absolute soil humidity with peat soils, the water availability remains close to constant over a wide soil humidity range, before the value then drops very fast.
For an optimal environmental simulation, soil water content and soil humidity are controlled for each plant individually to measure the response of the plants to specific environmental conditions.
Using LemnaTec scanalyzer conveyor systems and watering units to control soil water content and soil humidity
Soil Water Content - To provide similar basic growth conditions for plants, it is necessary to grow each plant or group of plants in a pot with a comparable amount of soil (based on dry weight). This is achieved by filling the pots with the same amounts of soil of a known, homogeneous soil humidity. Based on such data, the absolute amount of water in each pot can be measured and controlled by LemnaTec scanalyzer watering and weighing units. These systems can weigh the pots and plants one or several times per day. Shot weight changes during plant growth can be corrected by image-based biomass estimates, but in most cases the soil weight remains on a high level, in comparison with plant weight. Soil humidity distribution in different soil layers of transparent root columns can be assessed by near infrared imaging of the root columns, showing exactly where the plants extract the water from the soil.
If data is to be collected in short intervals , an additional option is to install soil humidity sensors in the individual pots and save resulting data to the central LemnaBase.