Vapour Sorption Analyser Automatically Generates Soil Water Isotherms

Labcell (UK) has introduced the Decagon Devices AquaLab Vapour Sorption Analyser (VSA) for generating soil water isotherms – also referred to as soil water characteristic curves, moisture release curves, water retention curves or pF curves. This instrument will be of interest to users in agriculture and horticulture, both for improving yields and in soil and plant research.

Compared with traditional methods of generating isotherms, the cost-effective VSA is much quicker and easier to use, bringing this type of analysis within the reach of many more users and for a wider range of applications.

Every soil has a unique moisture characteristic that determines water availability for plants. The traditional method for determining the soil moisture isotherm - which is a plot of the soil’s water potential content against its water content - involved using tensiometers at the wet end and, at the dry end, a laborious process of using thermocouple psychrometers or chilled mirror dewpoint hydrometers to measure water potential, and drying ovens and weighing scales to measure moisture content. Because of the time-consuming, laborious and, therefore, expensive way of generating isotherms, only a few data points are collected over a period of several days, so the isotherm is of very limited accuracy.

In contrast, the VSA is fully automatic and can generate an isotherm with up to 100 data points for each of the adsorption and desorption curves in as little as 24 to 48 hours. Users simply load the 10cc sample, enter a few parameters and wait for the wetting and drying curves to be generated. The combination of accuracy (+/-1MPa or +/-1%), speed and very wide measurement range (-10MPa to -475MPA), together with the instrument’s ease of use, make the VSA a far more attractive proposition than other analysers currently available. The instrument takes up very little space on the bench, and has a USB port so that data can easily be saved or transferred.

Because the VSA measures so many data points, the semi-log plot of water content versus matric potential is a straight line whose slope can be measured accurately. This slope indicates whether the soil will shrink or swell as it dries and takes up water, and it can also be used to calculate specific surface area for the soil.

In addition, the latest version of the VSA benefits from a static feature that enables humidity to be held constant so that studies can be made of the way a soil takes up water into its crystal structure (2:1 clays), and water content change can be monitored over time.